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Maternal thyroid hormone levels associated with child IQ

Issue: April 2015

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SAN DIEGO — Over the full range, maternal free thyroxine levels during early pregnancy appear to be associated with a decrease in childhood IQ levels, according to research presented here.

“There is consensus to treat subclinical hypothyroidism because it is generally believed that the potential benefits of treatment outweigh the potential risks of overtreatment,” Tim I. M. Korevaar, MD, a PhD student at Erasmus Medical Center in Rotterdam, Netherlands, said in a press release. “There was virtually no evidence in humans until now that mildly elevated levels of thyroid hormone could also be harmful.”

Tim I. M. Korevaar

Korevaar and colleagues evaluated maternal serum thyroid-stimulating hormone and free T₄ levels between pregnancy weeks 9 and 18 and child IQ at 6 years from 3,839 mother-child pairs from the Generation R Study to determine the association between maternal TSH and free T₄ levels and childhood IQ.

“According to our sensitivity analyses, brain development during early life may be affected by maternal free T₄ levels in up to 15% of all children in our study,” the researchers wrote. “Because high free T₄ levels have similar detrimental effects as low free T₄ levels, [levothyroxine] treatment aiming for high-normal thyroid function tests may not be completely without risks.”

Maternal free T₄ levels and child IQ were strongly associated in an inverse U-shaped manner (P = .003). There also was an association with the risk of borderline intellectual functioning with maternal free T₄ levels (P = .003).
A low free T₄ level at or below the eighth percentile was significantly associated with decreased mean child IQ (P ≤ .01). Conversely, high free T₄ levels yielded a significant decrease in mean child IQ at or above the 89th percentile (P ≤ .01).

Child IQ levels also were associated with TSH levels; however, this was not an independent association.

“Based on the data available, and particularly the lack of data on treatment effects, it is important to provide levothyroxine treatment during pregnancy with care,” Korevaar told Endocrine Today. – by Amber Cox

Reference:

Korevaar TIM, et al. OR11-4. Presented at: The Endocrine Society Annual Meeting; March 5-8, 2015; San Diego.

Disclosure: The researchers report no relevant financial disclosures.