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Free thyroxine levels higher in women treated for hypothyroidism

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August 4, 2015

Pregnant women undergoing treatment for hypothyroidism have higher levels of free thyroxine compared with pregnant women without hypothyroidism, according to recent study findings published in Thyroid.

“Findings from this study call attention to a possible dynamic interaction between thyroid gland output and maternal weight-related increases in peripheral deiodinase activity,” James E. Haddow, MD, of Women and Infants Hospital in Providence, RI, told Endocrine Today. “There are no implications for alterations in current management of hypothyroidism during pregnancy.”

Haddow and colleagues evaluated data from the First and Second Trimester Evaluation of Risk for Fetal aneuploidy (FaSTER) trial on pregnant women without hypothyroidism (controls; n = 9,267) and pregnant women treated for hypothyroidism (n = 306) with thyroid-stimulating hormone levels between the second and 98th reference percentiles. Researchers sought to determine whether free thyroxine levels are higher among pregnant women treated for hypothyroidism and if these levels are linked to weight.

James E. Haddow

James E. Haddow

The researchers compared free thyroxine levels between the groups at 11 to 14 and 15 to 18 weeks’ gestation and found that participants treated for hypothyroidism had higher median free thyroxine levels and percent of free thyroxine values at or above the 95th reference percentile compared with controls (P < .001). With increasing weight, median free thyroxine levels decreased monotonically; this was regardless of anti-thyroperoxidase antibody status.

Predictors of free thyroxine levels were identified as maternal age, maternal weight and treatment status.

“In conclusion, this study confirms that [free thyroxine] measurements are shifted upward among pregnant women with normal TSH who are being treated for hypothyroidism, consistent with reported findings in nonpregnant, hypothyroid adults,” the researchers wrote. “A new finding is our documentation of an inverse relationship between [free thyroxine] levels and weight during early pregnancy among the treated hypothyroid women, a pattern that has previously been noted only among reference women at the same time in pregnancy. Relatively lower [free thyroxine] levels among obese women being treated for hypothyroidism are unlikely to be explained by lower thyroid gland output. A more likely explanation might be that peripheral deiodinase activity is increased in the presence of higher weight.” – by Amber Cox

Disclosure: The researchers report no relevant financial disclosures.