A study group from the Mayo Clinic found that thyroid hormone treatment of subclinical hypothyroidism during pregnancy was associated with a lower risk of pregnancy loss, but a higher risk for adverse pregnancy outcomes such as premature birth and gestational diabetes.


Subclinical hypothyroidism (hypothyroidism proven by laboratory results without clinical symptoms) affects around 15% of pregnant women. The recent regulatory changes in laboratory TSH (thyroid stimulating hormone – an indicator of low thyroid hormone levels) threshold may have resulted in an overdiagnosis and overtreatment of hypothyroidism. A lower thyroid hormone level during pregnancy has been associated with adverse pregnancy outcomes such as pregnancy loss, placental abruption, premature rupture of membranes, and neonatal death.

An article was recently published in the British Medical Journal that analyzed data from the OptumLabs Data Warehouse database to assess the prevalence, effectiveness, and safety of thyroid hormone treatment of subclinical hypothyroidism among pregnant women. Researchers included 5405 pregnant women in the study with TSH 2.5-10mIU/L between 2010 and 2014. 15.6% received thyroid hormone treatment and the percentage of treated women increased each year. Comparing the treated and untreated groups, treated patients had a 38% lower risk of pregnancy loss, but higher odds of preterm delivery, gestational diabetes, and pre-eclampsia. However, the risk of pregnancy loss was only lower in treated women with higher TSH levels (meaning lower thyroid hormone levels) but not in women with lower TSH. Furthermore, the risk of gestational hypertension was higher among treated than untreated women with lower TSH levels.

In conclusion, it seems that thyroid hormone therapy of subclinical hypothyroidism lowers the risk of pregnancy loss, especially in patients with higher TSH levels, but is associated with a higher risk of adverse pregnancy outcomes. Further studies are needed to assess the safety of thyroid hormone treatment in pregnancy.


Written By: Dr. Fanni R. Eros

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