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 T 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 T, 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.