Maternal Thyroid Function at 11 to 13 Weeks of Gestation and Subsequent Fetal Death

Introduction

C linical hypothyroidism is associated with subfertility, and in those women who conceive there is a substantial risk of miscarriage (1,2). A study examining the outcome of pregnancies in women with primary hypothyroidism reported that the rate of miscarriage in 16 cases with overt hypothyroidism was 60%, whereas in 99 women made euthyroid with adequate thyroxine treatment there were no miscarriages (1). In another study of 29 pregnant women with overt hypothyroidism, the rate of fetal death during the second and third trimesters was nine times higher than in 552 euthyroid women (3).

In subclinical hypothyroidism there is contradictory evidence as to whether the rate of fetal death is increased or not. A prospective study of 63 pregnant women with hypothyroidism on thyroxine replacement reported that the rate of fetal loss at 7–20 weeks was significantly higher in women with thyroid-stimulating hormone (TSH) above 4 mU/L than in those with TSH at or below 4 mU/L (26.3% vs. 6.3%) (4). Another study measured TSH in sera obtained from women at 15–18 weeks as part of routine screening for neural tube and chromosomal defects and reported that the rate of fetal death between the time of enrollment and term was significantly higher in women with TSH at or above 6 mU/L than in those with TSH below 6 mU/L (3.8% vs. 0.9%) (5). Two other studies reported a lack of association between subclinical hypothyroidism and fetal death. Casey et al. assessed thyroid function in all pregnant women attending for routine care in their hospital at or before 20 weeks of gestation (6). The rate of fetal death between enrolment and delivery was 0.5% in both those with TSH at or above the 97.5th percentile and free thyroxine (FT4) within the normal range and in those with TSH values between the 5th and 95th percentiles. Cleary-Goldman et al. assessed thyroid function in women undergoing screening for chromosomal defects at 11–13 weeks (7). There were no significant differences in the rates of miscarriage before 24 weeks and perinatal death between women with serum TSH and FT4 between the 2.5th and 97.5th percentiles and those TSH at or above the 97.5th percentile and FT4 in the normal range (0.4% vs. 0.6% and 0% vs. 0.3%, respectively). Possible explanations for the contradictory results of the above studies are, first, the small number of fetal losses in the cases with thyroid deficiency, with only a total of 16 such losses in the four studies (4 –7), and, second, the timing of thyroid screening in the late first trimester or the second trimester when the majority of pregnancy losses had already occurred.

There is also controversy as to whether any possible association between subclinical hypothyroidism and fetal death is the direct consequence of the metabolic derangement or if it is mediated by the coexistence of antithyroid antibodies (8 –10). Studies in the first trimester reported that the risk of early miscarriage was 2–4 times higher in antithyroid antibody-positive women than in antibody-negative women (8). Antithyroid antibodies may exert a direct adverse effect on the pregnancy, they may serve as a marker for other autoimmune conditions that cause fetal death, women with thyroid autoimmunity may be euthyroid before pregnancy but develop subclinical or overt hypothyroidism during the first trimester, or such women suffer subfertility and become pregnant at an older age, which in itself is associated with increased risk of fetal loss.

The aim of our study was to investigate further the possible association between maternal thyroid dysfunction and fetal death in the second and third trimesters by comparing serum TSH, FT4, free triiodothyronine (FT3), and antithyroid antibody levels at 11–13 weeks of gestation in pregnancies ending in miscarriage or fetal death with those resulting in normal live births.

Materials and Methods

This was a prospective screening study for adverse obstetric outcomes in women attending for their routine first hospital visit in pregnancy. In this visit, which is held at 11⁺⁰ − 13⁺⁶ weeks of gestation, we record maternal characteristics, including age, ethnic origin (white, black, South Asian, East Asian, and mixed), cigarette smoking during pregnancy (yes or no), method of conception (spontaneous or assisted), medical history of chronic hypertension, parity (parous or nulliparous if no delivery beyond 23 weeks), weight, height, and body mass index (BMI). We then perform ultrasonography to confirm gestational age from the measurement of the fetal crown-rump length, to diagnose any major fetal abnormalities, and to measure fetal nuchal translucency thickness. We also measure maternal serum free beta human chorionic gonadotropin (β-hCG) and pregnancy associated plasma protein A (PAPP-A) (DELFIA EXPRESS analyzer; PerkinElmer) as part of screening for chromosomal abnormalities by a combination of fetal nuchal translucency thickness and serum biochemistry (11,12). Additionally, blood is collected for research and the separated plasma and serum are stored at −80°C for subsequent biochemical analysis. Written informed consent was obtained from the women agreeing to participate in the study, which was approved by King's College Hospital Ethics Committee.

In this study we retrospectively measured the maternal serum concentrations of FT3, FT4, TSH, anti-thyroperoxidase (anti-TPO), and anti-thyroglobulin (anti-Tg) at 11–13 weeks in 202 singleton pregnancies that subsequently resulted in miscarriage or fetal death (fetal loss group). The values were compared to the results of our previous study in 4318 singleton pregnancies with no history of thyroid disease, which did not develop pre-eclampsia and resulted in live birth after 34 weeks of phenotypically normal neonates with birth weight above the 5th centile (13). In that study there were 726 (16.8%) pregnancies in which the concentration of one or both antithyroid antibodies was 60 U/mL or more. Normal ranges for TSH, FT3, and FT4 were derived from the study of the 3592 pregnancies with no antithyroid antibodies (13).

The maternal serum concentrations of FT3, FT4, TSH, anti-TPO, and anti-Tg were measured by immunoassay using direct chemiluminometric technology (Siemens Advia Centaur assays; Siemens Healthcare Diagnostics Ltd.). The minimum detectable concentrations of FT3, FT4, TSH, anti-TPO, and anti-Tg were 0.3 pmol/L, 1.3 pmol/L, 0.003 mIU/L, 15 U/mL, and 30 U/mL, respectively. The inta-assay coefficients of variation were 3.08%, 2.35%, and 2.47% at FT3 concentrations of 2.9, 6.6, and 14.2 pmol/L, respectively; 4.69%, 2.31%, and 2.22% at FT4 concentrations of 6.1, 13.9, and 39.9 pmol/L, respectively; 2.48%, 2.44%, and 2.41% at TSH concentrations of 0.74, 5.65, and 18.98 mIU/L, respectively; 7.93%, 4.54%, and 6.26% at anti-TPO concentrations of 1.70, 10.01, 14.95 U/mL, respectively; and 5.5% and 2.9% at anti-Tg concentrations of 62 and 333 U/mL, respectively. If the serum concentration of anti-TPO and anti-Tg was <60 U/mL, which was the manufacturer's reference limit, the patients were considered to be antibody negative.

Results

The patients reported in this study were examined between December 2005 and May 2006. The gestational age distribution at the time of miscarriage or the diagnosis of fetal death in the fetal loss group is shown in Figure 1. The patient characteristics of the fetal loss and unaffected groups are compared in Table 1. In the fetal loss group, compared to the unaffected group, the median BMI was higher and there was a higher prevalence of black women, and women who conceived after receiving ovulation induction drugs.

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FIG. 1.

Gestational age distribution of miscarriage or fetal death.

In the fetal loss group, compared to the unaffected group, the median TSH MoM was increased, and the median FT3 MoM and FT4 MoM were decreased (Table 2). Linear regression analysis in the fetal loss group showed that there was no significant association between the gestation at fetal loss and TSH MoM (p = 0.654), FT3 MoM (p = 0.411), and FT4 MoM (p = 0.917). In the fetal loss group serum TSH was above the 97.5th centile of the normal range in 12 (5.9%) cases and the serum FT4 was below the 2.5th centile in 10 (5%) of cases. In 5 of the 10 cases with low FT4 serum TSH was high.

Multiple logistic regression analysis demonstrated that in the prediction of fetal loss there were significant contributions from black ethnic origin (odds ratio [OR] 4.102, 95% confidence interval [95% CI] 3.003–5.603, p < 0.001), use of ovulation drugs (OR 8.238, 95% CI 5.210–13.028, p < 0.001), BMI (OR 1.028, 95% CI 1.000–1.057, p = 0.05), and log FT4 MoM (OR 0.011, 95% CI 0.001–0.104, p < 0.001), but not TSH MoM (p = 0.208). If in the regression FT4 MoM is not included, then TSH MoM becomes significant. This is presumably the consequence of the good correlation between FT4 MoM and TSH MoM. The associations between TSH and FT3, TSH and FT4, and FT3 and FT4 in both the fetal loss and unaffected groups are shown in Table 3.

Discussion

This study has demonstrated that in pregnancies resulting in miscarriage or fetal death during the second and third trimesters, compared to those with normal outcome, the median maternal serum concentration of TSH is increased, FT4 is decreased, and the prevalence of high TSH and low FT4 is increased. In contrast, there were no significant differences between the groups in the median concentration of FT3 or in the prevalence of antithyroid antibody positivity.

In the fetal loss group, compared to the normal outcome group, more women were of black ethnic origin, the median maternal BMI was higher, and more pregnancies were conceived after ovulation induction. These findings are compatible with the results of previous studies on the rates of second trimester miscarriage and fetal death. Willinger et al. examined the stillbirth hazard in 5,138,122 singleton pregnancies from the National Center of Health Statistics and reported that in black, compared to white women, the risk of fetal death at 20–23 weeks was 2.75 times higher and the risk of death at 39–40 weeks was 1.57 times higher (15). Obesity is associated with an increased risk of several adverse outcomes. A systematic review reported that the rate of pregnancy loss before 20 weeks of gestation increases with maternal BMI (16). Similarly, a population-based cohort study demonstrated that the rate of late fetal death increases with maternal BMI (17). There is a scarcity of reports on the outcome of pregnancies conceived after the use of ovulation induction drugs without in vitro fertilization. Whether pregnancies conceived through assisted reproductive technology (ART) are at an increased risk of loss is inconclusive, and data on maternal age-, ART type-, and gestational age-specific risk of loss are limited. Farr et al. examined the outcome of 148,494 ART pregnancies and reported that the overall risk of pregnancy loss was 29% and the risk in both groups increased with maternal age. The risk of pregnancy loss in singletons after confirmation of a fetal heartbeat was about 15%, which is slightly higher than in naturally conceived pregnancies. The risk of fetal loss was 3.4% after 12 weeks of gestation and 1.2% after 20 weeks (18).

In our study the incidence of high TSH and/or low FT4 in the fetal loss group was higher than in the normal outcome group. The contradictory results of previous reports concerning the association between subclinical hypothyroidism and fetal death may be a consequence of the small number of cases of fetal loss in the hypothyroid group and methodological differences between the studies. The two studies reporting an increase in fetal loss in women with high TSH did not provide data on FT4, which may have been normal or decreased (4,5). In contrast, the studies reporting no significant difference in fetal loss between the high and low TSH groups included only women with normal FT4 (6,7).

The incidence of antithyroid antibody positivity for either anti-TPO or anti-Tg in the fetal loss group was not higher than in the normal outcome group. We have previously reported that in the antibody-positive group, compared to the antibody-negative group, the median TSH was higher and the median FT3 and FT4 were lower, and this effect was observed for both anti-TPO and anti-Tg antibodies (13). Our findings do not support the hypothesis that antithyroid antibodies exert a direct toxic effect on the pregnancy leading to fetal loss; however, the mechanism of fetal loss in the first trimester may be different from fetal losses in the second and third trimester. It is likely that the previously reported association between antithyroid autoimmunity and fetal loss (8) may be mediated by an underlying thyroid dysfunction. A study of euthyroid patients undergoing ART reported that the pregnancy and delivery rates were not different in the anti-TPO antibody-positive and antibody-negative groups (9). However, the antibody-positive women who failed to become pregnant or miscarried had higher TSH levels before ART than in those with a normal outcome. Another study examined whether anti-TPO antibody-positive patients have an increased risk of miscarriage and if this can be reduced by levothyroxine treatment (10). They reported that the miscarriage rate in anti-TPO antibody-positive women with no treatment (13.8%) was significantly higher than in antibody-negative women (3.5%) or in antibody-positive women treated with levothyroxine starting from the first trimester (2.4%) (10).

The strengths of our study are, first, examination of a large number of pregnancies resulting in miscarriage or fetal death; second, assessment of confounding factors in the prediction of fetal loss, including maternal characteristics and method of conception; third, use of normal ranges of thyroid function corrected for maternal characteristics, including age, ethnic origin, and BMI (13); and, fourth, assessment of thyroid function in the first trimester of pregnancy providing the option for therapeutic interventions in future studies to determine if the incidence of fetal loss can be reduced. A retrospective study of pregnancies in women with primary hypothyroidism treated with levothyroxine reported that the miscarriage rate in those that at the time of conception had overt or subclinical hypothyroidism was about 65%, whereas in the euthyroid group there were no miscarriages (1). The extent to which the fetal loss rate during the second and third trimesters can be reduced by the treatment of women found to have hypothyroidism at 11–13 weeks remains to be determined. The limitation of this study is that it does not include early miscarriages most of which occur before 11 weeks.

There are multiple causes of miscarriage and fetal death during the second and third trimesters of pregnancy. This study has demonstrated that previously undiagnosed hypothyroidism diagnosed at 11–13 weeks of gestation may be a contributing factor to about 5% of subsequent fetal losses. The extent to which the diagnosis of subclinical hypothyroidism and appropriate therapy can prevent fetal loss and the cost effectiveness of such strategy remain to be determined.