Endocrine Abstracts

Background: Subclinical hypothyroidism is common in pregnancy affecting about 5% of all pregnant women, and is associated with adverse pregnancy outcomes. There is a general consensus that subclinical hypothyroidism detected during pregnancy should be treated with L-thyroxine (Stagnaro-green et al. 2011, DeGroot et al. 2012). However, it is unclear whether the treatment should be limited only during the pregnancy or continued long-term. Therefore, we aimed to study whether subclinical hypothyroidism detected during pregnancy is reversible after pregnancy.

Subjects and methods: We analysed TSH, free T₄, free T₃ and thyroid peroxidase antibodies (TPO-Ab) on stored serum samples from 988 women at 28 weeks pregnancy. We carried out same tests on 523 of these women, who had no known thyroid disease or overt hypo- or hyperthyroidism during the pregnancy, on a visit mean (S.D.) 4.9 (1.6) years after delivery.

Results: Subclinical hypothyroidism in pregnancy (defined as TSH >3 mU/l for third trimester; Stagnaro-Green et al. 2011, DeGroot et al. 2012) was present in 65/523 (12.4%) of women. Of these, 48 (74%) women had normal thyroid function post-pregnancy; only 14 (21.5%) had persistently high TSH (defined as TSH >4.5 mU/l post-pregnancy) and a further 3 (4.6%) were on L-thyroxine. Those with TPO-Ab (P<0.001) or TSH above 5 mIU/l (P=0.03) in pregnancy were more likely to have persistently elevated TSH or be on L-thyroxine replacement outside pregnancy. 45/523 (8.6%) of patients had isolated maternal hypothyroxinaemia in pregnancy (defined as free T₄ below 10th centile without raised TSH). Only 2 (4.4%) of them had raised TSH outside pregnancy. The proportion of women with positive TPO-Ab more than doubled post-pregnancy compared to that in pregnancy (12 vs 5%).

Conclusions: The majority of cases of subclinical hypothyroidism in pregnancy are transient, so treatment of L-thyroxine in these cases may not be warranted outside of pregnancy.