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Experts query treatment of mild hypothyroidism in pregnancy

It cuts pregnancy loss but is linked to preterm delivery, gestational diabetes and pre-eclampsia

Louise Prime

Thursday, 26 January 2017

Pregnant women who take hormones to treat a mildly underactive thyroid have in some cases a lower risk of pregnancy loss – but they are also more likely to suffer complications including preterm delivery, gestational diabetes and pre-eclampsia, warned experts in The BMJ* this morning. They called for further investigation into the safety of thyroid hormone treatment in these women.

An international team of researchers, led from the Mayo Clinic in the US, explained that changes in thyroid physiology during pregnancy result in the normal range of thyroid stimulating hormone (TSH) being lower than among non-pregnant adults. They pointed out that using current diagnostic criteria (a fixed upper threshold for TSH concentration of 2.5 mIU/l during the first trimester and 3.0 mIU/l in the second and third), about one in seven pregnancies are thought to be affected by subclinical hypothyroidism, compared with 2-3% before these criteria were established, “raising the possibility of overdiagnosis”. Furthermore, they added, there has been insufficient evidence on whether thyroid hormone treatment improves pregnancy outcomes.

To estimate the potential effectiveness and safety of thyroid hormone treatment among pregnant women with subclinical hypothyroidism, they analysed data on 5,405 pregnant women who all had subclinical hypothyroidism, defined as untreated TSH concentration 2.5-10mIU/l, taken from a large US administrative database covering 1 January 2010 to 31 December 2014. They compared rates of pregnancy loss and other maternal and foetal pregnancy-related adverse outcomes among the 16% of participants treated with thyroid hormone, with rates among the untreated women.

They reported that overall, compared with the untreated group, treated women were significantly less likely to suffer pregnancy loss (adjusted odds ratio 0.62); but they also had higher odds of preterm delivery (1.60), gestational diabetes (1.37), and pre-eclampsia (1.61). Other pregnancy-related adverse outcomes were similar between the two groups.

They also found that the benefit of hormone treatment, in terms of cutting the risk of pregnancy loss, depended on women’s initial TSH levels. They calculated that pregnancy loss was significantly likely in treated women compared with untreated women (odds ratio 0.45) if their pre-treatment TSH concentration was 4.1-10mIU/l – but treatment was not associated with a significantly lower risk of pregnancy loss among women whose initial TSH level was 2.5-4.0 mIU/l.

They commented: “On the basis of our findings, continuing to offer thyroid hormone treatment to decrease the risk of pregnancy loss in pregnant women with TSH concentrations of 4.1-10.0 mIU/l is reasonable.”

But they added: “Owing to the smaller magnitude of effect in the group with TSH concentrations of 2.5-4.0 mIU/l, and in light of the possible increased risk of other adverse events, treatment may need to be withheld in this group and guidelines may need to be revised.”

* Maraka S, Mwangi R, McCoy RG, et al. Thyroid hormone treatment among pregnant women with subclinical hypothyroidism: US national assessment. BMJ 2017;356:i6865. 

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