The Risk of Preeclampsia According to High Thyroid Function in Pregnancy Differs by hCG Concentration

Context: During pregnancy, there is an increased demand for thyroid hormone. The pregnancy hormone human chorionic gonadotropin (hCG) is an important physiological stimulator of thyroid function. Already high-normal maternal free T4 concentrations are associated with a higher risk of preeclampsia. Objective: The objective of the investigation was to study our hypothesis that hCG concentrations can distinguish a physiological form of high thyroid function from a more pathological form of high thyroid function and that the risk of preeclampsia would differ accordingly. Design: TSH, free T4, hCG, or thyroperoxidase antibody concentrations were determined in pregnant women participating in a population-based prospective cohort study. Setting: The study was conducted in the general community. Participants: A nonselected sample of 5146 pregnant women participated in the study. Interventions: There were no interventions. Main Outcome Measure(s): Preeclampsia was measured. Results: Women with high hCG-associated high thyroid function did not have a higher risk of preeclampsia than women with normal thyroid function. In contrast, women with low hCG and high thyroid function had a 3.4- to 11.1-fold higher risk of preeclampsia. These risk estimates were amplified in women with a high body mass index. Women with a low hCG and suppressed TSH (<0.10 mU/L) had a 3.2- to 8.9-fold higher risk of preeclampsia. hCG was not associated with preeclampsia, and results remained similar after exclusion of thyroperoxidase antibody-positive women. Conclusion: This study suggests that, in contrast to women with a high hCG associated high thyroid function, women with low hCG and high thyroid function during pregnancy are at a higher risk of developing preeclampsia. The additional measurement of hCG may therefore help to distinguish a more pathological form of high thyroid function and women at a high risk of preeclampsia.