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Treating mild subclinical hypothyroidism in pregnancy: risks mostly outweigh benefits

By Bianca Nogrady
The risks may outweigh the benefits of treating mild subclinical hypothyroidism in most pregnant women, new research suggests.

A retrospective cohort study published in The BMJ examined the effect of thyroid hormone treatment in 5405 pregnant women with subclinical hypothyroidism – defined as an untreated thyroid-stimulating hormone (TSH) level between 2.5 and 10 mIU/L – 843 of whom were treated with thyroid hormone and 4562 were not. Compared with the untreated group, women who received thyroid hormone had a 38% lower risk of pregnancy loss, but a 60% greater risk of preterm delivery, 37% greater risk of gestational diabetes and 61% greater risk of pre­eclampsia. 

A subgroup analysis based on TSH concentration found the adjusted odds of pregnancy loss to be significantly lower in treated women if their TSH level was 4.1 to 10 mIU/L, but not significantly reduced in women with a pretreatment TSH level between 2.5 and 4.0 mIU/L. The risk of gestational hypertension was also significantly higher in treated women than in untreated women with a pre­ treatment TSH level of 2.5 to 4.0 mIU/L.

The researchers noted that their finding that the benefit of thyroid hormone use on pregnancy loss was observed only in women with pretreatment TSH levels of 4.1 to 10.0 mIU/L raises questions about the current guideline recommended threshold of 2.5 mIU/L for treating subclinical hypothyroidism when population reference ranges are unavailable.

Commenting on the study, endocrinologist Professor John Walsh said guidelines had previously recommended a TSH cut-off of 2.5 mIU/L for considering thyroid hormone treatment in pregnant women, although more recent guidelines have advocated locally derived TSH reference ranges, or a cut-off of 4.0 mIU/L if these are not available.

However, he told Endocrinology Today that this study suggests there may be no benefit for thyroxine treatment in women with TSH levels between 2.5 and 4.0 mIU/L.

‘That suggests we’ve been overtreating women and giving thyroid hormone unnecessarily,’ said Professor Walsh, Clinical Professor at Sir Charles Gairdner Hospital, WA.

Professor Walsh said there was one exception to this, namely a woman with a TSH level of 2.5 to 4.0 mIU/L who also has positive thyroid antibodies.

‘She may well have mild thyroid dysfunc­tion and here there is a case for treating because TSH may increase during pregnancy, so thyroxine may prevent it getting above 4.0 mIU/L, which is where harm lies.’
BMJ 2017; 356: i6865.

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