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

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.

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.

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.

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.