Background: Studies have shown that overt hypothyroidism is associated with a substantial risk of miscarriage. There is controversy as to whether subclinical hypothyroidism has the same effect and whether such effect is mediated by the presence of antithyroid antibodies. Our hypothesis is that maternal thyroid function in the first trimester is altered in pregnancies ending in miscarriage or fetal death.
Methods: Thyroid-stimulating hormone (TSH), free thyroxine (FT4), free triiodothyronine, anti-thyroperoxidase antibody, and anti-thyroglobulin antibody at 11-13 weeks of gestation were measured in 202 singleton pregnancies that subsequently resulted in miscarriage or fetal death, and the values were compared with the results of 4318 normal pregnancies.
Results: In the fetal loss group, compared to the unaffected group, there was an increase in median TSH multiple of the normal median (1.133 vs. 1.007 MoM), decrease in median FT4 MoM (0.958 vs. 0.992 MoM), and increase in the incidence of TSH above the 97.5th centile (5.9% vs. 2.5%) and FT4 below the 2.5th centile (5.0% vs. 2.5%). Logistic regression analysis demonstrated that in the prediction of fetal loss there were significant contributions from FT4 MoM, maternal black ethnic origin, history of chronic hypertension, and use of ovulation drugs. The prevalence of antithyroid antibody positivity was not significantly different in the fetal loss group compared to that of normal pregnancies (15.3% vs. 16.8%).
Conclusions: Impaired thyroid function may predispose to miscarriage and fetal death.