Association of Overt and Subclinical Hyperthyroidism During Weeks 4-8 with Adverse Pregnancy Outcomes

Although increasing data suggest that hyperthyroidism is associated with adverse pregnancy outcomes, there are only a few reports with different conclusions on whether the mild transient reduction in thyrotropin (TSH) with or without free thyroxine (FT4) elevation during the early stage of pregnancy also causes adverse pregnancy outcomes. We analyzed data from 3,783 women in this study from August 2011 to December 2013. Participants completed a questionnaire survey. Samples of blood were obtained in the 4th-8th week of pregnancy and their TSH, FT4, thyroid peroxidase antibody, and thyroglobulin antibody were measured. We divided the participants into overt hyperthyroidism group (OHG), subclinical hyperthyroidism group (SHG), and control group based on their blood results and followed up on their pregnancy outcomes. (1) The serum level of FT4 in the SHG was much higher than the control group (p < 0.05). No difference was found in the TSH between the OHG and SHG. The positive rate of autoimmune thyroid antibodies in the OHG (25.6%) was significantly higher than that in the SHG (14.2%) and control group (13.9%) (p < 0.05), whereas there was no difference between the SHG and control group. (2) The SHG had a lower incidence of miscarriage (1.7% vs. 7.2%; OR = 0.218, p = 0.016) than the control group, and the OHG had a higher incidence of placenta previa (3.3% vs. 0.8%; OR = 4.366, p = 0.039) than the control group. (3). We used a binary logistic regression to take other factors into consideration and found that subclinical hyperthyroidism was associated with a lower risk of abortion (OR = 0.206; 95% CI = 0.050-0.840; p = 0.028) but higher risk of preeclampsia (OR = 5.143; 95% CI = 1.463-18.076; p = 0.011) and placental abruption (OR = 4.676; 95% CI = 1.017-21.509; p = 0.048), and overt hyperthyroidism may increase the incidence of placenta previa (OR = 4.193; 95% CI = 1.222-14.382; p = 0.023). Subclinical hyperthyroidism during weeks 4-8 of pregnancy may be associated with the decreased incidence of abortion but might be a risk factor for preeclampsia and placental abruption. Meanwhile, pregnancy with overt hyperthyroidism may be an independent risk factor for placenta previa.