Thyroid function assays in infants

916

November, 1972 T h e Journal o[ P E D I A T R I C S

Thyroid function assays in infants Triiodothyronine-resin uptake (T~ resin), total thyroxine (T~), and #ee thyroxine index were measured in the sera o[ 341 in[ants o[ both sexes and are presented as normal ranges [or 12 age intervals [rom birth to 3 years. All parameters increased sharply [rom cord blood levels to maxima in the first two weeks, then declined to a constant value at 6 to 8 weeks [or T~ resin, and at 10 to 12 months for T~ and [ree thyroxine index. The data imply that the thyroxine-binding capacity is maximal in the newborn period, reducing to a steady value at the age o[ one year.

Mary T. O'Halloran, B.Sc., F.A.A.C.B.,* and Henry L. Webster, B.Sc., Ph.D., M.A.A.C.B., C a m p e r d o w n , N e w S o u t h Wales, Australia

T H E I N C R E A S I N G use of the ~25I triiodothyronine-resin uptake test (T3 resin) together with competitive binding assay of thyroxine using globulin-bound 12~I thyroxine (T4 assay) has raised the need for establishment of normal ranges for such data. Murray and associates 1 have provided T3 resin and T4 data for children of both sexes from 6 months to 15 years of age. In this hospital, it was considered essential that such information be extended with detail into the neonatal and early infancy period. Previous studies of protein-bound iodine and butanol-extractable iodine levels in the first weeks and months of lifC -4 had made it clear that marked departures from adult norms were to be expected. In this paper, the normal ranges for T, resin, T4, and free thyroxine index are presented in frequent age intervals from cord blood to 3 years with the intention of providFrom the Department o[ Biochemistry, Royal Alexandra Hospital [or Children. *Reprint address: Department of Biochemistry, Institute of Pathology, Royal Alexandra Hospital [or Children, Camperdown, New South Wales 2050, Australia.

Vol. 81, No. 5, pp. 916-919

ing a background for the assessment of early signs of thyroid disturbance. See related article, p. 982.

SUBJECTS AND M E T H O D S Cord blood samples from 64 infants were kindly provided by Dr. B. Storey, King George V Hospital, Sydney. Apart from rejection of hemolyzed sera, no other selection was made in this group. The remaining 277 infants examined were all out- or inpatients of the Royal Alexandra Hospital for Children. Each age group comprised 20 to 23 children, except for 62 in the one to two years category. The infants were made up of surgical patients, of those presenting with ailments which could not reasonably be expected to involve thyroid aberration, and of some for whom thyroid testing was routinely requested with the intention of excluding the diagnosis. T3 resin and T4 assays were made using Triosorb and Tetrasorb kits, respectively, marketed by Abbott Laboratories and made

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Thyroid [unction assays 9 1 7

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Fig. 1. available by Commonwealth X-ray and Radium Laboratories, Melbourne. Counts of ~25I were made with a well-crystal, scintillation counter and scaler-timer produced by EKCO Electronics Limited. Free thyroxine index and thyroxlne-binding globulin index were calculated as T3 resin x T4 and T3 resin/T4, respectively, for each infant and expressed .in arbitrary units. RESULTS

The group of 34i infants was made up of 161 females and 180 males. Irrespective of age, the values for T8 resin uptake and T4 were collated according to sex, and separate frequency-distribution bar plots were prepared. Visual comparison of these plots gave no indication of any difference of distribution between males and females. Accordingly, the results are presented without sex differentiation. The mean T3 resin (percentage standard) and T4 (micrograms of thyroxine per 100

ml.) as a function of age are shown in Fig. 1. The normal ranges (mean + 2 standard deviations) for these and for free thyroxine index, together with mean thyroxine-binding globulin index values, are shown in Table I. For T3 resin, the mean increases sharply above the cord blood value during the first two weeks, then falls to the cord level by about 6 to 8 weeks. From this age to 3 years it is seen that very little further change occurs, except for a barely perceptible upward trend. With respect to total thyroxine (T4), the cord blood, serum range (7.3 to 15.3 #g per 100 ml.) is already elevated above the adult levels (6.6 to 11.6 /~g per 100 ml.). 1 Within two weeks of birth this increases further to a maximum, then falls rapidly during the next four months, eventually reaching a minimum at about 9 to 12 months. From this age through to 3 years, the T4 vaRte shows a tendency to increase slowly. The free thyroxine index has been widely

9 18

O'Halloran and Webster

The Journal o[ Pediatrics November 1972

Table I. In vitro thyroid function tests on sera of infants from birth to 3 years of age

No.

Age Cord 0-13 days 14-27 days 28-41 days 42-55 days 8-11 wk. 12-15 wk. 16-23 wk. 24-35 wk. 9-11 mo. 12-23 mo. 24-36 mo.

4 in[ants 64 20 21 21 22 23 21 20 22 23 62 22

T~ resin uptake (% standard) Mean Range S 84 64-104 94 74-114 90 66-114 93 72-114 84 62-106 84 69-100 84 58-110 86 57-115 85 67-103 86 65-107 88 63-113 87 69-105

7", (#g thyroxine/ 100 mL) Mean Range 11.3 7.3-15.3 13.2 9.8-16.6 12.4 8.2-16.6 12.3 8.3-16.3 11.0 7.4-14.6 11.8 7.5-16.1 10.3 6.7-13.9 9.7 5.9-13.5 9.9 5.9-13.9 9.0 5.1-12.9 9.2 5.8-12.6 9.5 6.8-12.2

Free thyroxine index* Mean Range 9.3 5.9-12.7 12.3 9.1-15.5 10.9 6.1-15.7 11.5 7.3-15.7 8.9 4.7-13.1 10.2 6.9-13.5 8.3 5.0-11.6 8.2 3.9-12.5 8.5 4.6-12.4 7.7 3.4-12.0 8.2 4.8-11.6 8.4 5.3-11.5

Thyroxinebinding globulin~ index Mean 7.71 7.44 7.78 7.75 7.69 7.34 8.42 8.52 8.77 10.07 9.86 9.18

*Product o~ Ta resin a n d T4 values for each infant (in arbitrary units). ~Ta resin divided by "I"4 for each infant (in arbitrary units). :~Mean value + 2 S. D.

accepted, on theoretic grounds, as a direct function of the level of free thyroxine in the circulation. Definitive confirmation of this was provided in the recent report by Stein and Price 5 of a coefficient of correlation of 0.96 between free thyroxine index and measured concentration of free thyroxine. The means and normal ranges for this parameter, displayed in Table I, indicate a rapid increase from the cord blood value to a maximum in the first two weeks of life, followed by a rather steep decrease during the next three to four months. From this point a reduced rate of decrease occurs, the level finally becoming steady at 9 to 12 months of age. A gradual rise is then observed up to 3 years. The mean thyroxine-bindlng globulin index (a reciprocal function of the thyroxinebinding capacity) tends to rise from the neonatal level during the first year of life. This ratio may be compared to the value of 10.3 which is derived from the mean adult T3 resin and T4 data reported by Murray and associates. 1 DISCUSSION

The age range examined in our study Overlaps with that studied by Murray and associates? Comparison shows that for T3 resin means and deviations, the present vahles for

1 to 2 years (63 to 113 per cent, mean 88) are comparable with the 6 month to 6 year values (72 to 104 per cent, mean 88) found by these authors. For total thyroxine the present values for the 2 to 3 year group (6.8 to 12.2/~g per 100 ml., mean 9.5) are lower than those of Murray and associates 1 for the 6 month to 6 year age range norms (7.6 to 15.1/~g per 100 ml., mean 11.3). However, the results now presented show a tendency for T4 to be increasing during the 1 to 3 year age period, so that this apparent disparity may not be significant. The most interesting aspect of our data is in the period from birth to 12 months. Clearly, there is a marked spurt of thyroid activity in the first two weeks, if total circulating thyroxine concentration and free thyroxine index are taken as criteria. The T4 changes correlate with earlier reports by Danowski and associates, 2 and Durham and associates, s who presented data for serum protein-bound iodine and butanol-extractable iodine assays, respectively. In particular, the results of Danowski and associates 2 indicated that the proteln-bound iodine peak concentration is reached between 1 and 4 days of life. More recently, studies by Fisher and Odell 6 have shown that there is a marked transient increase in thyroid-stimulating hot-

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mone within 30 minutes of birth which reduces to cord blood levels by two to three days. Concomitant measurement of Ts resin uptake enables calculation of the free thyroxine index, data which further implies that the increased total thyroxine in the neonatal period is associated with correspondingly high levels of unbound thyroxine, a parameter generally accepted as the best readily measured indication of peripheral thyroid activity. Increasing information is available to suggest that the serum triiodothyronine concentration is probably of greater significance, but technical difficulties preclude its estimation on a routine basis. The peak of T3 resin uptake observed neonatally decreases to a stable value by about two months, a pattern of change which is similar to that reported by Marks and associates7 using the original triiodothyronine-red blood cell uptake technique of Hamolsky. During the steady period of Ts resin uptake (2 months to 3 years), the mean value of approximately 85 per cent is lower than the adult mean of 94 per cent (range 80 to 108 per cent) reported by Murray and associates. 1 It is noteworthy that this difference is the result of an extension of the range, so that the lower limit of normal is reduced from 80 per cent in adults 1 to approximately 60 per cent, while the upper limit of about 110 per cent has already attained the adult level. This is of particular interest in the assessment of hypothyroidism in early infancy. There is some disparity between the present data for T , and that of Danowski and associates 2 for the period 3 to 12 months of age. These authors concluded that the protein-bound iodine values finally reached a mininmm at about three months which then persisted to one year. In the present work, the total thyroxine levels (which are the most nearly equivalent to protein-bound iodine concentration) continued to fall, albeit at a reduced rate, reaching a minimum by the age of 9 to 12 months. On the other hand, the free thyroxine index values appear to indicate that the spurt of elevated thyroid

Thyroid function assays 9 1 9

activity in early infancy has stabilized by about three months. Consideration of the relationship between T3 and T , values, particularly over the period 6 weoks to 1 year, allows a theoretic deduction concerning the capacity of the thyroxine-binding sites of the plasma proteins. During this time interval there is an appreciable and steady decrease in the T , value. This factor, considered in isolation, should produce a corresponding decrease in the Ta resin uptake measurements. Apparently the observed Ts resin value is maintained at a constant level by virtue of a decrease in thyroxine-binding globulin from a maximum at least as early as 6 to 8 weeks to a minimum at one year of age. This is supported by computation of the thyroxine-binding globulin index s (T3 resin divided by T4) from the present data; this result suggests that the maximum thyroxine-binding capacity is in fact attained in the neonatal period. Definitive evidence for such changes would require direct estimation of thyroxine-binding globulin and thyroxine-binding prealbumin. REFERENCES

1. Murray, I. P. C., Joasoo, A., and Parkin, J.: In vitro thyroid tests in children, Med. J. Aust. 1: 77, 1971. 2. Danowski, T. S., Johnston, S. Y., Price, W. C., McKelvy, M., Stevenson, S. S., and McCluskey, E. R.: Protein-bound iodine in infants from birth to one year of age, Pediatrics 7: 240, 1951. 3. Durham, J. R., Cooke, R. E., Lancaster, J. W., and Man, E. B.: Serum butanol-extractable iodine values of children under ten years of age, Am. J. Dis. Child. 87: 468, 1954. 4. Pickering, D. E., Kontaxis, N. E., Benson, R. C., and Meechan, R. J.: Thyroid function in the perinatal period, Am. J. Dis. Child. 95: 616, 1958. 5. Stein, R. B., and Price, L.: Evaluation of adjusted total thyroxine (free thyroxine index) as a measure of thyroid function, J. Clin. E~ndocrinol. Metab. 34: 295, 1972. 6. Fisher, D. A., and Odell, W. D.: Acute release of thyrotropin in the newborn, J. Clin. Invest. 48: 1670, 1969. 7. Marks, J., Wolfson, J., and Klein, R.: Neonatal thyroid function: Erythrocyte Ts uptake in early infancy, J. PEDIATR. 58: 32, 1961. ~r 8. Marshall, J. S., Levy, R. P., and Steinberg; A. G.: Human thyroxine-blndlng globulin deficiency. A genetic study, N. Engl. J. Med. 274: 1469, 1966,