August 18, 1999
Children born to mothers with untreated hypothyroidism during pregnancy score lower on IQ tests than children of healthy mothers, according to a study conducted by Dr. James Haddow and partially funded by the National Institute of Child Health and Human Development (NICHD) and reported in the August 19 issue of the New England Journal of Medicine. However, children whose mothers were being treated for the condition scored almost the same as children born to healthy mothers. These findings suggest that early detection and treatment of hypothyroidism in pregnant women may be a critical part of prenatal care.
"We know that the health of the mother can be a key factor in her baby's health," said Duane Alexander, M.D., Director of the NICHD. "Fortunately, hypothyroidism is a condition that can be identified and treated during pregnancy. This study suggests that hypothyroidism might be added to the group of correctable maternal conditions that can influence the long-term health of the child," he said.
From 25,216 frozen serum samples obtained during pregnancy, researchers identified 62 women who had children between January 1987 and March 1990 and who, in retrospective analysis, were identified as having been hypothyroid during their pregnancies. These children were compared to a carefully matched group of 124 control children whose mothers' thyroid function in pregnancy was normal.
At the time of the study, the children ranged in age from 7 to 9 years. They participated in a series of 15 psychological tests relating to intelligence, attention, language, reading and school problems, and visual-motor performance.
The children born to mothers who were hypothyroid during pregnancy scored an average of 4 points lower in IQ tests than the control children, and 15 percent had IQ scores lower than 85, compared to only 5 percent of the control children who scored that low. Overall, the case children scored poorer on all 15 individual tests than the children born to healthy mothers.
Of the 62 women with hypothyroidism, 48 did not receive treatment during pregnancy for their condition. Their children's IQ scores averaged 7 points lower than children of mothers without the condition, and 19 percent had IQ scores below 85. However, the children born to mothers who were receiving treatment scored similarly to the control children, suggesting that treatment can help mitigate the adverse effects.
"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 James E. Haddow, M.D., lead author of the study. "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 thyroid gland is found in the neck and produces a hormone instrumental to many bodily functions. Hypothyroidism is a condition where the gland does not produce enough hormone. Signs and symptoms of hypothyroidism include feeling tired or without energy; coarse, brittle hair; thick, coarse skin; and a lowering of the metabolic rate.
Studies indicate that over a 10-year period, about 3% of women of childbearing age will develop the condition. However, in many women, hypothyroidism goes undetected because they do not yet have obvious physical signs or symptoms. Only 14 of the 62 women with hypothyroidism in this study had the condition detected and treated before their pregnancies, and on average the remaining women did not find out until 5 years after their pregnancies.
Doctors can find out if a woman has hypothyroidism by performing a blood test to measure levels of thyroid stimulating hormone (TSH), also known as serum thyrotropin. High TSH levels serve as an early warning system that the thyroid is not functioning adequately. The condition can then be treated with medication (thyroid hormone).
"There is an opportunity when women are pregnant and going for prenatal care to get a routine TSH test. It is a simple test for a condition with easy treatment. By taking action early, we might be able to help prevent adverse consequences to the child and the mother," said Haddow.
IQ Scores in Children with Healthy Mothers Compared to IQ Scores in Children Whose Mothers Were Hypothyroid During Pregnancy
Children with healthy mothers (Control) (124)
Average IQ scores : 107
% with IQ scores below 85 points : 5%
Children whose mothers were hypothyroid (Case)
Average IQ scores : 4 points lower than control % with IQ scores below 85 points : 15%
Mothers not treated during pregnancy (48)
Average IQ scores : 7 points lower than control % with IQ scores below 85 points : 19%
Mothers treated during pregnancy (14)
Average IQ scores : Similar to control % with IQ scores below 85 points : Similar to control