There have been a few studies regarding treating pregnant women with thyroid hormone replacement for subclinical hypothyroidism, and most of the results showed that there are multiple adverse outcomes associated with treatment. A recent study funded by the Mayo Clinic and published in BMJ was conducted over a 4-year period to elucidate the safety and efficacy of treating subclinical hypothyroidism in pregnancy.  

This retrospective cohort study consisting of 5,405 pregnant women between the ages of 18 to 55 years used data from a U.S. administrative database from January 2010 to December 2014. Approximately 4,562 women were not treated with thyroid hormone–replacement therapy, while 832 women (16%) were treated with levothyroxine. The remaining patients were treated with a thyroid-extract formulation or a combination of levothyroxine and liothyronine. The results indicate that pregnancy loss was less common in women who received treatment compared with those not treated (odds ratio 0.62, 95% CI 0.48-0.82). Women who were treated also had higher odds of preeclampsia (1.61, 1.10-2.37), preterm delivery (1.60, 1.14-2.24), and gestational diabetes (1.37, 1.05-1.79).

The researchers concluded that “Use of thyroid hormone was associated with decreased risk of pregnancy loss, but it was also associated with increased risk of preterm delivery, gestational diabetes, and preeclampsia; however, the benefit of thyroid hormone use on pregnancy loss was observed only among women with pretreatment thyroid-stimulating hormone (TSH) concentrations of 4.1-10.0 mIU/L, not those with concentrations of 2.5-4.0 mIU/L.” Based on this finding, the American Thyroid Association recently revised its guidelines to no longer treat thyroid peroxidase–negative women unless their serum TSH level is above 4.0 mIU/L.

This study contributes greater understanding of the benefits and risks of treating subclinical hypothyroidism in pregnancy. Further randomized, controlled, clinical studies are needed to confirm the observations from this study. In conclusion, before treating a pregnant woman for subclinical hypothyroidism, healthcare practitioners should consider the benefits versus the risks and use a patient-specific approach, monitoring the patient closely throughout the pregnancy for adverse events.  

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