Thyroid stimulating hormone (TSH) ≥2.5mU/l in early pregnancy: Prevalence and subsequent outcomes

Objective: There remains controversy over how women with abnormal thyroid function tests in pregnancy should be classified. In this study we assessed the proportion of women with thyroid stimulating hormone (TSH)≥2.5mU/l in a large obstetric cohort, and examined how many have gone on to develop thyroid disease in the years since their pregnancy.

Study design: 4643 women were recruited and samples taken in early pregnancy between 2007 and 2010. Thyroid function tests were analysed in 2014; in women with raised TSH computerised health records and prescription databases were used to identify thyroid disease detected since pregnancy.

Results: 58 women (1.5%) had a TSH over 5mU/l and 396 women (10.3%) had TSH between 2.5 and 5mU/l. Women with TSH>5mU/l delivered infants of lower birthweight than those with TSH<2.5mU/l; there were no other differences in obstetric outcomes between the groups. Of those who have had thyroid tests since their pregnancy, 78% of those with TSH>5mU/l and 19% of those with TSH between 2.5 and 5mU/l have gone on to be diagnosed with thyroid disease.

Conclusions: Using a TSH cut-off of 2.5mU/l in keeping with European and US guidelines means that over 12% of women in this cohort would be classified as having subclinical hypothyroidism. Treatment and monitoring of these women would have major implications for planning of obstetric services.