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Effects of dietary thyroid hormones on growth, plasma T3 and T4, and growth hormone in normal and hypothyroid chickens
GENERAL
AND
COMPARATIVE
ENDOCRINOLOGY
59,
9 l-99 (1985)
Effects of Dietary Thyroid Hormones on Growth, Plasma T, and and Growth Hormone in Normal and Hypothyroid Chickens F. C. LEUNG, Department
of Animal
J. E. TAYLOR, Physiology, Rahway,
Merck New
AND A. VAN Sharp Jersey
& Dohme 07065
T4,
IDERSTINE Research
Laboratories,
Accepted October 5, 1984 Cockerels and pullets fed with T, or T, for 2 weeks showed a decrease in both body weight gain and feed efficiency. The reduction in body weight gain and feed efficiency was dose related in cockerels where T, or T, were fed at 0.1, 1.0, and 10.0 ppm levels. T, and T, at 0.1 and 1.O ppm had no significant effects on growth or feed efficiency in pullets, but the lO.O-ppm level of T, and T, caused a reduction of - 55.24 and - 28.18%, respectively, in body weight gain as compared with control birds. T, was more active than T4 in reducing growth and was toxic when fed at 10.0 ppm both in cockerels and pullets. Both propylthiouracil (PTU)- and methimazole-treated cockerels showed a decrease in rates of gain. T, and T, at a dietary level of 0.1 ppm were equipotent in promoting growth in these PTUand methimazole-treated cockerels, but 10.0 ppm caused a further reduction in body weight gain. Plasma T, levels were found to be significantly higher in birds that were fed either T, or T,. Plasma T, levels were higher in T,-fed birds, but significantly lower in T,-fed birds as compared with controls. Both PTU- and methimazole-treated cockerels had significantly lower plasma T, and T, concentrations, but elevated plasma GH concentrations. Dietary T, and T, at 1.O and 10.0 ppm significantly lowered plasma GH concentrations. In summary, these results indicated that T, was more active than T, in reducing body weight gain in intact normal birds, but that they were equally potent in promoting growth in PTU- and methimazole-treated hypothyroid birds. Thyroid hormones (T, and T4) are also more active in decreasing growth in intact cockerels than in pullets. It appears that there is some peripheral conversion from T, to T,, and T, has a negative feedback on plasma T, concentrations. Since PTU-treated birds show elevated plasma GH concentrations, the decrease in body weight gain is probably due to the lowering of thyroid hormones rather than to an effect on pituitary GH release. 01985 Academic PXSS. hc.
It is known that thyroid hormones are necessary for normal growth and development in birds (Falconer, 1971; Ringer, 1976). In mammals, triiodothyronine (T,) has been shown to be more potent than thyroxine (T4) as determined by goiter prevention, sensitivity to hypoxia, and oxygen consumption (Barker, 1955). In the bird, however, studies of relative potency between T, and T, have been controversial. Shellabarger (1955) was the first to report that T, and T, were equally potent in preventing goiter, and Newcomer (1957) found that T, was as active as T4 in influencing oxygen consumption, heart rate, suffocation time, and feather growth in thiouraciltreated chicks. Singh et al. (1968) observed
that T, has the same effect as T, in suppressing the effects of methimazole as indicated by thyroid secretion rate, and Raheja and Snedecor (1970) found that T, and T, were equally potent in stimulating body weight, comb weight, and liver glycogen of propylthiouracil (PTU)-treated and radio/ thyroidectomized (RT) chicks. However, T, had been reported to be more potent than T, in preventing goiter in thiouraciltreated chicks (Newcomer, 1957; Mellen and Wentworth, 1959), in promoting oxygen consumption in cardiac muscle (Newcomer and Barrett, 1960) and in increasing metabolic rate (Singh et al., 1968). Srivastava and Turner (1967) reported that T, was more active than T,. T, was also reported 91 001~6480185 $1.50 Copyright 0 1985 by Academic Press, Inc. All rights of reproduction in any form reserved.
LEUNG,
92
TAYLOR,
to be more active than T, in stimulating intracellular accumulation of amino acids by embryonic chick bones (Adamson and Ingbar, 1967). May (1980) reported that T, was more active than T, in reducing growth and feed efficiency in intact birds, and T, was also found to be more active than T, in shortening survival time during heat stress (May, 1982). Recently, Harvey (1983) and Harvey et al. (1983) showed that thyroid hormones (T3 and T4) inhibited GH release in birds and that surgical thyroidectomy increased basal concentration of GH. Such interaction between thyroid hormones and GH is not observed in mammalian species. In the current series of experiments, we planned to investigate the effects of T, and T, on growth in normal and hypothyroid birds and its effects on plasma T,, T,, and GH concentrations. MATERIALS
AND METHODS
Hubbard x Hubbard cockerels and pullets were used in these experiments. Birds were individually caged in a temperature (25”)- and light (14 hr light-10 hr darkness)-controlled room. Pennfield Laboratory Broiler Feed-Pennfield 180 (Pennfield, Lancaster, Pa.) was used. Triiodothyronine (TX), thyroxine (T4), propylthiouracil (PTU), and methimazole were obtained from Sigma Chemical Company, St. Louis, Missouri. T, or T, was added to the Pennfield diet at 0.1, 1.O, and 10.0 ppm. PTU was added to the diet at 0.5% and methimazole was added to the drinking water at 0.05%. Feed and water were available ad libitum. Four- or Iiveweek-old birds were used in all experiments except in the methimazole-treated experiments in which 2week-old birds were used. The experimental period in all experiments was 2 weeks. All birds were weighed, feed consumption was measured, and plasma samples were collected via heart puncture two times a week for 2 weeks. Plasma samples were kept at -20” until assayed for T,, T,, and GH concentrations. Plasma T, and T, concentrations were determined by the T, and T, clasp radioimmunoassay kits (Squibb, Princeton, N.J.), and plasma GH concentrations were determined by a homologous RIA for chicken GH (Leung et al., 1984) with FLcGH-II as standard. The data were analyzed for statistical significance by the analysis of variance. The Student-NewmanKeuls test was used to analyze the differences among groups and differences were considered significant at P < 0.05.
AND VAN IDERSTINE
RESULTS
Effects of Thyroid Hormones The results of the and T4 on body weight and plasma T, and shown in Figs. 1 and
in Cockerels
effects of dietary T, gain, feed efficiency, T, in cockerels are 2. T, fed at 1.0 and
A 0 Control 0 T,.O.i ppm
2
I 1I I
3
7
IO
14
Days
FIG. 1. Effects of T, on body weight gain, feed efficiency, and plasma T, and T, in 4-week-old cockerels. N = 9 per group. *P = cO.05, compared with controls.
EFFECTS
OF THYROID
HORMONES
IN CHICKEN
93
body weight gain and feed efficiency were significantly decreased in birds fed T, at 10.0 ppm. Birds that were fed either T, or T, at 1 .O and 10.0 ppm showed a significant increase in plasma T, levels. Plasma T, concentrations were significantly lower in birds fed with T, at 0.1, 1.0, and 10.0 ppm, and the decrease was dose related. Plasma T, concentrations were significantly elevated in the birds fed with T, at 1.0 and 10.0 ppm. The slight elevation of plasma T, concentrations in birds fed with 10.0 ppm of T, was probably due to peripheral conversion of T, to T,. Effects of Thyroid Hormones
100
Days
FIG. 2. Effects of T4 on body weight gain, feed efficiency, and T4 in Sweek-old cockerels. N = 9 per group. *P = cO.05, compared with controls. **Values over the limit of the assay.
10.0 ppm caused significant dose-related reductions in body weight gain. Birds fed T, at 10.0 ppm had poorer feed efficiency than birds on control diet and T, was also found to cause fatalities. Eight out of nine birds were dead by the end of the experiment. Birds fed T, at 0.1 and 1 .O ppm showed no significant difference in growth and feed efficiency as compared with controls, but
in Pullets
The effects of dietary T, and T, on body weight gain, feed efficiency, and plasma T, and T, in pullets are shown in Figs. 3 and 4. Birds fed T, or T, at 0.1 and 1.0 ppm showed no significant effects on body weight gain or feed efficiency, but birds fed 0.1 ppm of T, or T, showed a slight increase in body weight gain, 3.03 and 4.99%, respectively. T, or T, fed at 10.0 ppm caused significant reductions in body weight gain and feed efficiency. T, at 10.0 ppm also caused fatalities with five of nine birds dying by the end of the experiment. Plasma T, and T, concentrations in the pullets were similar to those found in the cockerels. Plasma T, concentrations were significantly elevated in birds fed T, at 1.0 and 10.0 ppm and birds fed T, at 10.0 ppm. Plasma T, concentrations were also higher in T,-fed birds, but birds fed T, at 1.0 and 10.0 ppm showed a significant decrease in plasma T, concentrations. Effects of Thyroid Hormones
on GH
In a separate series of experiments which were similar to those described in the above sections, the effects of thyroid hormones on body weight gain and feed efficiency in cockerels and pullets were the same as were reported in the above sections (data
I
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I
Plosmo T, (rig/ml) g g $
I
I
I 0
g
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0
-NW*
Plarma T3 (rig/ml)
Plasma T3 (rig/ml 1
(feed/gain)
-R)cIIPCn
Feed. Efficiency (feed/gains)
Feed Efficiency
I
l
DOOD
Body Weight Gain (gms 1
Body Weight Gain (gms)
EFFECTS
OF THYROID
HORMONES
IN CHICKEN
95
TABLE 1 EFFECTS OF DIETARY T, AND T4 ON PLASMA GH CONCENTRATIONSIN MALE AND FEMALE CHICKENS GH (&ml) Treatment (dose level) Control 0.1 ppm 1.0 ppm 10.0 ppm
T,-fed Male 40.9 38.0 30.1 37.0
k k t k
3.1 2.2 1.1* 2.8
T,-fed Male (9) (9) (9) (6)
51.9 47.8 35.4 32.8
+ f k *
T,-fed Female
4.8 6.5 3.5* 1.6*
(9) (9) (9) (9)
29.0 21.2 22.4 19.5
f * k k
5.2 4.4 1.9 3.2*
T,-fed Female (9) (9) (9) (6)
26.8 23.1 23.6 20.5
f t r e
2.5 2.3 1.8 1.3*
(9) (9) (9) (9)
( ) = Number of birds. a Mean ? SEM. * P < 0.05 as compared with control. Note.
Effects of Thyroid Hormones in PTU- and Methimazole-Treated Cockerels
The effects of thyroid hormones on body weight gain, plasma T,, T,, and GH concentrations are shown in Table 2. PTU fed at 0.5% caused a significant decrease in body weight gain and plasma T, and T, concentrations but GH concentrations were elevated. The addition of T, and T, at 0.1 ppm in the diet to the PTU-treated birds improved their growth by 22.0 and 21.0%, respectively, and plasma T, and T, and GH concentrations were similar to those found in control birds. When birds were fed with T, or T4 at 1.O and 10.0 ppm, plasma T, and T, concentrations were elevated. T, and T, at doses of 1.0 and 10.0 ppm produced a significant retardation in body weight gain. Plasma T4 concentrations in birds fed T3 at 1.0 and 10.0 ppm were significantly lower than controls and PTU-treated birds probably due to a negative feedback of T, at the pituitary and/or hypothalamus. Birds receiving these hyperthyroid doses of T, or T, also had significantly lower plasma GH concentrations when compared with birds which received control or PTU diet. Similar results were obtained with 2-week-old cockerels that were treated with methimazole (Table 3). Methimazole at 0.05% in the drinking water significantly lowered body weight gain and plasma T, and T, concen-
trations. The addition of T3 or T, at 0.1 ppm in the diet of the methimazole-treated birds improved growth by 42.0 and 58.0%, respectively, and plasma T, concentrations were also significantly higher than in methimazole-treated birds. Plasma T, and T, concentrations were still significantly lower than those found in control birds. Doses of 1 and 10 ppm of T, and T4 produced higher circulating thyroid hormones resulting in further retardation in body weight gain as compared to control and methimazoletreated birds. DISCUSSION
Our present findings clearly demonstrate that the potency differences between T, and T, depend on the physiological state of the thyroid gland in the animals used in these experiments. Our results agree well with previous investigators that T, was equally potent as T, when birds were in a hypothyroid state induced by thyroidectomy or goitrogen treatment. Also, experiments indicating that T, was more active than T, were conducted with presumably euthyroid birds. It is possible that in hypothyroid states T4 is readily converted into T3 and thus yields biological responses similar to those of T, itself. Since both T, and T, appear to have a very short and similar half-life, a range between 2 and 8 hr (Astier,
7
+ T, (10.0
2
+ T, (0.1
+ T, (1.0
+ T, (10.0
PTU
Experiment Control PTU
PTU
PTU
PTU
’ All values b PTU was c Percentage * P < 0.05 ** P < 0.05
7 5
+ T, (1 .O ppm)
PTU
7
5
7
HORMONES
are means 2 SEM. added into the diet at 0.5%. change as compared with control, as compared with control. as compared with PTU control.
ppm)
ppm)
ppm)
ppm)
7
+ T, (0.1
PTU 7
7 7
N
THYROID
1
ppm)
OF DIETARY
Experiment Control PTUb
Treatment
EFFECTS gain (g)
856.4 644.0 (-24.8) 690.0 (- 19.4) 601.0 (- 29.8) 382.1 (-55.4)
802.1** 617.1* (-23.1y 657.9* (- 18.0) 520.0* (-35.2) 322.1*,** (-59.8)
wt.
WEIGHT
in parentheses.
Body
ON BODY
T,,
T,,
81.23
107.2
120.5
126.1 64.7
290.3
218.1
219.7 104.7
ml)
AND GH
+
+
4.8*
18.3
If- 14.3**
7.66
1.95
2.43 1.27
0.41
-
-t- 15.8** t 18.6*
0.61
0.82
1.10 0.89 0.0.5*
0.04** 0.02*
ml)”
PTU-TREATED
0.06*,**
0.90*,** >50.0
t
2 0.34
-t 0.09** k 0.14*
f
k 0.04*%**
i
f ”
T, (pg/lOO
IN ~-WEEK-OLD
2 75.7
A 14.4
f 20.4** ? 16.2*
T, (ng/lOO
TABLE 2 GAIN, PLASMA
5.9
15.5
27.4
31.0 38.1
23.60
21.61
41.44
32.54 48.45
GH
+
5
t
It k
f
?
It
zt k
2.1*,**
2.7*,*”
5.7
8.2 18.6
2.55*~**
1.60*,**
3.98
1.39** 9.66*
(r&ml)
COCKERELS
n
E 2 2
ij
2
% u
$
2
2
“3
EFFECTS
OF THYROID
HORMONES
97
IN CHICKEN
TABLE 3 EFFECTSOF DIETARY THYROID HORMONES ON BODY WEIGHT GAIN AND PLASMA T, AND T, IN ~-WEEK-OLD METHIMAZOLE-TREATED COCKERELS Treatment Experiment 1 Control Methimazoleb
N
Body wt. gain (g)
T, (ng/lOO ml)
T4 (141~
6 7
511.3** 356.9* (-30.2)= 423.0* (-17.3) 338.0* (-33.9) 238.2*,** (-53.4)
246.3 e 33.2** 40.9 +- 3.4*
2.10 -+ 0.03** 1.36 k 0.05*
133.9 2 13.2*,**
1.27 + 0.04*
347.4 * 39.g**
1.16 + 0.02*
1310.1 f 20.9*,**
1.21 k 0.04*
165.5 k 10.3** 53.3 If: 6.6*
2.12 2 0.10** 1.48 2 0.03*
119.6 AZ 8.3*,**
1.41 2 0.04*
184.1 2 15.4**
12.23 k 2.59*,**
262.3 r 36.8*,**
>50.0
Methimazole
+ T, (0.1 ppm)
5
Methimazole
+ T, (1 .O ppm)
7
Methimazole
+ T, (10.0 ppm)
6
Experiment 2 Control Methimazole
7 7
Methimazole
+ T, (0.1 ppm)
8
Methimazole
+ T, (1.0 ppm)
7
Methimazole
+ T, (10.0 ppm)
5
490.6** 352.2* (-28.2) 432.5** (-11.8) 375.6* (-23.4) 296.0* (-39.7)
ml)’
a All values are means f SEM. b Methimazole was added into the water at 0.05%. c Percentage change as compared with control, in parentheses. * P < 0.05 as compared with control. ** P < 0.05 as compared with methimazole treated at 0.05%.
1973; Assenmacher, 1973; Davison, 1978), the change in biological potency between T, and T, in different physiological states could simply be due to the differences in monodeiodinase activity. Recently, Scanes et al. (1983) showed that the hypothyroid nature found in sex-linked dwarf chickens was due to depressed liver T,-5’ monodeiodinase activity. Reverse T, was found in the circulation of chickens (Thommes and Hylka, 1977; Premachandra, 1977; May, 1977), and May (1980) found most of the T,, when given at higher doses, was converted into rT,. Reverse T, was not measured in the present experiments, but the small increase of plasma T, in the T,-fed groups suggests that at least some of the T, was converted to
rT,, thereby preventing accumulation of toxic levels of T, in the circulation. The interaction between thyroid hormones and gonadal hormones had not been previously studied extensively. Our findings that T, and T, were more active in the male than in the female agree well with the findings of Srivastava and Turner (1967) and May (1980). Thyroid hormones were also found to be more potent in lowering circulating GH concentrations in cockerels than in pullets suggesting that there may be differences between cockerels and pullets as to their sensitivity to thyroid hormones. Thyroid hormones were shown in the present studies to lower GH concentrations both in normal and PTU-treated cockerels and in normal pullets. These findings are
98
LEUNG,
TAYLOR,
consistent with those reported by Harvey (1983) and Harvey et al. (1983). Such interaction between thyroid hormones and growth is quite interesting. In mammals, it is evident that thyroid hormones stimulate GH synthesis and release in vitro (Samuels et al., 1976), and marked reduction in pituitary GH content in hypothyroid animals is also well documented (Simpson et al., 1950; Solomon and Greep, 1959). The decrease in body weight in hypothyroid animals is thought to be mediated through pituitary GH in mammals. In birds, we observed an increase of basal GH concentrations in the hypothyroid state. That is consistent with the findings of Scanes et al. (1976) and Chiasson et al. (1979). Thus, the reduction in body weight gain seen in hypothyroid birds could not be due to the lack of GH nor could the gain in body weight in the thyroid hormone-supplemented hypothyroid birds be due to GH, since they showed an inverse relationship between GH and body weight. It is evident that T, is more active than T, in intact birds. T, also lowers plasma T4 concentrations in both intact birds and hypothyroid birds suggesting that T, might act via a negative feedback to either the hypothalamus and/or pituitary. Thus T, apparently could be the main biological active hormone in birds as well as mammals. In summary, we have clearly demonstrated that the differences in T, and T, potency in birds reported in the previous literature could be due to the physiological state of the thyroid gland. In euthyroid birds, T, is more active in reducing body weight gain and feed efficiency than T,, but they are equally potent in hypothyroid birds. T, has a negative feedback on plasma T, concentrations in both intact and hypothyroid birds. Exogenous thyroid hormones are more effective in males than in females, and T, and T4 decrease GH concentrations in both intact and hypothyroid birds.
AND VAN IDERSTINE
ACKNOWLEDGMENTS We thank Ms. R. Serio and Cathy Ball for expert technical assistance and Ms. S. Zalink and Ms. S. Sieliecki for expert secretarial assistance. We also thank Dr. J. Brooks for reviewing the manuscript.
REFERENCES Adamson, L. F., and Ingbar, S. H. (1967). Some properties of the stimulatory effect of thyroid hormones on amino acid transport by embryonic chick bone. Endocrinology 81, 1372- 1378. Assenmacher, 1. (1973). The peripheral endocrine glands. In “Avian Biology,” Vol. 3. (D. S. Farner and J. R. Kings, eds.), pp 183-286. Academic Press, New York. Astier, H. (1973). Presence dune fraction importante de proteines iodees “non hormonales” dans le P.B.I. des oiseaux. Compt. Rend. Acad. Sci. 276, 793-796. Barker, S. B. (1955). Thyroid. Anna. Rev. Physiol. 17, 417-442. Chiasson, R. B., Sharp, P. J., Klandoft, J., Scanes, C. G., and Harvey, S. (1979). The effect of rapeseed meal and methimazole on levels of plasma hormones in growing broiler cockerels. PO&. Sci.
58,
1575-1583.
Davison, T. F. (1978). The turnover of thyroxine in the plasma of the domestic fowl (Gal/us domesticus). Gen.
Comp.
Endocrinology
36,
380-387.
Falconer, I. R. (1971). The thyroid glands. In “Physiology and Biochemistry of the Domestic Fowl” (D. J. Bell and B. M. Freeman, eds.), pp. 459472. Academic Press, London/New York. Fisher, D. A., Hoath, S., and Lakshmanan, J. (1982). The thyroid hormone effects on growth and development may be mediated by growth factors. Endocrinol.
Exp.
16, 259-
172.
Gavin, L., Castle, J., McMahon, F., Martin, P., Hammond, M., and Cavalieri, R. R. (1977). Extrathyroidal conversion of thyroxine to 3,3’,5’-triiodothyronine (reverse-T,) and to 3,5,3’-triiodothyronine (T3) in humans. J. Clin. Endocrinol. Metab. 44, 733-742. Harvey, S. (1983). Thyroid hormones inhibit growth hormone secretion in domestic fowl (Gallus domesticus).
1. Endocrinol.
96, 329-334.
Harvey, S., Sterling, R. J., and Klandorf, H. (1983). Concentrations of triiodothyronine, growth hormone, and luteinizing hormone in the plasma of thyroidectomized fowl (Gal/us domesticus). Gen. Comp.
Endocrinol.
50, 275-281.
Jallageas, M., and Assenmacher, I. (1972). Efects de la photoperiode et du taux d’androgene circulant sur la function thyroidiene du canard. Gen. Comp.
Endocrinol.
19, 331-340.
EFFECTS
OF THYROID
King, D. B., and King, C. R. (1973). Thyroidal influence on early muscle growth of chickens. Gen. Comp. Endocrinol. 21, 517-529. King, D. B., and King, C. R. (1976). Thyroidal intluence on gastrocnemius and sartorius muscle growth in young white leghorn cockerels. Gen. Comp. Endocrinol. 29, 473-479. Leung, E C., Taylor, J. E., Steelman, S. L., Bennett, C. D., Rodkey, J. A., Long, R. A., Serio, R., Weppelman, R. M., and Olson, G. (1984). Purification and properties of chicken growth hormone and the development of a homologous radioimmunoassay. Gen. Comp. Endocrinol. 56, 389400. May, J. D. (1977). Thyroxine, 3,5,3’-triiodothyronine and 3,3’,5’-triiodothyronine concentrations in broiler serum. Proc. South. Beg. Aviun Environ. Physiol. Bioeng. Stud. Group 13, 15-20. May, J. D. (1980). Effect of dietary thyroid hormone on growth and feed efficiency of broilers. Poult. Sci. 59, 888-892. May, J. E. (1982). Effect of dietary thyroid hormone on survival time during heat stress. Pot&. Sci. 61, 706-709. Mellen, W. J., and Wentworth, B. C. (1959). Thyroxine vs. triiodothyronine in the fowl. Pot&. Sci. 38, 228-230. Newcomer, W. S. (1957). Relative potencies of thyroxine and triiodothyronine based on various criteria in thiouracil-treated chickens. Amer. J. Physiol. 190, 413-418. Newcomer, W. S., and Barrett, P. A. (1960). Effects of various analogues of thyroxine on oxygen uptake of cardiac muscle from chicks. Endocrinology 66, 409-415. Newcomer, W. S. (1974). Diurnal rhythms of thyroid function in chicks. Gen. Comp. Endocrinol. 24, 65-73. Premachandra, B. N., Lang, S., Andrada, J. A., and Kite, J. H., Jr. (1977). Reverse triiodothyronine in the chicken. Life Sci. 21, 205-212. Raheja, K. L., and Snedecor, J. G. (1970). Comparison of subnormal multiple doses of L-thyroxine and L-triiodothyronine in propylthiouracil-fed and radiothyroidectomized chicks (Gallus domesticus). Comp. Biochem. Physiol. 37, 555-563. Ringer, R. K. (1976). Thyroids. In “Avian Physi-
HORMONES
IN CHICKEN
99
ology” (P. D. Sturkie, ed.), 3rd Ed., pp. 348-358. Springer-Verlag, New York. Samuels, H. H., Stanley, F., and Shapiro, L. E. (1976). Dose dependent depletion of nuclear receptors by L-triiodothyronine: Evidence for a role in induction of growth hormone synthesis in cultured GH cells. Proc. Natl. Acad. Sci. USA 73, 3877-3881. Scanes, C. G., Gales, L., Harvey, S., Chadwick, A., and Newcomer, W. S. (1976). Endocrine status of chicks of the obese strain. Gen. Comp. Endocrinol. 30, 419-423. Scanes, C. G., Marsh, J., Decuypere, E., and Rudas, P. (1983). Abnormalities in the plasma concentrations of thyroxine, tri-iodothyronine and growth hormone in sex-linked dwarf and autosomal dwarf white leghorn domestic fowl (Gallus domesticus). .I. Endocrinol. 97, 127- 135. Shellabarger, C. J. (1955). A comparison of triiodothyronine and thyroxine in the chick goiter-prevention tests. Pot&. Sci. 34, 1437-1440. Simpson, M. E., Asling, C. N., and Evans, H. M. (1950). Some endocrine influences on skeletal growth and differentiation. Yale J. Biol. Med. 23, l-27. Singh, A., Reineke, E. P., and Ringer, R. K. (1968). Influence of thyroid status of the chick on growth and metabolism, with observations on several parameters of thyroid function. Poult. Sci. 47, 212219. Snapir, N., Robinzon, B., Hoffman, Y., and Berman, A. (1982). Adenohypophyseal cytology of chemically and surgically thyroidectomized cockerels. Poult. Sci. 61, 1720-1728. Solomon, J., and Greep, R. D. (1959). The effects of alterations in thyroid function on the pituitary growth hormone content and acidophil cytology. Endocrinology 65, 158-164. Srivastava, L. S., and ‘Buner, C. W. (1967). Comparison of biological activity of injected and orally administered L-thyroxine, t,-triiodothyronine and thyroprotein in fowls. Pro. Sot. Exp. Biol. Med. 126, 157-161. Thommes, R. C., and Hylka, V. W. (1977). Plasma iodothyronines in the embryonic and immediate post-hatch chick. Gen. Comp. Endocrinol. 32, 417-422.
AND
COMPARATIVE
ENDOCRINOLOGY
59,
9 l-99 (1985)
Effects of Dietary Thyroid Hormones on Growth, Plasma T, and and Growth Hormone in Normal and Hypothyroid Chickens F. C. LEUNG, Department
of Animal
J. E. TAYLOR, Physiology, Rahway,
Merck New
AND A. VAN Sharp Jersey
& Dohme 07065
T4,
IDERSTINE Research
Laboratories,
Accepted October 5, 1984 Cockerels and pullets fed with T, or T, for 2 weeks showed a decrease in both body weight gain and feed efficiency. The reduction in body weight gain and feed efficiency was dose related in cockerels where T, or T, were fed at 0.1, 1.0, and 10.0 ppm levels. T, and T, at 0.1 and 1.O ppm had no significant effects on growth or feed efficiency in pullets, but the lO.O-ppm level of T, and T, caused a reduction of - 55.24 and - 28.18%, respectively, in body weight gain as compared with control birds. T, was more active than T4 in reducing growth and was toxic when fed at 10.0 ppm both in cockerels and pullets. Both propylthiouracil (PTU)- and methimazole-treated cockerels showed a decrease in rates of gain. T, and T, at a dietary level of 0.1 ppm were equipotent in promoting growth in these PTUand methimazole-treated cockerels, but 10.0 ppm caused a further reduction in body weight gain. Plasma T, levels were found to be significantly higher in birds that were fed either T, or T,. Plasma T, levels were higher in T,-fed birds, but significantly lower in T,-fed birds as compared with controls. Both PTU- and methimazole-treated cockerels had significantly lower plasma T, and T, concentrations, but elevated plasma GH concentrations. Dietary T, and T, at 1.O and 10.0 ppm significantly lowered plasma GH concentrations. In summary, these results indicated that T, was more active than T, in reducing body weight gain in intact normal birds, but that they were equally potent in promoting growth in PTU- and methimazole-treated hypothyroid birds. Thyroid hormones (T, and T4) are also more active in decreasing growth in intact cockerels than in pullets. It appears that there is some peripheral conversion from T, to T,, and T, has a negative feedback on plasma T, concentrations. Since PTU-treated birds show elevated plasma GH concentrations, the decrease in body weight gain is probably due to the lowering of thyroid hormones rather than to an effect on pituitary GH release. 01985 Academic PXSS. hc.
It is known that thyroid hormones are necessary for normal growth and development in birds (Falconer, 1971; Ringer, 1976). In mammals, triiodothyronine (T,) has been shown to be more potent than thyroxine (T4) as determined by goiter prevention, sensitivity to hypoxia, and oxygen consumption (Barker, 1955). In the bird, however, studies of relative potency between T, and T, have been controversial. Shellabarger (1955) was the first to report that T, and T, were equally potent in preventing goiter, and Newcomer (1957) found that T, was as active as T4 in influencing oxygen consumption, heart rate, suffocation time, and feather growth in thiouraciltreated chicks. Singh et al. (1968) observed
that T, has the same effect as T, in suppressing the effects of methimazole as indicated by thyroid secretion rate, and Raheja and Snedecor (1970) found that T, and T, were equally potent in stimulating body weight, comb weight, and liver glycogen of propylthiouracil (PTU)-treated and radio/ thyroidectomized (RT) chicks. However, T, had been reported to be more potent than T, in preventing goiter in thiouraciltreated chicks (Newcomer, 1957; Mellen and Wentworth, 1959), in promoting oxygen consumption in cardiac muscle (Newcomer and Barrett, 1960) and in increasing metabolic rate (Singh et al., 1968). Srivastava and Turner (1967) reported that T, was more active than T,. T, was also reported 91 001~6480185 $1.50 Copyright 0 1985 by Academic Press, Inc. All rights of reproduction in any form reserved.
LEUNG,
92
TAYLOR,
to be more active than T, in stimulating intracellular accumulation of amino acids by embryonic chick bones (Adamson and Ingbar, 1967). May (1980) reported that T, was more active than T, in reducing growth and feed efficiency in intact birds, and T, was also found to be more active than T, in shortening survival time during heat stress (May, 1982). Recently, Harvey (1983) and Harvey et al. (1983) showed that thyroid hormones (T3 and T4) inhibited GH release in birds and that surgical thyroidectomy increased basal concentration of GH. Such interaction between thyroid hormones and GH is not observed in mammalian species. In the current series of experiments, we planned to investigate the effects of T, and T, on growth in normal and hypothyroid birds and its effects on plasma T,, T,, and GH concentrations. MATERIALS
AND METHODS
Hubbard x Hubbard cockerels and pullets were used in these experiments. Birds were individually caged in a temperature (25”)- and light (14 hr light-10 hr darkness)-controlled room. Pennfield Laboratory Broiler Feed-Pennfield 180 (Pennfield, Lancaster, Pa.) was used. Triiodothyronine (TX), thyroxine (T4), propylthiouracil (PTU), and methimazole were obtained from Sigma Chemical Company, St. Louis, Missouri. T, or T, was added to the Pennfield diet at 0.1, 1.O, and 10.0 ppm. PTU was added to the diet at 0.5% and methimazole was added to the drinking water at 0.05%. Feed and water were available ad libitum. Four- or Iiveweek-old birds were used in all experiments except in the methimazole-treated experiments in which 2week-old birds were used. The experimental period in all experiments was 2 weeks. All birds were weighed, feed consumption was measured, and plasma samples were collected via heart puncture two times a week for 2 weeks. Plasma samples were kept at -20” until assayed for T,, T,, and GH concentrations. Plasma T, and T, concentrations were determined by the T, and T, clasp radioimmunoassay kits (Squibb, Princeton, N.J.), and plasma GH concentrations were determined by a homologous RIA for chicken GH (Leung et al., 1984) with FLcGH-II as standard. The data were analyzed for statistical significance by the analysis of variance. The Student-NewmanKeuls test was used to analyze the differences among groups and differences were considered significant at P < 0.05.
AND VAN IDERSTINE
RESULTS
Effects of Thyroid Hormones The results of the and T4 on body weight and plasma T, and shown in Figs. 1 and
in Cockerels
effects of dietary T, gain, feed efficiency, T, in cockerels are 2. T, fed at 1.0 and
A 0 Control 0 T,.O.i ppm
2
I 1I I
3
7
IO
14
Days
FIG. 1. Effects of T, on body weight gain, feed efficiency, and plasma T, and T, in 4-week-old cockerels. N = 9 per group. *P = cO.05, compared with controls.
EFFECTS
OF THYROID
HORMONES
IN CHICKEN
93
body weight gain and feed efficiency were significantly decreased in birds fed T, at 10.0 ppm. Birds that were fed either T, or T, at 1 .O and 10.0 ppm showed a significant increase in plasma T, levels. Plasma T, concentrations were significantly lower in birds fed with T, at 0.1, 1.0, and 10.0 ppm, and the decrease was dose related. Plasma T, concentrations were significantly elevated in the birds fed with T, at 1.0 and 10.0 ppm. The slight elevation of plasma T, concentrations in birds fed with 10.0 ppm of T, was probably due to peripheral conversion of T, to T,. Effects of Thyroid Hormones
100
Days
FIG. 2. Effects of T4 on body weight gain, feed efficiency, and T4 in Sweek-old cockerels. N = 9 per group. *P = cO.05, compared with controls. **Values over the limit of the assay.
10.0 ppm caused significant dose-related reductions in body weight gain. Birds fed T, at 10.0 ppm had poorer feed efficiency than birds on control diet and T, was also found to cause fatalities. Eight out of nine birds were dead by the end of the experiment. Birds fed T, at 0.1 and 1 .O ppm showed no significant difference in growth and feed efficiency as compared with controls, but
in Pullets
The effects of dietary T, and T, on body weight gain, feed efficiency, and plasma T, and T, in pullets are shown in Figs. 3 and 4. Birds fed T, or T, at 0.1 and 1.0 ppm showed no significant effects on body weight gain or feed efficiency, but birds fed 0.1 ppm of T, or T, showed a slight increase in body weight gain, 3.03 and 4.99%, respectively. T, or T, fed at 10.0 ppm caused significant reductions in body weight gain and feed efficiency. T, at 10.0 ppm also caused fatalities with five of nine birds dying by the end of the experiment. Plasma T, and T, concentrations in the pullets were similar to those found in the cockerels. Plasma T, concentrations were significantly elevated in birds fed T, at 1.0 and 10.0 ppm and birds fed T, at 10.0 ppm. Plasma T, concentrations were also higher in T,-fed birds, but birds fed T, at 1.0 and 10.0 ppm showed a significant decrease in plasma T, concentrations. Effects of Thyroid Hormones
on GH
In a separate series of experiments which were similar to those described in the above sections, the effects of thyroid hormones on body weight gain and feed efficiency in cockerels and pullets were the same as were reported in the above sections (data
I
I
$j
I
I
Plosmo T, (rig/ml) g g $
I
I
I 0
g
w
0
-NW*
Plarma T3 (rig/ml)
Plasma T3 (rig/ml 1
(feed/gain)
-R)cIIPCn
Feed. Efficiency (feed/gains)
Feed Efficiency
I
l
DOOD
Body Weight Gain (gms 1
Body Weight Gain (gms)
EFFECTS
OF THYROID
HORMONES
IN CHICKEN
95
TABLE 1 EFFECTS OF DIETARY T, AND T4 ON PLASMA GH CONCENTRATIONSIN MALE AND FEMALE CHICKENS GH (&ml) Treatment (dose level) Control 0.1 ppm 1.0 ppm 10.0 ppm
T,-fed Male 40.9 38.0 30.1 37.0
k k t k
3.1 2.2 1.1* 2.8
T,-fed Male (9) (9) (9) (6)
51.9 47.8 35.4 32.8
+ f k *
T,-fed Female
4.8 6.5 3.5* 1.6*
(9) (9) (9) (9)
29.0 21.2 22.4 19.5
f * k k
5.2 4.4 1.9 3.2*
T,-fed Female (9) (9) (9) (6)
26.8 23.1 23.6 20.5
f t r e
2.5 2.3 1.8 1.3*
(9) (9) (9) (9)
( ) = Number of birds. a Mean ? SEM. * P < 0.05 as compared with control. Note.
Effects of Thyroid Hormones in PTU- and Methimazole-Treated Cockerels
The effects of thyroid hormones on body weight gain, plasma T,, T,, and GH concentrations are shown in Table 2. PTU fed at 0.5% caused a significant decrease in body weight gain and plasma T, and T, concentrations but GH concentrations were elevated. The addition of T, and T, at 0.1 ppm in the diet to the PTU-treated birds improved their growth by 22.0 and 21.0%, respectively, and plasma T, and T, and GH concentrations were similar to those found in control birds. When birds were fed with T, or T4 at 1.O and 10.0 ppm, plasma T, and T, concentrations were elevated. T, and T, at doses of 1.0 and 10.0 ppm produced a significant retardation in body weight gain. Plasma T4 concentrations in birds fed T3 at 1.0 and 10.0 ppm were significantly lower than controls and PTU-treated birds probably due to a negative feedback of T, at the pituitary and/or hypothalamus. Birds receiving these hyperthyroid doses of T, or T, also had significantly lower plasma GH concentrations when compared with birds which received control or PTU diet. Similar results were obtained with 2-week-old cockerels that were treated with methimazole (Table 3). Methimazole at 0.05% in the drinking water significantly lowered body weight gain and plasma T, and T, concen-
trations. The addition of T3 or T, at 0.1 ppm in the diet of the methimazole-treated birds improved growth by 42.0 and 58.0%, respectively, and plasma T, concentrations were also significantly higher than in methimazole-treated birds. Plasma T, and T, concentrations were still significantly lower than those found in control birds. Doses of 1 and 10 ppm of T, and T4 produced higher circulating thyroid hormones resulting in further retardation in body weight gain as compared to control and methimazoletreated birds. DISCUSSION
Our present findings clearly demonstrate that the potency differences between T, and T, depend on the physiological state of the thyroid gland in the animals used in these experiments. Our results agree well with previous investigators that T, was equally potent as T, when birds were in a hypothyroid state induced by thyroidectomy or goitrogen treatment. Also, experiments indicating that T, was more active than T, were conducted with presumably euthyroid birds. It is possible that in hypothyroid states T4 is readily converted into T3 and thus yields biological responses similar to those of T, itself. Since both T, and T, appear to have a very short and similar half-life, a range between 2 and 8 hr (Astier,
7
+ T, (10.0
2
+ T, (0.1
+ T, (1.0
+ T, (10.0
PTU
Experiment Control PTU
PTU
PTU
PTU
’ All values b PTU was c Percentage * P < 0.05 ** P < 0.05
7 5
+ T, (1 .O ppm)
PTU
7
5
7
HORMONES
are means 2 SEM. added into the diet at 0.5%. change as compared with control, as compared with control. as compared with PTU control.
ppm)
ppm)
ppm)
ppm)
7
+ T, (0.1
PTU 7
7 7
N
THYROID
1
ppm)
OF DIETARY
Experiment Control PTUb
Treatment
EFFECTS gain (g)
856.4 644.0 (-24.8) 690.0 (- 19.4) 601.0 (- 29.8) 382.1 (-55.4)
802.1** 617.1* (-23.1y 657.9* (- 18.0) 520.0* (-35.2) 322.1*,** (-59.8)
wt.
WEIGHT
in parentheses.
Body
ON BODY
T,,
T,,
81.23
107.2
120.5
126.1 64.7
290.3
218.1
219.7 104.7
ml)
AND GH
+
+
4.8*
18.3
If- 14.3**
7.66
1.95
2.43 1.27
0.41
-
-t- 15.8** t 18.6*
0.61
0.82
1.10 0.89 0.0.5*
0.04** 0.02*
ml)”
PTU-TREATED
0.06*,**
0.90*,** >50.0
t
2 0.34
-t 0.09** k 0.14*
f
k 0.04*%**
i
f ”
T, (pg/lOO
IN ~-WEEK-OLD
2 75.7
A 14.4
f 20.4** ? 16.2*
T, (ng/lOO
TABLE 2 GAIN, PLASMA
5.9
15.5
27.4
31.0 38.1
23.60
21.61
41.44
32.54 48.45
GH
+
5
t
It k
f
?
It
zt k
2.1*,**
2.7*,*”
5.7
8.2 18.6
2.55*~**
1.60*,**
3.98
1.39** 9.66*
(r&ml)
COCKERELS
n
E 2 2
ij
2
% u
$
2
2
“3
EFFECTS
OF THYROID
HORMONES
97
IN CHICKEN
TABLE 3 EFFECTSOF DIETARY THYROID HORMONES ON BODY WEIGHT GAIN AND PLASMA T, AND T, IN ~-WEEK-OLD METHIMAZOLE-TREATED COCKERELS Treatment Experiment 1 Control Methimazoleb
N
Body wt. gain (g)
T, (ng/lOO ml)
T4 (141~
6 7
511.3** 356.9* (-30.2)= 423.0* (-17.3) 338.0* (-33.9) 238.2*,** (-53.4)
246.3 e 33.2** 40.9 +- 3.4*
2.10 -+ 0.03** 1.36 k 0.05*
133.9 2 13.2*,**
1.27 + 0.04*
347.4 * 39.g**
1.16 + 0.02*
1310.1 f 20.9*,**
1.21 k 0.04*
165.5 k 10.3** 53.3 If: 6.6*
2.12 2 0.10** 1.48 2 0.03*
119.6 AZ 8.3*,**
1.41 2 0.04*
184.1 2 15.4**
12.23 k 2.59*,**
262.3 r 36.8*,**
>50.0
Methimazole
+ T, (0.1 ppm)
5
Methimazole
+ T, (1 .O ppm)
7
Methimazole
+ T, (10.0 ppm)
6
Experiment 2 Control Methimazole
7 7
Methimazole
+ T, (0.1 ppm)
8
Methimazole
+ T, (1.0 ppm)
7
Methimazole
+ T, (10.0 ppm)
5
490.6** 352.2* (-28.2) 432.5** (-11.8) 375.6* (-23.4) 296.0* (-39.7)
ml)’
a All values are means f SEM. b Methimazole was added into the water at 0.05%. c Percentage change as compared with control, in parentheses. * P < 0.05 as compared with control. ** P < 0.05 as compared with methimazole treated at 0.05%.
1973; Assenmacher, 1973; Davison, 1978), the change in biological potency between T, and T, in different physiological states could simply be due to the differences in monodeiodinase activity. Recently, Scanes et al. (1983) showed that the hypothyroid nature found in sex-linked dwarf chickens was due to depressed liver T,-5’ monodeiodinase activity. Reverse T, was found in the circulation of chickens (Thommes and Hylka, 1977; Premachandra, 1977; May, 1977), and May (1980) found most of the T,, when given at higher doses, was converted into rT,. Reverse T, was not measured in the present experiments, but the small increase of plasma T, in the T,-fed groups suggests that at least some of the T, was converted to
rT,, thereby preventing accumulation of toxic levels of T, in the circulation. The interaction between thyroid hormones and gonadal hormones had not been previously studied extensively. Our findings that T, and T, were more active in the male than in the female agree well with the findings of Srivastava and Turner (1967) and May (1980). Thyroid hormones were also found to be more potent in lowering circulating GH concentrations in cockerels than in pullets suggesting that there may be differences between cockerels and pullets as to their sensitivity to thyroid hormones. Thyroid hormones were shown in the present studies to lower GH concentrations both in normal and PTU-treated cockerels and in normal pullets. These findings are
98
LEUNG,
TAYLOR,
consistent with those reported by Harvey (1983) and Harvey et al. (1983). Such interaction between thyroid hormones and growth is quite interesting. In mammals, it is evident that thyroid hormones stimulate GH synthesis and release in vitro (Samuels et al., 1976), and marked reduction in pituitary GH content in hypothyroid animals is also well documented (Simpson et al., 1950; Solomon and Greep, 1959). The decrease in body weight in hypothyroid animals is thought to be mediated through pituitary GH in mammals. In birds, we observed an increase of basal GH concentrations in the hypothyroid state. That is consistent with the findings of Scanes et al. (1976) and Chiasson et al. (1979). Thus, the reduction in body weight gain seen in hypothyroid birds could not be due to the lack of GH nor could the gain in body weight in the thyroid hormone-supplemented hypothyroid birds be due to GH, since they showed an inverse relationship between GH and body weight. It is evident that T, is more active than T, in intact birds. T, also lowers plasma T4 concentrations in both intact birds and hypothyroid birds suggesting that T, might act via a negative feedback to either the hypothalamus and/or pituitary. Thus T, apparently could be the main biological active hormone in birds as well as mammals. In summary, we have clearly demonstrated that the differences in T, and T, potency in birds reported in the previous literature could be due to the physiological state of the thyroid gland. In euthyroid birds, T, is more active in reducing body weight gain and feed efficiency than T,, but they are equally potent in hypothyroid birds. T, has a negative feedback on plasma T, concentrations in both intact and hypothyroid birds. Exogenous thyroid hormones are more effective in males than in females, and T, and T4 decrease GH concentrations in both intact and hypothyroid birds.
AND VAN IDERSTINE
ACKNOWLEDGMENTS We thank Ms. R. Serio and Cathy Ball for expert technical assistance and Ms. S. Zalink and Ms. S. Sieliecki for expert secretarial assistance. We also thank Dr. J. Brooks for reviewing the manuscript.
REFERENCES Adamson, L. F., and Ingbar, S. H. (1967). Some properties of the stimulatory effect of thyroid hormones on amino acid transport by embryonic chick bone. Endocrinology 81, 1372- 1378. Assenmacher, 1. (1973). The peripheral endocrine glands. In “Avian Biology,” Vol. 3. (D. S. Farner and J. R. Kings, eds.), pp 183-286. Academic Press, New York. Astier, H. (1973). Presence dune fraction importante de proteines iodees “non hormonales” dans le P.B.I. des oiseaux. Compt. Rend. Acad. Sci. 276, 793-796. Barker, S. B. (1955). Thyroid. Anna. Rev. Physiol. 17, 417-442. Chiasson, R. B., Sharp, P. J., Klandoft, J., Scanes, C. G., and Harvey, S. (1979). The effect of rapeseed meal and methimazole on levels of plasma hormones in growing broiler cockerels. PO&. Sci.
58,
1575-1583.
Davison, T. F. (1978). The turnover of thyroxine in the plasma of the domestic fowl (Gal/us domesticus). Gen.
Comp.
Endocrinology
36,
380-387.
Falconer, I. R. (1971). The thyroid glands. In “Physiology and Biochemistry of the Domestic Fowl” (D. J. Bell and B. M. Freeman, eds.), pp. 459472. Academic Press, London/New York. Fisher, D. A., Hoath, S., and Lakshmanan, J. (1982). The thyroid hormone effects on growth and development may be mediated by growth factors. Endocrinol.
Exp.
16, 259-
172.
Gavin, L., Castle, J., McMahon, F., Martin, P., Hammond, M., and Cavalieri, R. R. (1977). Extrathyroidal conversion of thyroxine to 3,3’,5’-triiodothyronine (reverse-T,) and to 3,5,3’-triiodothyronine (T3) in humans. J. Clin. Endocrinol. Metab. 44, 733-742. Harvey, S. (1983). Thyroid hormones inhibit growth hormone secretion in domestic fowl (Gallus domesticus).
1. Endocrinol.
96, 329-334.
Harvey, S., Sterling, R. J., and Klandorf, H. (1983). Concentrations of triiodothyronine, growth hormone, and luteinizing hormone in the plasma of thyroidectomized fowl (Gal/us domesticus). Gen. Comp.
Endocrinol.
50, 275-281.
Jallageas, M., and Assenmacher, I. (1972). Efects de la photoperiode et du taux d’androgene circulant sur la function thyroidiene du canard. Gen. Comp.
Endocrinol.
19, 331-340.
EFFECTS
OF THYROID
King, D. B., and King, C. R. (1973). Thyroidal influence on early muscle growth of chickens. Gen. Comp. Endocrinol. 21, 517-529. King, D. B., and King, C. R. (1976). Thyroidal intluence on gastrocnemius and sartorius muscle growth in young white leghorn cockerels. Gen. Comp. Endocrinol. 29, 473-479. Leung, E C., Taylor, J. E., Steelman, S. L., Bennett, C. D., Rodkey, J. A., Long, R. A., Serio, R., Weppelman, R. M., and Olson, G. (1984). Purification and properties of chicken growth hormone and the development of a homologous radioimmunoassay. Gen. Comp. Endocrinol. 56, 389400. May, J. D. (1977). Thyroxine, 3,5,3’-triiodothyronine and 3,3’,5’-triiodothyronine concentrations in broiler serum. Proc. South. Beg. Aviun Environ. Physiol. Bioeng. Stud. Group 13, 15-20. May, J. D. (1980). Effect of dietary thyroid hormone on growth and feed efficiency of broilers. Poult. Sci. 59, 888-892. May, J. E. (1982). Effect of dietary thyroid hormone on survival time during heat stress. Pot&. Sci. 61, 706-709. Mellen, W. J., and Wentworth, B. C. (1959). Thyroxine vs. triiodothyronine in the fowl. Pot&. Sci. 38, 228-230. Newcomer, W. S. (1957). Relative potencies of thyroxine and triiodothyronine based on various criteria in thiouracil-treated chickens. Amer. J. Physiol. 190, 413-418. Newcomer, W. S., and Barrett, P. A. (1960). Effects of various analogues of thyroxine on oxygen uptake of cardiac muscle from chicks. Endocrinology 66, 409-415. Newcomer, W. S. (1974). Diurnal rhythms of thyroid function in chicks. Gen. Comp. Endocrinol. 24, 65-73. Premachandra, B. N., Lang, S., Andrada, J. A., and Kite, J. H., Jr. (1977). Reverse triiodothyronine in the chicken. Life Sci. 21, 205-212. Raheja, K. L., and Snedecor, J. G. (1970). Comparison of subnormal multiple doses of L-thyroxine and L-triiodothyronine in propylthiouracil-fed and radiothyroidectomized chicks (Gallus domesticus). Comp. Biochem. Physiol. 37, 555-563. Ringer, R. K. (1976). Thyroids. In “Avian Physi-
HORMONES
IN CHICKEN
99
ology” (P. D. Sturkie, ed.), 3rd Ed., pp. 348-358. Springer-Verlag, New York. Samuels, H. H., Stanley, F., and Shapiro, L. E. (1976). Dose dependent depletion of nuclear receptors by L-triiodothyronine: Evidence for a role in induction of growth hormone synthesis in cultured GH cells. Proc. Natl. Acad. Sci. USA 73, 3877-3881. Scanes, C. G., Gales, L., Harvey, S., Chadwick, A., and Newcomer, W. S. (1976). Endocrine status of chicks of the obese strain. Gen. Comp. Endocrinol. 30, 419-423. Scanes, C. G., Marsh, J., Decuypere, E., and Rudas, P. (1983). Abnormalities in the plasma concentrations of thyroxine, tri-iodothyronine and growth hormone in sex-linked dwarf and autosomal dwarf white leghorn domestic fowl (Gallus domesticus). .I. Endocrinol. 97, 127- 135. Shellabarger, C. J. (1955). A comparison of triiodothyronine and thyroxine in the chick goiter-prevention tests. Pot&. Sci. 34, 1437-1440. Simpson, M. E., Asling, C. N., and Evans, H. M. (1950). Some endocrine influences on skeletal growth and differentiation. Yale J. Biol. Med. 23, l-27. Singh, A., Reineke, E. P., and Ringer, R. K. (1968). Influence of thyroid status of the chick on growth and metabolism, with observations on several parameters of thyroid function. Poult. Sci. 47, 212219. Snapir, N., Robinzon, B., Hoffman, Y., and Berman, A. (1982). Adenohypophyseal cytology of chemically and surgically thyroidectomized cockerels. Poult. Sci. 61, 1720-1728. Solomon, J., and Greep, R. D. (1959). The effects of alterations in thyroid function on the pituitary growth hormone content and acidophil cytology. Endocrinology 65, 158-164. Srivastava, L. S., and ‘Buner, C. W. (1967). Comparison of biological activity of injected and orally administered L-thyroxine, t,-triiodothyronine and thyroprotein in fowls. Pro. Sot. Exp. Biol. Med. 126, 157-161. Thommes, R. C., and Hylka, V. W. (1977). Plasma iodothyronines in the embryonic and immediate post-hatch chick. Gen. Comp. Endocrinol. 32, 417-422.