Subclinical hypothyroidism is an early and mild form of hypothyroidism. In general, thyroid hormone treatment is routinely used to treat subclinical hypothyroidism, but its efficacy and safety in pregnant women remain controversial. This study aims to evaluate the safety and efficacy of thyroid hormone treatment among women with subclinical hypothyroidism.

This retrospective cohort study included a total of 5,405 pregnant women with subclinical hypothyroidism, with untreated thyroid-stimulating hormone (TSH) concentration 2.5-10 mIU/L. The exposure to thyroid hormone therapy was evaluated, and the primary outcome of the study was pregnancy loss, along with other pre-specified pregnancy-related adverse outcomes.

Of 5,405 pregnant women with subclinical hypothyroidism, 843 (mean TSH concentration 4.8 mIU/L) were treated with thyroid hormone therapy, and the remaining 4,562 women with a mean baseline TSH concentration (3.3 mIU/L) were not treated. The findings suggested that pregnancy loss was less common among women treated with thyroid hormone therapy (10.6%), as compared with untreated women (13.6%). When compared with untreated women, treated women had lower odds of pregnancy-related outcomes, like preeclampsia and gestational diabetes.

The research concluded that thyroid hormone treatment was associated with a reduced risk of pregnancy-related adverse outcomes in women with subclinical hypothyroidism.