Hypothyroidism Doesn't Always Call for Treatment in Pregnant Women

Hypothyroidism, or underactive thyroid, typically calls for hormone therapy. But researchers from the Mayo Clinic are questioning if that should be the case for pregnant women.

One hormonal change that takes place naturally when a woman becomes pregnant is in the level of thyroid-stimulating hormone (TSH). While pregnant women usually have a TSH level ranging from 0.4 to 4.0 milli-international units per liter, some international guidelines say that that level should not go above 2.5 to 3.0 milli-international units per liter. Therefore, when the TSH level goes beyond that recommended limit, it’s classified as subclinical hypothyroidism, or mildly underactive thyroid.

The Mayo Clinic researchers said that when a pregnant woman is left untreated for subclinical hypothyroidism, it could cause pregnancy loss, placental abruption, premature rupture of membranes, and neonatal death. However, that does not mean that every woman with the condition should be treated.

“It seemed likely that treating subclinical hypothyroidism would reduce the chance of these deadly occurrences. But we know that treatment brings other risks, so we wanted to find the point at which benefits outweighed risks,” lead author, Spyridoula Maraka, MD, an endocrinologist, said in a news release.

Using data from 5,405 pregnant women with subclinical hypothyroidism, the team set out to identify at what point treatment should be implemented. A total of 843 pregnant women had an average TSH level of 4.8 milli-international units per liter, and were then put on the thyroid hormone drug levothyroxine. The other 4,562 had an average TSH level of 3.3 milli-international units per liter and were not being treated.

The women in the treated group were 38% less likely to have a miscarriage than the untreated women. However, treated women with TSH levels from 2.5 to 4.0 milli-international units per liter had significantly higher risk of gestational hypertension, which could lead to preeclampsia. The treated women were also more likely to have preterm delivery and gestational diabetes.

No adverse outcomes were reported with different pretreatment TSH levels and subsequent thyroid use.

“Unsurprisingly, we found that women with higher levels of pre-treatment TSH—between 4.1 and 10 milli-international units per liter—benefitted most from treatment,” Maraka continued. “This group’s much lower likelihood of experiencing pregnancy loss was what brought the average down — and creates a good argument for updated clinical guidelines.” Results from women with these higher TSH levels were similar to untreated women.

But are there cases in which pregnant women with subclinical hypothyroidism should not be treated? Overtreatment may be possible in this group, said Juan Brito Campana, MBBS, an endocrinologist at Mayo Clinic. It might be best to leave the condition untreated if TSH levels fall from 2.5 to 4.0 milli-international units per liter.

The American Thyroid Association updated guidelines for treating thyroid disease in pregnant women on January 6, which mirrored results from the study.

The news release was provided by the Mayo Clinic.

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