Low thyroid-stimulating hormone and its persistence beyond the first trimester of pregnancy

To investigate predictors of low thyroid-stimulating hormone (TSH) persisting beyond the first trimester and associated pregnancy outcomes.

We retrospectively assessed the association between low first-trimester TSH and obstetric outcomes of singleton pregnancies in southern Israel between 2001 and 2011. We included women with first-trimester TSH and at least one other THS measurement. Clinical data were from maternity and community medical records.

Among 3761 women, 185 (4.9%) had a TSH of 0.10 mIU/L or less in the first trimester. Multiple of the median human chorionic gonadotropin was higher in women with TSH of 0.40 mIU/L or less versus 0.41-4.0 mIU/L, but was not associated with higher rates of persistent low TSH. Maternal age (odds ratio [OR] 1.08, 95% confidence interval [CI] 1.00-1.16; per yearly increment), free thyroxine (FT4; OR 1.11, 95% CI 1.03-1.20; per ng/dL increment), and TSH of 0.10 mIU/L or less (OR 3.06, 95% CI 1.38-6.80 vs 0.21-0.40 mIU/L) were independent predictors of persistent low TSH. No adverse pregnancy outcomes occurred in women with low first-trimester TSH.

Low TSH persisting beyond the first trimester was more common with increasing maternal age, elevated FT4, and TSH below 0.10 mIU/L, but was not associated with obstetric complications.