Low thyroid levels during pregnancy may lower child's IQ

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August 18, 1999
Web posted at: 5:18 p.m. EDT (2118 GMT)

(CNN) -- The IQs of children born to mothers with untreated underactive thyroid glands are lower than those of children born to healthy mothers by up to seven points, according to a study in this week's New England Journal of Medicine.

"Hypothyroidism (an underactive thyroid) in pregnant women can adversely affect their child's subsequent performance on neuropsychological tests," wrote researchers from the Foundation for Blood Research in Scarborough, Maine.

Scientists found the child's IQ is affected "even when the pregnant woman's hypothyroidism is mild." Women with mild hypothyroidism may display none of the usual symptoms of the condition, which include fatigue, weight gain and dry hair and skin.

The thyroid is a dual-lobed gland that secretes the hormones that control metabolism, growth and development. It is located just under the voice box.

The research team, led by Dr. James Haddow, looked at the IQ scores of 7- to 9-year old children born to 124 healthy women, and children in the same age range born to 62 women with hypothyroidism. The children took part in 15 psychological tests of intelligence, attention, language, reading and school problems, and visual-motor performance.

On average, the authors found children born to mothers with underactive thyroids scored four points lower than children with mothers with normal thyroid glands during pregnancy. Fifteen percent had IQ scores under 85, compared to 5 percent of the children with healthy mothers. An average IQ score is 100.

Of the 62 thyroid-deficient women, 48 received no treatment during pregnancy for their condition. The children of those mothers scored an average of 7 points lower than children of mothers without the condition. Nineteen percent had IQ scores below 85.

The children of the mothers who were treated for hypothyroidism scored similarly to the children of healthy mothers.

"These findings suggest that early detection and treatment for hypothyroidism of the mother during pregnancy might be an important factor in the intelligence and well-being of her child," said Haddow. "However, these data do not allow us to determine whether detection and treatment must be accomplished prior to the pregnancy to be effective, or whether they will be effective if done early in pregnancy."

The 14 women with hypothyroidism who received treatment during pregnancy had been diagnosed and treated before they became pregnant. The women who went without treatment did not find out they had the condition until an average of five years after the pregnancy.

The study was funded by the National Institute of Child Health and Human Development and by Knoll Pharmaceutical Co., a unit of German conglomerate BASF A.G. that makes thyroid treatment medicine.

Haddow's group concluded, "Systematic screening for hypothyroidism early in pregnancy may be worthwhile, even when the degree of deficiency is mild and does not cause immediate clinical manifestations in the woman."

In an accompanying editorial, journal deputy editor Dr. Robert Utiger suggested that before screening routinely for underactive thyroid, there should be a push to correct the problem by adding iodine, which is necessary for the production of thyroid hormones, to prenatal vitamins and increasing the amount of iodine added to salt and other foods.

"The beneficiaries would be not only pregnant women and their offspring, but everyone," he said.

The number of women with hypothyroidism differs greatly worldwide. It is 2.5 percent in the United States but only 0.3 in Japan, according to Utiger.

Reuters contributed to this report.