In the ‘largest analysis to date’, researchers have determined that subclinical hypothyroidism is associated with preterm birth but have stopped short of recommending screening.

It is well-known that overt thyroid disease in pregnancy is linked with preterm birth, as defined as delivery before 37 weeks. But whether milder forms of thyroid disease make a difference has been controversial.

Consequently, research published in JAMA looked at 19 prospective cohort studies that had investigated this issue. Over 47,000 pregnant women were included. What the reviewers found was that three types of mild thyroid abnormality were linked with prematurity. These were isolated elevated TSH (which increased the risk of 29% compared with euthyroid women), isolated decreased thyroxine levels (which increased the risk by 46%),  and women who were euthyroid but had thyroid antibodies (TPO positive) who were 33% more likely to have a preterm birth.