DOI: 10.1111/j.1471-0528.2008.01673.x

Maternal medicine

www.blackwellpublishing.com/bjog

Establishment of reference range for thyroid hormones in normal pregnant Indian women RK Marwaha,a S Chopra,b S Gopalakrishnan,a B Sharma,a RS Kanwar,a A Sastry,a S Singha a Division of Endocrinology and Thyroid Research, Institute of Nuclear Medicine and Allied Sciences, Delhi, India b Department of Obstetrics and Gynecology, Armed Forces Clinic, New Delhi, India Correspondence: Dr RK Marwaha, Division of Endocrinology and Thyroid Research, Institute of Nuclear Medicine and Allied Sciences, Brig. SK Mazumdar Marg, Timarpur, Delhi 110054, India. Email [email protected]

Accepted 3 January 2008.

Background Interpretation of thyroid function tests during

pregnancy needs trimester-related reference intervals from pregnant populations with minimal risk for thyroid dysfunction. While India has become iodine sufficient after two decades of salt iodisation, there is no normative data for thyroid function from healthy pregnant women of this country. Aims and objectives To determine trimester-specific reference

ranges for free triiodothyronine (FT3), free thyroxine (FT4) and thyrotropin (TSH) from healthy pregnant Indian women. Design Cross-sectional study in a reference population of pregnant

women. Setting Primary care level obstetric department in India. Population Women with uncomplicated pregnancy in any trimester. Methods Five hundred and forty-one apparently healthy pregnant women with uncomplicated single intrauterine gestations reporting to the Armed Forces Clinic in any trimester were consecutively recruited. Clinical examination, thyroid ultrasound for echogenicity and nodularity and estimation of FT3, FT4, TSH and antithyroid antibodies (antithyroperoxidase [anti-TPO] and antithyroglobulin [anti-Tg]) using electrochemiluminescence technique were carried out. From this entire sample, a disease- and risk-free reference population was obtained by excluding those with any known factor that could affect thyroid function or those who were being treated for thyroid dysfunction.

Main outcome measure None. Results Of the 541 consecutive pregnant women in different trimesters enrolled for the study, 210 women were excluded. The composition of reference population comprising 331 women was 107 in first trimester, 137 in second trimester and 87 in third trimester. The 5th and 95th percentiles values were used to determine the reference ranges for FT3, FT4 and TSH. The trimester-wise values in the first, second and third trimesters were: FT3 (1.92–5.86, 3.2–5.73 and 3.3–5.18 pM/l), FT4 (12–19.45, 9.48–19.58 and 11.32–17.7 pM/l) and TSH (0.6–5.0, 0.44–5.78 and 0.74–5.7 iu/ml), respectively. Analysis of mean, median values for FT3, FT4 and TSH between each trimester showed no significant difference in FT3 and TSH values (95% CI). However, FT4 showed significant variation between trimesters with values decreasing with advancing gestational age (P value: first versus second = 0.015, first versus third = 0.003 and second versus third = not significant). Women with antibody positivity and hypoechogenicity of thyroid gland had significantly higher TSH values when compared with women with antibody negativity and normoechogenicity. Conclusions Reference ranges of FT3, FT4 and TSH have been

established for pregnant Indian women using 5th and 95th percentiles. Keywords Electrochemiluminescence, pregnancy, reference range,

thyroid hormones, trimester specific.

Please cite this paper as: Marwaha R, Chopra S, Gopalakrishnan S, Sharma B, Kanwar R, Sastry A, Singh S. Establishment of reference range for thyroid hormones in normal pregnant Indian women. BJOG 2008;115:602–606.

Introduction Optimal functioning of thyroid gland is essential at all stages of life, including pregnancy and fetal development. Sufficient evidence is available showing that thyroid dysfunction during pregnancy can affect not only the maternal outcome but also the neuropsychological development of fetus.1–8 Since the reported prevalence of thyroid disorders during pregnancy

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ranges from 2 to 5% in pregnant women, it is important that these are detected early for timely intervention.4,9 Physiological changes such as haemodilution, increased serum thyroxine-binding globulin and increased human chorionic gonadotrophin especially in the first trimester could affect functioning of thyroid gland and interpretation of thyroid function tests.10,11 These variations during pregnancy necessitate the formation of trimester-related reference

ª 2008 The Authors Journal compilation ª RCOG 2008 BJOG An International Journal of Obstetrics and Gynaecology

Trimester-specific reference ranges in pregnant Indian women

intervals from pregnant populations with minimal risk for thyroid dysfunction.12 Although several studies are available from different regions of the world,13–15 it is essential to develop norms for Indian population due to ethnic variations, different environmental conditions and more so after two decades of salt iodisation programme.16–18

Aims and objectives To determine trimester-specific reference ranges for free thyroid hormones (free triiodothyronine [FT3], free thyroxine [FT4]) and thyrotropin (TSH) from healthy pregnant women.

Materials and methods Setting The study was conducted by the Division of Endocrine Research at the Institute of Nuclear Medicine and Allied Sciences (INMAS), Delhi, in collaboration with obstetric outpatient department of Armed Forces Clinic (AFC), New Delhi. The obstetric department at the AFC is a primary care provider for families of armed forces personnel from different regions of the country. The Institutional Ethics Committee at INMAS approved the study protocol, and the women were recruited after prior informed consent.

Subjects Healthy women with uncomplicated single intrauterine gestations consuming iodised salt in any trimester were consecutively recruited. History was taken to ascertain any chronic illness, thyroid illness in the past or present, medications (current and past) and family history of thyroid illness. Obstetric history was elicited to know duration of gestation, gravida, para and number of abortions. Physical examination included anthropometry, goitre grading19 and general and systemic examinations. Ultrasound of thyroid was carried out in the supine position with the neck extended with a 7.5-MHz transducer (LogiQ a 100 MP; Wipro GE, Bangalore, India) for echogenicity and nodularity. Fasting venous samples were drawn, serum separated and stored at –20C for analysis of thyroid hormones and antithyroid antibodies. From the entire population, a reference population was identified by using the following exclusion criteria: 1 History of hyperemesis gravidarum, thyroid illness or use of thyroid medications and abortions. 2 Family history of thyroid illness. 3 Presence of goitre, grade 1 or 2. 4 Presence of hypoechogenicity/nodularity of thyroid on ultrasound. 5 Presence of antithyroid antibodies. 6 Overt hypothyroidism (OH) or hyperthyroidism. The values of serum FT3, FT4 and TSH from this group were used to derive reference intervals for pregnant women.

Lab methods FT3, FT4, TSH and antibodies (antithyroperoxidase [antiTPO] and antithyroglobulin [anti-Tg]) were estimated by the electrochemiluminescence (ECL) technique using commercially available kits from Roche Diagnostics (Mannheim, Germany) with Elecsys 1010 analyzer. The analytical sensitivity and total precision values for FT3, FT4 and TSH assays were 0.400 pM/l and 2.2%, 0.30 pM/l and 2.7% and 0.005 miu/ml and 2.2%, respectively. The laboratory reference ranges were: FT3 (3.7–7.2 pM/l), FT4 (12.0–23.0 pM/l) and TSH (0.27–4.2 miu/ml). The intra-assay coefficients of variation for the assays were 4.7, 1.4 and 1.9%. The corresponding values for intra-assay coefficient of variation (CV), total precision and analytical sensitivity for anti-TPO and anti-Tg were 5.6%, 7.6, 115 iu/ml, respectively.

Statistics All numerical data were entered into MS Excel 5.0 (Microsoft Inc, Seattle, WA, USA) and analysed using the Stata 9.1 (Stata Corp., College Station, TX, USA). Data for FT3 and FT4 are expressed as mean ± SD, median (range) and 5th to 95th percentiles. TSH was normalised using log transformation as it did not follow normal distribution and summarised as geometric mean (95% CI). Comparison among different groups were performed using Student’s t test/one-way analysis of variance followed by Bonferroni post hoc analysis and were considered significant at P < 0.05

Results Total study population consisted of 541 women (first trimester: 172, second trimester: 215 and third trimester: 154) (234 primigravidas and 307 multigravida). History of abortions and thyroid illness in the family were present in 134 (24.7%) and 20 (3.7%) women, respectively. Clinical examination showed goitre in 111 (20%) and hypoechogenicity was noted in 104 women (19.2%) on ultrasound. Anti-TPO and anti-Tg were elevated in 43 (7.9%) and 16 (2.9%) women, respectively. Using the kit reference values, OH was found in 7 (1.3%) women—1 in the first trimester, 2 in the second trimester and 4 in the third trimester. Subclinical hypothyroidism (SCH) was noted in 78 (14.2%) women—32 in the first trimester, 27 in the second trimester and 26 in the third trimester. Subclinical hyperthyroidism was found in 11 (2%) women—2 in the first trimester, 5 in the second trimester and 4 in the third trimester. No significant difference was observed in the prevalence of thyroid dysfunction and autoimmunity between women with or without goitre, as shown in Table 1. Median TSH was significantly higher among pregnant women with TPO positivity and hypoechoic thyroid glands compared with those with TPO

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Table 1. Distribution of thyroid dysfunction and autoimmunity in women with or without goitre Goitre

Anti-TPO positive

Anti-Tg antibody positive

OH

SCH

Overt hyperthyroidism

Subclinical hyperthyroidism

32 (7.4) 11 (9.9)

10 (2.3) 6 (5.4)

7 (1.6) 0 (0)

60 (13.95) 18 (16.2)

0 0

9 (2.1) 2 (1.8)

Absent (n 5 430), n (%) Present (n 5 111), n (%)

negativity and normoechoic thyroid glands. This justifies exclusion of these women from the reference population.

Reference population After applying the exclusion criteria, 65 women in the first trimester, 78 in the second trimester and 67 in the third trimester were excluded from the study. The remaining 331 women constituted the reference population (first trimester: 107, second trimester: 137 and third trimester: 87). The mean ± SD, median and 5th and 95th percentiles for FT3, FT4 and TSH were determined in each trimester of pregnancy. The reference intervals for each trimester are shown in Table 2. Analysis of these parameters between each trimester showed no significant difference in FT3 and TSH values. However, FT4 showed significant variation between trimesters with values decreasing with advancing gestational age. Difference was primarily seen between first and second trimesters and first and third trimesters but not between second and third trimesters. No definite trend was seen in FT3 and TSH between the trimesters unlike in the case of FT4, where it showed a decreasing trend. When the trimester-wise 95th percentile of TSH from the reference population was applied to the total population, the number of women with SCH decreased to 36 (6.6%) from 78 (14.4%).

Comparison of thyroid function parameters between different groups When the normal versus excluded groups were compared, no significant difference in values of mean FT3, FT4 and median

TSH was observed. Although, the median TSH values in the third trimester were higher in the excluded group, it did not achieve statistical significance (2.6 miu/ml [0.6–18.3] versus 2.1 miu/ml [0.2–9.5], P = 0.11). However, within the excluded group, the subgroups with anti-TPO antibody positivity and those with hypoechoic thyroid glands had significantly higher median TSH values in comparison with those with anti-TPO negativity and normoechoic thyroid glands (3.5 versus 2.2 miu/ml, P = 0.0002, hypoechoic versus normoechoic glands: 2.55 versus 2.21, P = 0.03). No significant difference was seen in the values of FT3, FT4 and TSH between primigravidas and multigravidas and those with or without abortions.

Discussion Five hundred and forty-one pregnant women selected for the study were evaluated for thyroid function and autoimmunity by ECL method. The currently available methods to estimate FT3 and FT4 are all binding protein sensitive to some extent in a method-specific manner, but the fallacy has been reported to be least with ECL, which has been shown to be highly sensitive.20,21 The determination of normal values (appropriate reference intervals) has been described in the recommendations of International Federation of Clinical Chemistry and few other studies, the central feature of which is that the reference population should be appropriately described and criteria for selection made clear.22,23

Table 2. Trimester-wise values for mean, median, 5th and 95th centiles for FT3, FT4 and TSH from reference population

FT3

FT4

TSH

Trimesters

Mean  SD

Median (range)

5th

95th

I II III I II III I II III

4.36  1.08 4.34  0.78 4.15  0.64 14.9  2.35 14.0  2.33 13.76  2.35 2.42  1.65 2.49  1.9 2.6  1.9

4.4 (0.37–6.58) 4.3 (2.7–7.69) 4.1 (2.93–5.92) 14.46 (8.04–22) 13.4 (9.26–22.12) 13.28 (9.54–27.02) 2.1 (0.04–10.8) 2.4 (0.026–10.85) 2.1 (0.2–9.55)

1.92 3.2 3.3 12 9.48 11.3 0.6 0.435 0.74

5.86 5.7 5.18 19.45 19.58 17.71 5.0 5.78 5.7

FT3: P 5 0.167, NS; FT4: P 5 0.0019; TSH: P 5 0.641, Not significant. Comparison of FT4: I vs II, P 5 0.015; I vs III, P 5 0.003; II vs III, P 5 Not significant.

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Trimester-specific reference ranges in pregnant Indian women

The National Health and Nutrition Examination Survey in the USA had used exclusion criteria like personal or family history of thyroid illness, goitre and antibody positivity for an adult reference population but not ultrasound.24 Hypoechogenicity of thyroid gland has been shown to be associated with not only thyroid dysfunction but also thyroid autoimmunity.25,26 A recent study establishing new reference intervals for TSH and thyroid hormones in healthy adults using ECL method based on National Academy of Clinical Biochemistry criteria has also used thyroid ultrasound to rule out thyroid pathology.27 In view of the above, we decided to obtain reference intervals for FT3, FT4 and TSH in pregnant women using strict exclusion criteria including thyroid ultrasound. We believe that the reference population should be as exclusive as possible as the purpose of reference intervals is to decide on thyroid status of a pregnant woman in whom thyroid functions cannot be compromised. This is because of well-established adverse outcomes in mother and fetus arising from thyroid dysfunction during pregnancy. This has been substantiated in our study showing significantly higher TSH values in those with anti-TPO antibody positivity and hypoechoic thyroid glands. Similar observations have been made by several other workers.25,28 Analysis of our cross-sectional data on mean FT3, FT4 and median TSH showed significant decrease in FT4 with advancing pregnancy, while no significant difference was seen in values of FT3 or TSH between the trimesters. However, sequential evaluation in three trimesters and urinary iodine estimation would have been ideal for interpretation of thyroid function in pregnancy. There are several studies available on pregnant women among which some are longitudinal in the same study group, while others are cross-sectional. They also vary on the assay method used for estimation of thyroid hormones. In one cross-sectional study of 522 pregnant women from Japan using ECL kits, the authors showed significant decrease in both FT3 and FT4 and increase in TSH with advancing pregnancy.15 Another study from India evaluated 124 pregnant women using radioimmunoassay showed increase in TSH progressively with each trimester.29 When mean serum triiodothyronine (T3) and thyroxine (T4) were analysed, both values increased from first to second trimester, but declined from second to third trimester. A longitudinal study through the trimesters from a group of pregnant women in Sweden used immunoassay and mass spectrometry to analyse thyroid function. Mean value of FT4 decreased by about 15% from first to second trimester, while it remained stable from second to third trimester. However, mean TSH increased with advancing gestational age from first to second trimester, while it remained stable from second to third trimester.13 Another longitudinal study used immunoassay for thyroid function to arrive at the trimester-specific reference range,

although only paired samples from first and second trimesters of 20 pregnant Asian women were studied.30 They found TSH to increase from first to second trimester, while FT4 decreased with progress in pregnancy. FT3 did not change between the first and second trimesters. Ethnic variations remain a concern, although studies that address this aspect are scant. One study31 showed TSH in Asian women to be lower than that in Caucasian women during the second trimester of pregnancy. In their opinion, Asian women are more prone to gestational thyrotoxicosis, although we did not find any case of overt hyperthyroidism. A recent study on thyroid function carried out only in the second trimester of gestation showed higher prevalence of raised TSH in whites compared with Asians and blacks.16 However, Price et al.30 did not find any difference in the thyroid hormone changes between pregnant Asian and pregnant Caucasian women. In a longitudinal study on pregnant Chinese women, the authors found a decrease in FT4 and FT3 by about 25% with gestational age from peak to nadir.32 Whereas, TSH was suppressed in the first trimester and rose subsequently with advancing pregnancy, the rise correlating with decrease in FT3 and FT4, as also shown in other studies.33,34

Conclusion The following reference intervals for FT3, FT4 and TSH determined for each trimester of pregnancy (FT3 [1.92–5.86, 3.2– 5.73 and 3.3–5.18 pM/l], FT4 [12–19.45, 9.48–19.58 and 11.32– 17.7 pM/l] and TSH [0.6–5.0, 0.44–5.78 and 0.74–5.7 iu/ml]) are recommended for evaluation of pregnant Indian women.

Funding The study was supported financially by the Defence Research and Development Organisation, under the Ministry of Defence, Government of India.

Ethics approval The study has been approved by the institutional ethics committee of INMAS, Delhi, India.

Contribution to authorship R.K.M. contributed to conceptualising the study, clinical evaluation and preparation of manuscript. S.C. assisted in the recruitment and clinical evaluation of the subjects. S.G. contributed to conceptualising the study, clinical evaluation and preparation of manuscript. B.S. contributed in conceptualisation, data collection, analysis and preparation of manuscript. R.S.K. assisted in clinical evaluation and ultrasound examination. A.S. and S.S. helped in collection of biochemical samples and laboratory evaluation. j

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References 1 Man EB, Jones WS. Thyroid function in human pregnancy. V. Incidence of maternal serum low butanol-extractable iodine and of normal gestational TBG and TBPA capacities; retardation of 8-month–old infants. Am J Obstet Gynecol 1969;104:898–908. 2 Davis LE, Leveno KJ, Cunningham FG. Hypothyroidism complicating pregnancy. Obstet Gynecol 1988;72:108–12. 3 Leung AS, Millar LK, Koonings PP, Montoro M, Mestman JH. Perinatal outcome in hypothyroid pregnancies. Obstet Gynecol 1993;81: 349–53. 4 Allen WC, Haddow JE, Palomaki GE, Williams JR, Mitchell ML, Hermos RJ, et al. Maternal thyroid deficiency and pregnancy complications: implications for population screening. J Med Screen 2000;7: 127–30. 5 Glinoer D, Soto MF, Bourdoux P, Lejeune PB, Delange F, Lemone M, et al. Pregnancy in patients with mild thyroid abnormalities: maternal and neonatal repercussions. J Clin Endocrinol Metab 1991;73:421–7. 6 Morreale de Escobar G, Obregon MJ, Escobar del Rey F. Is neuropsychological development related to maternal hypothyroidism or maternal hypothyroxinemia? J Clin Endocrinol Metab 2000;85:3975–87. 7 Haddow JE, Palomaki GE, Allan WC, William JR, Knight GJ, Gagnon J, et al. Maternal thyroid deficiency during pregnancy and subsequent neuropsychological development of the child. N Engl J Med 1999;341: 549–55. 8 Pop VJ, Kuijpens JL, van Baar AL, Verkerk G, van Son MM, de Vijlder JJ, et al. Low maternal free thyroxine concentrations during early pregnancy are associated with impaired psychomotor development in infancy. Clin Endocrinol 1999;50:149–55. 9 Klein RZ, Haddow JE, Faix JD, Brown RS, Hermos RJ, Pulkkinen A, et al. Prevalence of thyroid deficiency in pregnant women. Clin Endocrinol 1991;35:41–6. 10 Burrow GN, Fisher DA, Larsen PR. Maternal and fetal function. N Engl J Med 1994;331:1072–8. 11 Glinoer D. The regulation of thyroid function in pregnancy: pathways of endocrine adaptation from physiology to pathology. Endocr Rev 1997;18:404–33. 12 Mandel SJ, Spencer CA, Hollowell JG. Are detection and treatment of thyroid insufficiency in pregnancy feasible? Thyroid 2005;15:44–53. 13 Soldin OP, Tractenberg RE, Hollowell JG, Jonklass J, Janicic N, Solding SJ. Trimester-specific changes in maternal thyroid hormone, thyrotropin, and thyroglobulin concentrations during gestation: trends and associations across trimesters in iodine sufficiency. Thyroid 2004;14: 1084–90. 14 Quinn FA, Gridasov GN, Vdovenko SA, Krasnova NA, Vodopianova NV, Epiphanova MA, et al. Prevalence of abnormal thyroid stimulating hormone and thyroid peroxidase antibody-positive results in a population of pregnant women in the Samara region of the Russian Federation. Clin Chem Lab Med 2005;43:1223–6. 15 Kurioka H, Takahashi K, Miyazaki K. Maternal thyroid function during pregnancy and puerperal period. Endocr J 2005;52:587–91. 16 La’ulu SL, Roberts WL. Second-trimester reference intervals for thyroid tests: the role of ethnicity. Clin Chem 2007;53:1658–64. 17 Marwaha RK, Tandon N, Gupta N, Karak AK, Verma K, Kochupillai N. Residual goitre in the postiodization phase: iodine status, thiocyanate exposure and autoimmunity. Clin Endocrinol (Oxf) 2003;59:672–81.

606

18 Gopalakrishnan S, Singh SP, Prasad WR, Jain SK, Ambardar VK, Sankar R. Prevalence of goitre and autoimmune thyroiditis in schoolchildren in Delhi, India after two decades of salt iodisation. J Pediatr Endocrinol Metab 2006;19:889–93. 19 WHO, UNICEF, ICCIDD. Indicators for Assessing Idd and Their Control Through Salt Iodization. Geneva, Switzerland: World Health Organization, 1994. WHO/NUT/94.6. 20 Blackburn GF, Shah HP, Kenten JH, Leland J, Kamin RA, Link J, et al. Electrochemiluminescence detection for development of immunoassays and DNA probe assays for clinical diagnostics. Clin Chem 1991; 37:1534–9. 21 Sonan M, Hiraoka K, Yamada E, Watanabe S, Kobayashi M. Fundamental and clinical evaluation of TSH and thyroid hormone measurement by electrochemiluminescence immunoassay system Modular Analytics (EE). Jpn J Med Pharm Sci 2001;46:759–71. 22 Solberg HE. International Federation of Clinical Chemistry. Scientific committee, Clinical Section. Expert Panel on Therapy of Reference Values and International Committee for Standardization in Haematology Standing Committee on Reference Values. Approved recommendations (1986) on the theory of reference values. Part I. The concept of reference values. Clin Chim Acta 1987;165:111–18. 23 Demers LM, Spencer CA. Laboratory medicine practice guidelines: laboratory support for the diagnosis and monitoring of thyroid disease. Clin Endocrinol (Oxf) 2003;58:138–40. 24 Hollowell JG, Staechling NW, Flanders WD, Gunter EW, Spencer CA, Braverman LE. Serum TSH, T4 and thyroid antibodies in the United States population (1998–1994): National Health and Nutrition Examination Survey (NHANES III). J Clin Endocrinol Metab 2002;87:489–99. 25 Pedersen OM, Aardal NP, Larssen TB, Varhaug JE, Myking O, Vik-Mo H. The value of ultrasonography in predicting autoimmune thyroid disease. Thyroid 2000;10:251–9. 26 Marwaha RK, Tandon N, Kanwar R, Ganie MA, Bhattacharya B, Reddy DHK, et al. Evaluation of the role of ultrasonography in diagnosis of lymphocytic thyroiditis in goitrous children. Indian Pediatr (in press). 27 Kratzsch J, Fiedler GM, Leichtle A, Bru¨gel M, Buchbinder S, Otto L, et al. New reference intervals for thyrotropin and thyroid hormones based on National Academy of Clinical Biochemistry criteria and regular ultrasonography of the thyroid. Clin Chem 2005;51:1480–6. 28 Hawkins BR, Cheah PS, Dawkins RL, Whittingham S, Burger HG, Patel Y, et al. Diagnostic significance of thyroid microsomal antibodies in randomly selected population. Lancet 1980;2:1057–9. 29 Kumar A, Gupta N, Nath T, Sharma JB, Sharma S. Thyroid function tests in pregnancy. Indian J Med Sci 2003;57:252–8. 30 Price A, Owen O, Cresswell J, Catch I, Rutter S, Barik S, et al. Comparison of thyroid function in pregnant and non-pregnant Asian and western Caucasian women. Clin Chim Acta 2001;308:91–8. 31 Price A, Davies R, Heller SR, Milford-Ward A, Weetman AP. Asian women are at increased risk of gestational thyrotoxicosis. J Clin Endocrinol Metab 1996;81:1160–3. 32 Panesar NS, Li CY, Rogers MS. Reference intervals for thyroid hormones in pregnant Chinese women. Ann Clin Biochem 2001;38:329–32. 33 Parker JH. Amerlex free triiodothyronine and free thyroxine levels in normal pregnancy. Br J Obstet Gynaecol 1985;92:1234–8. 34 Gow SM, Kellet HA, Seth J, Sweeting VM, Toft AD, Beckett GJ. Limitations of new thyroid function tests in pregnancy. Clin Chim Acta 1985;152:325–33.

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