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Detection of Thyroid Dysfunction in Early Pregnancy: Universal Screening or Targeted High-Risk Case Finding?

Department of Endocrinology (B.V., B.S.), Peninsula Medical School, Royal Devon & Exeter Hospital, Exeter EX2 5DW, United Kingdom; and Departments of Endocrinology (S.A., M.B., R.B.), Clinical Biochemistry (J.D.), and Obstetrics (S.H.), James Cook University Hospital, Middlesbrough TS4 3BW, United Kingdom

Address all correspondence and requests for reprints to: Dr. Bijay Vaidya, Department of Endocrinology, Royal Devon, Exeter Hospital, Exeter EX2 5DW, United Kingdom. E-mail: bijay.vaidya{at}pms.ac.uk.

	Abstract

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Context: Maternal subclinical hypothyroidism during pregnancy is associated with various adverse outcomes. Recent consensus guidelines do not advocate universal thyroid function screening during pregnancy but recommend testing high-risk pregnant women with a personal history of thyroid or other autoimmune disorders or with a family history of thyroid disorders.

Objective: The objective of the study was to assess efficacy of the targeted high-risk case-finding approach in identifying women with thyroid dysfunction during early pregnancy.

Design/Setting: This was a single-center cohort study.

Patients/Outcome Measures: We prospectively analyzed TSH, free T₄ and free T₃ in 1560 consecutive pregnant women during their first antenatal visit (median gestation 9 wk). We tested thyroperoxidase antibodies in 1327 (85%). We classified 413 women (26.5%), who had a personal history of thyroid or other autoimmune disorders or a family history of thyroid disorders, as a high-risk group. We examined whether testing only such a high-risk group would pick up most pregnant women with thyroid dysfunction.

Results: Forty women (2.6%) had raised TSH (>4.2 mIU/liter). The prevalence of raised TSH was higher in the high-risk group [6.8 vs. 1% in the low-risk group, relative risk (RR) 6.5, 95% confidence interval (CI) 3.3–12.6, P < 0.0001]. Presence of personal history of thyroid disease (RR 12.2, 95% CI 6.8–22, P < 0.0001) or other autoimmune disorders (RR 4.8, 95% CI 1.3–18.2, P = 0.016), thyroperoxidase antibodies (RR 8.4, 95% CI 4.6–15.3, P < 0.0001), and family history of thyroid disorders (RR 3.4, 95% CI 1.8–6.2, P < 0.0001) increased the risk of raised TSH. However, 12 of 40 women with raised TSH (30%) were in the low-risk group.

Conclusion: Targeted thyroid function testing of only the high-risk group would miss about one third of pregnant women with overt/subclinical hypothyroidism.

	Introduction

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MATERNAL SUBCLINICAL hypothyroidism during early pregnancy has been shown to be associated with impaired neuropsychological development of children and several other adverse outcomes, including premature birth, preeclampsia, breech delivery, and increased fetal mortality (1, 2, 3, 4, 5). These findings have triggered a debate about whether all pregnant women should be screened for hypothyroidism. The recent consensus guidelines from an expert panel sponsored by the American Thyroid Association, the American Association of Clinical Endocrinologists, and The Endocrine Society did not advocate universal screening of thyroid function during pregnancy but recommended aggressive case finding in high-risk pregnant women who have a family or personal history of thyroid disorders, a personal history of type 1 diabetes or other autoimmune disorders, or clinical features suggestive of a thyroid disorder (6). We examined efficacy of this targeted high-risk case-finding approach in identifying women with thyroid dysfunction during early pregnancy.

	Subjects and Methods

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Subjects

Between June 2002 and July 2003, we invited pregnant women who were attending the James Cook University Hospital (Middlesbrough, UK) for their first antenatal check-up to have screening blood tests for thyroid function and thyroid antibodies. Their demographic and clinical details were collected as a part of routine antenatal care and were recorded on the local maternity database. We specifically asked the women about personal and family history (in first and second degree relatives) of thyroid disorders; personal and family history of other autoimmune diseases; and current and past treatment with antithyroid drugs, T₄, radioiodine, or thyroid surgery. Duration of gestation was calculated from last menstrual period and verified by ultrasonography.

The local research ethics committee approved the study, and all participating women gave informed written consent.

Analysis of thyroid function and thyroid antibodies

Serum concentrations of TSH, free T₄ (FT4), and free T₃ (FT3) were measured by the fully automated electrochemiluminescent immunoassay, run on the Modular E 170 analyzer (Roche Diagnostics Ltd., Lewes, UK). The between-batch coefficient of variations for TSH, FT4, and FT3 were 2.9, 4.5, and 5.5%, respectively. Thyroperoxidase antibodies (TPOAbs) were analyzed by a manual semiquantitative microtiter plate agglutination method using the Serodia kit (Fujirebio Inc., Tokyo, Japan). A reactive pattern detected at a final dilution of 1 in 1600 or greater was considered positive.

The manufacturers’ population reference ranges for TSH, FT4, and FT3 were 0.27–4.2 mIU/liter, 12–23 pmol/liter, and 4–7.8 pmol/liter, respectively. Women, who were found to have abnormal thyroid function at screening were reviewed in the joint antenatal endocrine clinic. We offered T₄ replacement or increased the dose if already on T₄ replacement for women with TSH greater than 4.2 mIU/liter.

In recent years, it has been suggested that trimester-specific reference ranges should be used for the assessment of thyroid function in pregnancy (7, 8). Therefore, we also carried out a post hoc analysis of our cohort to define first trimester-specific reference ranges for TSH, FT4, and FT3. The results of women in the first trimester (up to wk 12) who had no personal or family history of thyroid disease and who were negative for TPOAbs were used for the analysis. The TSH, FT4, and FT3 results were square root transformed to enable normal distribution, and therefore, the reference ranges were based on the squared 95% confidence intervals.

Statistical analysis

We used ² test for statistical comparisons (SPSS version 11.5; SAS Institute, Cary, NC).

	Results

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Study population

We screened thyroid function in 1560 pregnant women. TPOAbs were checked in 1327 (85%). Demographic characteristics of these pregnant women are shown in Table 1. Mean age of the women was 27 yr and median duration of gestation was 9 wk. The majority (91.4%) were whites; 4% were South Asian (Indian, Pakistani, Bangladeshi, and Sri-Lankan origin). Eighty-nine women (5.7%) reported a history of thyroid disorder; 42 hypothyroidism (overt or subclinical), 25 hyperthyroidism (overt or subclinical), six goiter/thyroid nodule, and 16 unspecified. At screening, 35 (2.2%) were on T₄ (median dose 100 µg; range 50–200 µg); 946 (60.6%) and 59 (3.8%) were taking folic acid and multivitamin tablets, respectively.

Prevalence of raised TSH

Forty women (2.6%) had raised TSH (>4.2 mIU/liter), 16 of whom also had low FT4 (<12 pmol/liter) (Table 2). Of the 16 women with raised TSH and low FT4, eight had TPOAbs (TPOAbs unknown in four). Overall, eight women had TSH greater than 10 mIU/liter, five of whom had low FT4.

We found that 7.8% of the women with normal TSH had low FT4 (Table 2).

Prevalence of fully suppressed TSH

A fully suppressed TSH level (<0.03 mIU/liter) was found in 29 women (1.9%); 11 of them also had raised FT4 and/or FT3 (Table 2). There was no significant difference in the prevalence of fully suppressed TSH between the high-risk and low-risk groups (Table 3).

Prevalence of thyroid dysfunction in women on T₄ replacement

Thirty-five women were on T₄ replacement at recruitment; eight of them (22.9%) had raised TSH (Table 2). Two women had fully suppressed TSH.

First trimester-specific reference ranges for TSH, FT4, and FT3

The post hoc analysis of our cohort defined first trimester-specific reference ranges for TSH, FT4, and FT3 as 0.09–3.03 mIU/liter, 10.6–20.4 pmol/liter, and 3.4–7.1 pmol/liter, respectively. An analysis of the data using these trimester-specific reference ranges showed 98 women (6.3%) as having raised TSH (>3.03 mIU/liter); 54 of 413 in the high-risk group (13.1%), compared with 44 of 1147 in the low-risk group (3.8%) had raised TSH (RR 3.41, 95% CI 3.16–3.67, P < 0.0001). Overall, 44 of 98 women with raised TSH (44.9%) belonged to the low-risk group. The analysis also showed that 25 of the 1560 women (1.6%) as having hypothyroxemia (normal TSH but a low FT4 level); none of these women were positive for TPOAbs.

	Discussion

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This prospective screening of thyroid function in a cohort of unselected pregnant women shows that high-risk women (with a personal or family history of thyroid disorders or a personal history of other autoimmune diseases) have more than a 6-fold increased risk of hypothyroidism (subclinical or overt) during early pregnancy. However, testing only the high-risk pregnant women, as the consensus guidelines recommend (6), would miss about one third of women with hypothyroidism. Subclinical hypothyroidism during early pregnancy is common, affecting about 2.5% pregnant women (5, 9). Therefore, with the growing evidence for an association between maternal subclinical hypothyroidism and adverse pregnancy outcomes but lack of intervention trials showing beneficial effect of T₄ in preventing these adverse outcomes, the controversy between targeted high-risk case-finding and universal screening continues (10, 11, 12). The consensus guidelines recommend the use of T₄ in pregnant women with subclinical hypothyroidism, justified on the basis of potential benefit to risk ratio (6). Our study shows that, without universal screening, a significant number of such pregnant women with thyroid dysfunction will not be picked up. Furthermore, our previous audit has highlighted the difficulty in implementing targeted screening: despite the development and circulation of local guidelines, less than 20% of the high-risk pregnant women in the district were screened for thyroid dysfunction (13).

Several factors affect thyroid function tests during various stages of pregnancy. FT4 increases with suppression of TSH in response to placental human chorionic gonadotrophin during the first trimester, whereas FT4 tends to decrease in late gestation (3, 14). This is likely to be the cause for the high prevalence of suppressed TSH in our cohort. Furthermore, increased serum thyroid-binding globulin and decreased albumin during pregnancy result in assay-dependent variations in FT4 levels (15). These observations have led to the call for using trimester- and assay-specific reference ranges for thyroid function tests in pregnancy (7, 8). If the trimester-specific reference range is used, 6.3% pregnant women in our cohort will be considered to have hypothyroidism. Whereas there will be less of a controversy to use the trimester-specific reference range in titrating the dose of T₄ in pregnant women on T₄ replacement, further studies are needed to determine the threshold level of TSH at which initiation of T₄ replacement should be considered.

There is also an uncertainty regarding the most appropriate initial screening test for thyroid dysfunction in pregnancy. The consensus guidelines recommend using TSH as the initial test (6), whereas others have stressed the importance of testing FT4 by highlighting the fact that FT4 (and FT3) is responsible for thyroid hormone action and that maternal hypothyroxinemia (normal TSH but low FT4) is associated with neuropsychological deficit in the offspring (16, 17). In our study, 7.8% (1.6% if we use the trimester specific reference ranges) of pregnant women had hypothyroxemia. The cause of maternal hypothyroxemia is not fully understood, but iodine deficiency is thought to be a major factor (18). Although urinary iodine was not analyzed in the present cohort, a previous study in this same population has shown that 7 and 40% pregnant women have urinary iodine excretion of less than 50 µg/liter (suggestive of dietary iodine deficiency) and 50–100 µg/liter (suggestive of borderline iodine deficiency) (19).

Nearly one quarter of hypothyroid women on T₄ replacement in this study had raised TSH at their first antenatal visit. Given the fact that the fetus relies entirely on maternal thyroid hormones for its development until about 13 wk gestation, it is critical to ensure adequate T₄ replacement in pregnant women during the first trimester. Hypothyroid pregnant women on T₄ require an increased dose from as early as the fifth week of gestation to maintain optimum T₄ replacement (20). Some recommend a 30% increase in the T₄ dose as soon as the pregnancy is confirmed, with further dose adjustments based on TSH measurements (20). In addition, through education of all hypothyroid women in the reproductive age, every attempt should be made to ensure an adequate T₄ replacement before a planned pregnancy.

There are several limitations of our study. First, our study was based on single thyroid function test at screening. The data on subsequent thyroid function tests during the pregnancy were not collected systematically. Second, we relied on patients’ recall in ascertaining personal and family history of thyroid and other autoimmune disorders and have not verified by reviewing case records. Finally, our cohort may not represent other populations with different ethnic mix and iodine intake.

In conclusion, this study shows that targeted thyroid function testing of only high-risk pregnant women would miss nearly one third of women with overt/subclinical hypothyroidism during early pregnancy.

	Acknowledgments

 
We gratefully acknowledge the help of Patricia Wood and Elaine McGeary in recruitment and Elaine Hall and Barbara Woodward in organizing the databases.

	Footnotes

 
This study was presented in part at the 13th International Thyroid Congress, Buenos Aires, Argentina, October 30 to November 4, 2005.

Disclosure: The authors have nothing to disclose.

First Published Online October 10, 2006

Abbreviations: CI, Confidence interval; FT3, free T₃; FT4, free T₄; RR, relative risk; TPOAb, thyroperoxidase antibody.

Received August 11, 2006.

Accepted October 4, 2006.

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