(Boston)—While severe maternal hypothyroidism (low thyroid hormone levels) in pregnancy is known to increase risks of adverse pregnancy outcomes, it is unclear whether mild (subclinical) hypothyroidism causes similarly adverse pregnancy complications. It is also not clear whether maternal hypothyroidism in pregnancy increases risks of gestational diabetes.
A new study in the journal Lancet Diabetes and Endocrinology (Osinga et al "Association of gestational thyroid function and thyroid autoimmunity with gestational diabetes mellitus: a systematic review and individual-participant meta-analysis") investigated a potential association between maternal thyroid function in pregnancy and risk of gestational diabetes. The study, which used data from a large sample of patients from several different studies, found low free thyroxine (FT4) levels in pregnancy was associated with increased risk of gestational diabetes (6.5% vs 3.5% in those with normal FT4 levels). However, patients with mild subclinical hypothyroidism (high thyrotropin, or TSH, and normal FT4 level) were not at increased risk of gestational diabetes.
"Since TSH is used as the screening test for thyroid dysfunction, it is unclear whether these new findings should change the current practice of thyroid function screening in pregnancy," says Sun Young Lee MD, MSc, assistant professor of medicine at Boston University Chobanian & Avedisian School of Medicine in an accompanying commentary, (https://www.thelancet.com/journals/landia/article/PIIS2213-8587(25)00126-3/fulltext). "Currently available laboratory tests can also result in falsely low FT4 levels in pregnancy because of interference from normal changes in protein levels in pregnancy," adds Lee who also is an endocrinologist at Boston Medical Center.
Presently, routinely checking thyroid hormone levels in pregnancy is not recommended. In her commentary, Lee points out that other clinical trials have not shown that treatment of mild maternal hypothyroidism in pregnancy decreases risks of pregnancy complications. "It is unclear whether the findings of this study would advocate for universal screening and treatment of mild subclinical maternal hypothyroidism."
Lee believes further clinical trials are needed to study the potential benefit of levothyroxine treatment of low FT4 levels in pregnancy in decreasing risks of pregnancy. In the meantime, she feels more vigilant screening of gestational diabetes in patients with known thyroid disease is important.
