- Email: [email protected]
Method specific second-trimester reference intervals for thyroid-stimulating hormone and free thyroxine
Available online at www.sciencedirect.com
Clinical Biochemistry 42 (2009) 750 – 753
Method specific second-trimester reference intervals for thyroid-stimulating hormone and free thyroxine Rechelle Silvio a , Karly J. Swapp a , Sonia L. La'ulu b , Kara Hansen-Suchy a , William L. Roberts c,⁎ a
c
Department of Clinical Laboratory Sciences, Weber State University, Ogden, UT, USA b ARUP Institute for Clinical and Experimental Pathology, Salt Lake City, UT, USA Department of Pathology, University of Utah Health Sciences Center, Salt Lake City, UT, USA
Received 24 October 2008; received in revised form 4 December 2008; accepted 8 December 2008 Available online 24 December 2008
Abstract Objective: To determine second trimester reference intervals for TSH and FT4. Design: Samples from 3102 subjects were tested for TPO and Tg antibodies. Methods: Elecsys E170 reference intervals for TSH and FT4 were determined using antibody-negative samples. Results: Second trimester reference intervals for TSH and FT4 were 0.18–4.07 mIU/L and 9.5–15.8 pmol/L, respectively. The Elecsys E170 TSH results were positively biased compared to ARCHITECT i2000SR results for these same samples. Conclusions: Method-specific reference intervals are required for TSH and FT4. © 2008 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved. Keywords: Elecsys E170; Free thyroxine; Thyrotropin; Reference interval; Pregnancy
Introduction Hyperthyroidism occurs in 0.2% of pregnancies and Graves' disease accounts for 95% of cases [1]. Severe maternal hyperthyroidism is associated with stillbirth, preterm delivery, intrauterine growth retardation, pre-eclampsia, and heart failure [2]. Hypothyroidism is present in 0.1 to 0.3% of pregnancies [1]. Maternal hypothyroidism is associated with miscarriage, pre-eclampsia, placental abruption, growth retardation, prematurity and stillbirths and fetuses often have impaired neurologic development [1]. Worldwide, iodine deficiency is the most common cause of hypothyroidism. In the United States, Hashimoto's thyroiditis accounts for most cases of hypothyroidism in pregnancy [1]. Thyroid function changes during pregnancy make diagnosis of thyroid disorders difficult. Guidelines published in 2005 for diagnosis of thyroid disease during pregnancy recommend trimester specific reference intervals for thyrotropin (TSH) and free thyroxine (FT4) [3].
These authors assert “Inasmuch as between-method biases for TSH are minimal, as future studies progress in pregnancy, there is no need for method-specific TSH ranges.” They also recommend that FT4 reference intervals during pregnancy should be determined for each method [3]. Recent clinical practice guidelines recommend a TSH upper reference limit of 3 mIU/L in the second and third trimesters or “trimester-specific normal TSH ranges” but do not specify that these be method specific [4]. We previously published second trimester reference intervals for thyroid tests on the ARCHITECT i2000SR analyzer [5]. Recently, a second trimester reference interval for FT4 was determined for the Modular E170 analyzer [6]. In this study we sought to answer three questions. First, can a single TSH reference interval be used for multiple methods? Second, is the FT4 reference interval method-specific? Third, are second trimester FT4 reference intervals determined at two North American sites using Modular E170 analyzers comparable? Methods
⁎ Corresponding author. ARUP Laboratories, 500 Chipeta Way, Salt Lake City, UT 84108, USA. Fax: +1 801 584 5207. E-mail address: [email protected] (W.L. Roberts).
Residual samples (n = 3102) with sufficient volume for additional testing that had been collected at multiple sites
0009-9120/$ - see front matter © 2008 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.clinbiochem.2008.12.004
R. Silvio et al. / Clinical Biochemistry 42 (2009) 750–753
throughout the US from subjects 14 to 20 weeks gestation for maternal serum screening were used. The iodine status of these subjects was unknown. These samples were stored frozen and analyzed for thyroid peroxidase antibodies (TPO Ab), thyroglobulin antibodies (Tg Ab), TSH, FT4, total T4, free triiodothyronine, and total triiodothyronine by ARCHITECT i2000SR methods [5]. Samples were retrieved from frozen storage, thawed, mixed, and checked for clots prior to testing for TSH and FT4 using a Modular E170 analyzer and Elecsys reagents (Roche Diagnostics, Indianapolis, IN) according to the manufacturer's instructions. A single reagent lot was used for TSH and FT4. All studies with human subject samples were approved by the University of Utah Institutional Review Board. Samples with TPO Ab and/or Tg Ab concentrations that were higher than the upper reference limits were excluded from reference interval determinations. The central 95% reference interval for TSH was determined non-parametrically using EP Evaluator Release 8 software (David G. Rhoads Associates, Kennett Square, PA). Results from samples that had normal TSH results after this analysis were used to determine the central 95% non-parametric reference interval for FT4. Analysis of method comparison data from the Elecsys E170 and ARCHITECT i2000SR methods was performed using Analyse-It, version 2.05 (Analyse-It Software, Leeds, UK). Results Approximately 14% of the 3102 samples available for this study were positive for either TPO Ab or Tg Ab, or both. When both TPO Ab and Tg Ab positive samples were excluded, the Elecsys E170 TSH reference interval was 0.18–4.07 mIU/L (n = 2660). The reference interval for FT4 determined using subjects negative for both TPO Ab and Tg Ab and with normal TSH results was 9.5–15.8 pmol/L (n = 2528). The 95% confidence limits for the upper and lower reference limits for both analytes are shown (Table 1). A recent Canadian study used a similar approach [6]. These authors did not test for Tg Ab and used a second trimester TSH reference interval of 0.1– 3.0 mIU/L. They found the Elecsys E170 second trimester FT4 reference interval for 242 subjects was 9.7–17.5 pmol/L. The lower reference limit was slightly higher than ours while the upper reference limit was substantially higher. To investigate possible causes for the difference between our FT4 upper
751
reference limit and the one from the Canadian study, we reanalyzed our data. First, we included all subjects regardless of their Tg Ab results (third column of Table 1). Second, we included all subjects regardless of their Tg Ab results and used a TSH reference interval of 0.1–3.0 mIU/L (fourth column of Table 1). Finally, we included all subjects regardless of their Tg Ab results, used a TSH reference interval of 0.1–3.0 mIU/L, and analyzed the data parametrically to exactly replicate the conditions of the Canadian study (fifth column of Table 1). Only the use of parametric analysis made a significant difference in the FT4 lower reference limit. None of these conditions changed the upper reference limit. The TSH and FT4 results previously obtained with the ARCHITECT i2000SR were compared with the current results (Fig. 1). The TSH methods showed a strong correlation with r = 0.97. The correlation between FT4 methods was not as good (r = 0.82). These samples had been through another freeze–thaw cycle so we wanted to confirm the stability of FT4 and exclude sample instability as a possible explanation for FT4 upper reference limit differences between our study and the Canadian study. Therefore, we re-analyzed 60 samples for FT4 with the ARCHITECT i2000 method using a different lot of calibrators and reagents. The mean for the original ARCHITECT results was 12.04 pmol/L and the mean for the repeat results was 11.96 pmol/L (p = 0.43), indicating no significant difference with two additional freeze–thaw cycles. Discussion The second trimester Elecsys E170 TSH reference interval of 0.18–4.07 mIU/L was different from the previously determined ARCHITECT i2000SR reference interval of 0.15–3.11 mIU/L [5]. The 95% confidence interval for the lower reference limit overlaps with the ARCHITECT i2000SR lower reference limit. However, the 95% confidence interval for the Elecsys E170 upper reference limit does not overlap with that of the ARCHITECT i2000SR. The Elecsys E170 reference interval for FT4 of 9.5–15.8 pmol/L was different from the ARCHITECT i2000SR reference interval of 9.3–15.2 pmol/L. The FT4 upper, but not lower, reference limits were significantly different between these two methods. Given the rather poor correlation between these two FT4 methods (r = 0.82), it is surprising that the FT4 reference limits agree as well as they do.
Table 1 Summary of Elecsys E170 second trimester reference intervals Subject exclusion criteria and statistical analysis method Analyte TPO Ab and TG Ab positive subjects excluded, non-parametric TSH 0.18–4.07 mIU/L a (0.14–0.27) b, (3.90–4.35) n = 2660 FT4 9.5–15.8 pmol/L c (9.3–9.6), (15.6–16.0) n = 2528 a b c
TPO Ab positive subjects excluded, non-parametric 0.19–4.19 mIU/L (0.15–0.27), (3.97–4.40) n = 2722 9.5–15.8 pmol/L (9.3–9.6), (15.6–16.0) n = 2588
TPO Ab positive subjects excluded, TSH TPO Ab positive subjects excluded, 0.1–3.0 mIU/L, non-parametric TSH 0.1–3.0 mIU/L, parametric 0.1–3.0 mIU/L 0.1–3.0 mIU/L
9.5–15.8 pmol/L (9.3–9.6), (15.6–16.0) n = 2424
The central 95% reference intervals are indicated. The 95% confidence intervals for the lower and upper reference limits are indicated in parentheses. Samples with TSH values within the indicated reference interval were used to determine the FT4 reference interval.
9.3–15.6 pmol/L (9.2–9.4), (15.5–15.7) n = 2424
752
R. Silvio et al. / Clinical Biochemistry 42 (2009) 750–753
Fig. 1. Comparison of Modular E170 and ARCHITECT i2000SR results for TSH and FT4. The thick line is the regression line, the thin line is the line of identity, and the arrow indicates a statistical outlier that was repeated and confirmed by both methods. In panel A, TSH method comparison data are shown. Five samples with TSH values N12 mIU/L were excluded from this analysis. The regression equation was y = 1.32x − 0.05, r = 0.97, Sy/x = 0.26, and p b 0.001. In panel B, FT4 method comparison data are shown. Nine samples with FT4 values N21 pmol/L were excluded from this analysis. The regression equation was y = 1.12x − 1.02, r = 0.82, Sy/x = 1.01, and p b 0.001.
The positive bias observed for TSH for the Modular E170 versus the ARCHITECT i2000SR method has been previously reported [7]. The differences between these two TSH methods appear to be independent of calibration and reagent lot and relatively stable over time. Further harmonization of TSH methods is required if method independent reference intervals for TSH are to be considered during pregnancy and in nonpregnant patients. Currently, proper interpretation of TSH values requires method-dependent reference intervals. A TSH therapeutic range for levothyroxine replacement in non-
pregnant patients has been set at 0.5–2.00 mIU/L [8]. This recommendation may need to be reviewed given the TSH intermethod differences we observed. ReferenceintervalsforFT4inpregnancyalsoneedtobemethod specific, although the reference intervals determined by the two methodsweevaluatedwereactuallymoresimilarthanthoseforTSH. Since we analyzed only a subset of the samples from the original ARCHITECT i2000SR study, we re-analyzed the ARCHITECT datausingthissamesubset.NochangeswerenotedineithertheTSHor FT4referenceintervals(datanotshown). It is not clear why the upper reference limit for FT4 that we determined was lower than that found in the Canadian study [6]. Although the Canadian study did not test for Tg Ab, removing Tg Ab results from our data analysis did not change the FT4 reference interval. However, it did slightly but not significantly increase the upper reference limit for TSH from 4.07 to 4.19 mIU/L (Table 1). Similarly, the choice of normal subjects based on TSH did not significantly affect the FT4 reference interval (Table 1) as previously suggested [6]. Reference results were normally distributed and parametric statistics were used in the Canadian study. In the current study, which included a larger number of subjects, we used non-parametric statistics. When our data was re-examined using parametric statistics, we found a FT4 reference interval of 9.3–15.6 pmol/L. This lower reference limit was significantly different from the nonparametric lower reference limit. However, this difference in data analysis between studies cannot explain the difference in upper reference limits. There are three other possible explanations for the difference between Elecsys E170 reference intervals from the two studies. First, different gestational ages were used. Our study included subjects from 14 to 20 weeks of gestation while the Canadian study included weeks 19 to 21. It is unlikely that this difference explains the difference in FT4 upper reference limits since the upper reference limits decreases slightly with increasing gestational age [5]. The second is geographic and/or ethnic differences. Our study included much of the US, while the Canadian study enrolled subjects exclusively from Toronto. A third possible explanation is analytical differences in the FT4 methods, perhaps due to lot-tolot variability or calibration differences. Neither of these latter two explanations can be excluded. In summary, method specific reference intervals are required during pregnancy for both TSH and FT4. TSH methods clearly are not adequately harmonized for method independent reference intervals, despite an earlier assurance that they were well harmonized [3]. Subject exclusion criteria, such as the presence of thyroid antibodies, may be important factors affecting second trimester TSH reference intervals. The FT4 reference interval is not sensitive to the presence of Tg Ab or the TSH cut points that are selected. Other factors including geographic location, ethnicity, and intra-method variability likely play a significant role. Acknowledgments We gratefully acknowledge Katie Ludwig for sample collection and de-identification. Reagents and an analyzer for
R. Silvio et al. / Clinical Biochemistry 42 (2009) 750–753
this study were provided by Roche Diagnostics and the ARUP Institute for Clinical and Experimental Pathology provided additional support. References [1] Neale DM, Cootauco AC, Burrow G. Thyroid disease in pregnancy. Clin Perinatol 2007;34:543–57. [2] Davis LE, Lucas MJ, Hawkins GD, et al. Thyrotoxicosis complicating pregnancy. Am J Obstet Gynecol 1989;160:63–70. [3] Mandel SJ, Spencer CA, Hollowell JG. Are detection and treatment of thyroid insufficiency in pregnancy feasible? Thyroid 2005;15:44–53.
753
[4] Abalovich M, Amino N, Barbour LA, et al. Management of thyroid dysfunction during pregnancy and postpartum: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab 2007;92:S1–S47. [5] La'ulu SL, Roberts WL. Second-trimester reference intervals for thyroid tests: the role of ethnicity. Clin Chem 2007;53:1658–64. [6] Gong Y, Hoffman BR. Free thyroxine reference interval in each trimester of pregnancy determined with the Roche Modular E-170 electrochemiluminescent immunoassay. Clin Biochem 2008;41:902–6. [7] Rawlins ML, Roberts WL. Performance characteristics of six thirdgeneration assays for thyroid-stimulating hormone. Clin Chem 2004;50: 2338–44. [8] Baloch Z, Carayon P, Conte-Devolx B, et al. Laboratory medicine practice guidelines. Laboratory support for the diagnosis and monitoring of thyroid disease. Thyroid 2003;13:3–126.
Clinical Biochemistry 42 (2009) 750 – 753
Method specific second-trimester reference intervals for thyroid-stimulating hormone and free thyroxine Rechelle Silvio a , Karly J. Swapp a , Sonia L. La'ulu b , Kara Hansen-Suchy a , William L. Roberts c,⁎ a
c
Department of Clinical Laboratory Sciences, Weber State University, Ogden, UT, USA b ARUP Institute for Clinical and Experimental Pathology, Salt Lake City, UT, USA Department of Pathology, University of Utah Health Sciences Center, Salt Lake City, UT, USA
Received 24 October 2008; received in revised form 4 December 2008; accepted 8 December 2008 Available online 24 December 2008
Abstract Objective: To determine second trimester reference intervals for TSH and FT4. Design: Samples from 3102 subjects were tested for TPO and Tg antibodies. Methods: Elecsys E170 reference intervals for TSH and FT4 were determined using antibody-negative samples. Results: Second trimester reference intervals for TSH and FT4 were 0.18–4.07 mIU/L and 9.5–15.8 pmol/L, respectively. The Elecsys E170 TSH results were positively biased compared to ARCHITECT i2000SR results for these same samples. Conclusions: Method-specific reference intervals are required for TSH and FT4. © 2008 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved. Keywords: Elecsys E170; Free thyroxine; Thyrotropin; Reference interval; Pregnancy
Introduction Hyperthyroidism occurs in 0.2% of pregnancies and Graves' disease accounts for 95% of cases [1]. Severe maternal hyperthyroidism is associated with stillbirth, preterm delivery, intrauterine growth retardation, pre-eclampsia, and heart failure [2]. Hypothyroidism is present in 0.1 to 0.3% of pregnancies [1]. Maternal hypothyroidism is associated with miscarriage, pre-eclampsia, placental abruption, growth retardation, prematurity and stillbirths and fetuses often have impaired neurologic development [1]. Worldwide, iodine deficiency is the most common cause of hypothyroidism. In the United States, Hashimoto's thyroiditis accounts for most cases of hypothyroidism in pregnancy [1]. Thyroid function changes during pregnancy make diagnosis of thyroid disorders difficult. Guidelines published in 2005 for diagnosis of thyroid disease during pregnancy recommend trimester specific reference intervals for thyrotropin (TSH) and free thyroxine (FT4) [3].
These authors assert “Inasmuch as between-method biases for TSH are minimal, as future studies progress in pregnancy, there is no need for method-specific TSH ranges.” They also recommend that FT4 reference intervals during pregnancy should be determined for each method [3]. Recent clinical practice guidelines recommend a TSH upper reference limit of 3 mIU/L in the second and third trimesters or “trimester-specific normal TSH ranges” but do not specify that these be method specific [4]. We previously published second trimester reference intervals for thyroid tests on the ARCHITECT i2000SR analyzer [5]. Recently, a second trimester reference interval for FT4 was determined for the Modular E170 analyzer [6]. In this study we sought to answer three questions. First, can a single TSH reference interval be used for multiple methods? Second, is the FT4 reference interval method-specific? Third, are second trimester FT4 reference intervals determined at two North American sites using Modular E170 analyzers comparable? Methods
⁎ Corresponding author. ARUP Laboratories, 500 Chipeta Way, Salt Lake City, UT 84108, USA. Fax: +1 801 584 5207. E-mail address: [email protected] (W.L. Roberts).
Residual samples (n = 3102) with sufficient volume for additional testing that had been collected at multiple sites
0009-9120/$ - see front matter © 2008 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.clinbiochem.2008.12.004
R. Silvio et al. / Clinical Biochemistry 42 (2009) 750–753
throughout the US from subjects 14 to 20 weeks gestation for maternal serum screening were used. The iodine status of these subjects was unknown. These samples were stored frozen and analyzed for thyroid peroxidase antibodies (TPO Ab), thyroglobulin antibodies (Tg Ab), TSH, FT4, total T4, free triiodothyronine, and total triiodothyronine by ARCHITECT i2000SR methods [5]. Samples were retrieved from frozen storage, thawed, mixed, and checked for clots prior to testing for TSH and FT4 using a Modular E170 analyzer and Elecsys reagents (Roche Diagnostics, Indianapolis, IN) according to the manufacturer's instructions. A single reagent lot was used for TSH and FT4. All studies with human subject samples were approved by the University of Utah Institutional Review Board. Samples with TPO Ab and/or Tg Ab concentrations that were higher than the upper reference limits were excluded from reference interval determinations. The central 95% reference interval for TSH was determined non-parametrically using EP Evaluator Release 8 software (David G. Rhoads Associates, Kennett Square, PA). Results from samples that had normal TSH results after this analysis were used to determine the central 95% non-parametric reference interval for FT4. Analysis of method comparison data from the Elecsys E170 and ARCHITECT i2000SR methods was performed using Analyse-It, version 2.05 (Analyse-It Software, Leeds, UK). Results Approximately 14% of the 3102 samples available for this study were positive for either TPO Ab or Tg Ab, or both. When both TPO Ab and Tg Ab positive samples were excluded, the Elecsys E170 TSH reference interval was 0.18–4.07 mIU/L (n = 2660). The reference interval for FT4 determined using subjects negative for both TPO Ab and Tg Ab and with normal TSH results was 9.5–15.8 pmol/L (n = 2528). The 95% confidence limits for the upper and lower reference limits for both analytes are shown (Table 1). A recent Canadian study used a similar approach [6]. These authors did not test for Tg Ab and used a second trimester TSH reference interval of 0.1– 3.0 mIU/L. They found the Elecsys E170 second trimester FT4 reference interval for 242 subjects was 9.7–17.5 pmol/L. The lower reference limit was slightly higher than ours while the upper reference limit was substantially higher. To investigate possible causes for the difference between our FT4 upper
751
reference limit and the one from the Canadian study, we reanalyzed our data. First, we included all subjects regardless of their Tg Ab results (third column of Table 1). Second, we included all subjects regardless of their Tg Ab results and used a TSH reference interval of 0.1–3.0 mIU/L (fourth column of Table 1). Finally, we included all subjects regardless of their Tg Ab results, used a TSH reference interval of 0.1–3.0 mIU/L, and analyzed the data parametrically to exactly replicate the conditions of the Canadian study (fifth column of Table 1). Only the use of parametric analysis made a significant difference in the FT4 lower reference limit. None of these conditions changed the upper reference limit. The TSH and FT4 results previously obtained with the ARCHITECT i2000SR were compared with the current results (Fig. 1). The TSH methods showed a strong correlation with r = 0.97. The correlation between FT4 methods was not as good (r = 0.82). These samples had been through another freeze–thaw cycle so we wanted to confirm the stability of FT4 and exclude sample instability as a possible explanation for FT4 upper reference limit differences between our study and the Canadian study. Therefore, we re-analyzed 60 samples for FT4 with the ARCHITECT i2000 method using a different lot of calibrators and reagents. The mean for the original ARCHITECT results was 12.04 pmol/L and the mean for the repeat results was 11.96 pmol/L (p = 0.43), indicating no significant difference with two additional freeze–thaw cycles. Discussion The second trimester Elecsys E170 TSH reference interval of 0.18–4.07 mIU/L was different from the previously determined ARCHITECT i2000SR reference interval of 0.15–3.11 mIU/L [5]. The 95% confidence interval for the lower reference limit overlaps with the ARCHITECT i2000SR lower reference limit. However, the 95% confidence interval for the Elecsys E170 upper reference limit does not overlap with that of the ARCHITECT i2000SR. The Elecsys E170 reference interval for FT4 of 9.5–15.8 pmol/L was different from the ARCHITECT i2000SR reference interval of 9.3–15.2 pmol/L. The FT4 upper, but not lower, reference limits were significantly different between these two methods. Given the rather poor correlation between these two FT4 methods (r = 0.82), it is surprising that the FT4 reference limits agree as well as they do.
Table 1 Summary of Elecsys E170 second trimester reference intervals Subject exclusion criteria and statistical analysis method Analyte TPO Ab and TG Ab positive subjects excluded, non-parametric TSH 0.18–4.07 mIU/L a (0.14–0.27) b, (3.90–4.35) n = 2660 FT4 9.5–15.8 pmol/L c (9.3–9.6), (15.6–16.0) n = 2528 a b c
TPO Ab positive subjects excluded, non-parametric 0.19–4.19 mIU/L (0.15–0.27), (3.97–4.40) n = 2722 9.5–15.8 pmol/L (9.3–9.6), (15.6–16.0) n = 2588
TPO Ab positive subjects excluded, TSH TPO Ab positive subjects excluded, 0.1–3.0 mIU/L, non-parametric TSH 0.1–3.0 mIU/L, parametric 0.1–3.0 mIU/L 0.1–3.0 mIU/L
9.5–15.8 pmol/L (9.3–9.6), (15.6–16.0) n = 2424
The central 95% reference intervals are indicated. The 95% confidence intervals for the lower and upper reference limits are indicated in parentheses. Samples with TSH values within the indicated reference interval were used to determine the FT4 reference interval.
9.3–15.6 pmol/L (9.2–9.4), (15.5–15.7) n = 2424
752
R. Silvio et al. / Clinical Biochemistry 42 (2009) 750–753
Fig. 1. Comparison of Modular E170 and ARCHITECT i2000SR results for TSH and FT4. The thick line is the regression line, the thin line is the line of identity, and the arrow indicates a statistical outlier that was repeated and confirmed by both methods. In panel A, TSH method comparison data are shown. Five samples with TSH values N12 mIU/L were excluded from this analysis. The regression equation was y = 1.32x − 0.05, r = 0.97, Sy/x = 0.26, and p b 0.001. In panel B, FT4 method comparison data are shown. Nine samples with FT4 values N21 pmol/L were excluded from this analysis. The regression equation was y = 1.12x − 1.02, r = 0.82, Sy/x = 1.01, and p b 0.001.
The positive bias observed for TSH for the Modular E170 versus the ARCHITECT i2000SR method has been previously reported [7]. The differences between these two TSH methods appear to be independent of calibration and reagent lot and relatively stable over time. Further harmonization of TSH methods is required if method independent reference intervals for TSH are to be considered during pregnancy and in nonpregnant patients. Currently, proper interpretation of TSH values requires method-dependent reference intervals. A TSH therapeutic range for levothyroxine replacement in non-
pregnant patients has been set at 0.5–2.00 mIU/L [8]. This recommendation may need to be reviewed given the TSH intermethod differences we observed. ReferenceintervalsforFT4inpregnancyalsoneedtobemethod specific, although the reference intervals determined by the two methodsweevaluatedwereactuallymoresimilarthanthoseforTSH. Since we analyzed only a subset of the samples from the original ARCHITECT i2000SR study, we re-analyzed the ARCHITECT datausingthissamesubset.NochangeswerenotedineithertheTSHor FT4referenceintervals(datanotshown). It is not clear why the upper reference limit for FT4 that we determined was lower than that found in the Canadian study [6]. Although the Canadian study did not test for Tg Ab, removing Tg Ab results from our data analysis did not change the FT4 reference interval. However, it did slightly but not significantly increase the upper reference limit for TSH from 4.07 to 4.19 mIU/L (Table 1). Similarly, the choice of normal subjects based on TSH did not significantly affect the FT4 reference interval (Table 1) as previously suggested [6]. Reference results were normally distributed and parametric statistics were used in the Canadian study. In the current study, which included a larger number of subjects, we used non-parametric statistics. When our data was re-examined using parametric statistics, we found a FT4 reference interval of 9.3–15.6 pmol/L. This lower reference limit was significantly different from the nonparametric lower reference limit. However, this difference in data analysis between studies cannot explain the difference in upper reference limits. There are three other possible explanations for the difference between Elecsys E170 reference intervals from the two studies. First, different gestational ages were used. Our study included subjects from 14 to 20 weeks of gestation while the Canadian study included weeks 19 to 21. It is unlikely that this difference explains the difference in FT4 upper reference limits since the upper reference limits decreases slightly with increasing gestational age [5]. The second is geographic and/or ethnic differences. Our study included much of the US, while the Canadian study enrolled subjects exclusively from Toronto. A third possible explanation is analytical differences in the FT4 methods, perhaps due to lot-tolot variability or calibration differences. Neither of these latter two explanations can be excluded. In summary, method specific reference intervals are required during pregnancy for both TSH and FT4. TSH methods clearly are not adequately harmonized for method independent reference intervals, despite an earlier assurance that they were well harmonized [3]. Subject exclusion criteria, such as the presence of thyroid antibodies, may be important factors affecting second trimester TSH reference intervals. The FT4 reference interval is not sensitive to the presence of Tg Ab or the TSH cut points that are selected. Other factors including geographic location, ethnicity, and intra-method variability likely play a significant role. Acknowledgments We gratefully acknowledge Katie Ludwig for sample collection and de-identification. Reagents and an analyzer for
R. Silvio et al. / Clinical Biochemistry 42 (2009) 750–753
this study were provided by Roche Diagnostics and the ARUP Institute for Clinical and Experimental Pathology provided additional support. References [1] Neale DM, Cootauco AC, Burrow G. Thyroid disease in pregnancy. Clin Perinatol 2007;34:543–57. [2] Davis LE, Lucas MJ, Hawkins GD, et al. Thyrotoxicosis complicating pregnancy. Am J Obstet Gynecol 1989;160:63–70. [3] Mandel SJ, Spencer CA, Hollowell JG. Are detection and treatment of thyroid insufficiency in pregnancy feasible? Thyroid 2005;15:44–53.
753
[4] Abalovich M, Amino N, Barbour LA, et al. Management of thyroid dysfunction during pregnancy and postpartum: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab 2007;92:S1–S47. [5] La'ulu SL, Roberts WL. Second-trimester reference intervals for thyroid tests: the role of ethnicity. Clin Chem 2007;53:1658–64. [6] Gong Y, Hoffman BR. Free thyroxine reference interval in each trimester of pregnancy determined with the Roche Modular E-170 electrochemiluminescent immunoassay. Clin Biochem 2008;41:902–6. [7] Rawlins ML, Roberts WL. Performance characteristics of six thirdgeneration assays for thyroid-stimulating hormone. Clin Chem 2004;50: 2338–44. [8] Baloch Z, Carayon P, Conte-Devolx B, et al. Laboratory medicine practice guidelines. Laboratory support for the diagnosis and monitoring of thyroid disease. Thyroid 2003;13:3–126.