About one out of every 50 pregnant women will experience a condition known as hypothyroidism, in which an underactive thyroid gland doesn't produce enough of that thyroid hormone necessary for proper fetal brain development. The condition can be diagnosed with a simple blood test and treated with a once-a-day hormone pill.
The study found that 19 percent of the children born to mothers with thyroid deficiency had IQ scores of 85 or lower, compared with only 5 percent of those born to mothers without such problems.
"These children who score in this range may face lifelong developmental challenges," said the study's lead author, Dr. James Haddow of the Foundation for Blood Research in Scarborough, Maine. "It's the kind of cutoff that tells you you're going to have more troubles in school and more troubles in life in general."
Symptoms of hypothyroidism include tiredness, muscle weakness, cramps, constipation, a slow heart rate, dry and flaky skin, hair loss and a deep and husky voice.
Hypothyroidism affects between 6 million and 7 million Americans of all ages, but more than half of those cases are undiagnosed, according to the American Association of Clinical Endocrinologists.
Doctors don't know exactly what causes hypothyroidism, but many link it to a lack of iodine, which is found in seafood, dairy products, bread and some table salt.
Untreated hypothyroidism previously had been associated with early miscarriages and stillbirths. Thyroid disease can cause serious problems for a woman, even if she is not pregnant. The condition can raise the risk of heart disease and worsen osteoporosis and infertility.
For years, doctors have also known that a deficiency in thyroid hormone in both the mother and fetus could cause developmental problems in the child. But researchers were not sure whether the child would be affected if the deficiency were found in the mother alone. In this study, the children had normal levels of the hormone as newborns.
Haddow and his colleagues believe widespread screening of pregnant mothers could help prevent retardation and other severe problems with brain development in their children.
Haddow said decreases in a child's mental abilities can result even when the mother's hypothyroidism is mild and she shows no symptoms.
In the study, researchers compared 62 women with a deficiency of thyroid hormone to 124 women with normal levels, then studied the intelligence of their 7- to 9-year-old children.
The doctors looked at the youngsters' language and reading abilities, school performance, visual and motor skills and ability to pay attention.
The children of the women with hormone deficiencies sored an average of four points lower in IQ than the other youngsters.
In an accompanying editorial, Dr. Robert Utiger, an endocrinologist and deputy editor of the journal, said iodine in the American diet has declined substantially in the past 15 years because of lower salt intake and a reduction in the amount of iodine added to bread and animal feed. He suggested that efforts be made to ensure that "all prenatal vitamins and, indeed, all vitamin products contain iodine, by increasing the amount of iodine added to salt, or by adding iodine to other foods."