A study has found that treating mild hypothyroidism during pregnancy does not lead to improved cognitive functioning in children through five years of age. The trial, published in the New England Journal of Medicine, also found no differences in rates of preterm birth, stillbirth, miscarriage and other outcomes between women who received the treatment and those who did not.

While significantly lowered maternal thyroid function during pregnancy has long been linked to adverse outcomes – including impaired neurodevelopmental delay in the baby – some observational studies have suggested that even subclinical levels of hypothyroidism may be associated with increased risks. As such, several organizations have called for routine screening and treatment of subclinical hypothyroidism in pregnant women. Up until now, however, there had been no published trials investigating the benefit of such practices.

The current study was designed to determine if addressing hypothyroidism that is below the current threshold for treatment during pregnancy in fact led to improved IQ scores in children.

The trial, conducted at 15 medical centers across the country, included 677 pregnant women with subclinical hypothyroidism, indicated by elevated levels of the hormone thyrotropin (TSH), and 526 with hypothyroxinemia, characterized by normal TSH levels but an abnormally low blood concentration of thyroxine (T4), the main hormone secreted by the thyroid gland. Some studies had also linked hypothyroxinemia in pregnancy to adverse cognitive outcomes.

The women in the trial were randomized to receive either levothyroxine, a common thyroid medication, or a placebo during pregnancy. After birth, their children then underwent developmental and behavioral testing every year up to the age of five.

At the end of the study, the investigators found no significant difference in IQ levels or other measures of neurodevelopment, behavior, attention deficits or hyperactivity between the children of women who received treatment and those who received the placebo. The rate of adverse pregnancy and neonatal outcomes – such as gestational diabetes, preterm birth and low Apgar scores – were also similar in the two groups.

“This trial should put a halt to routine screening and treatment of subclinical hypothyroidism during pregnancy,” Peaceman said. “The result will be not only less intervention and cost, but also less anxiety for pregnant women about having a condition that needs treatment.”