Experts are calling for wider thyroid screening of women of childbearing age following a report linking undetected thyroid disease during pregnancy to lower IQ in children.
"The message is that we are not detecting thyroid disease and it is costing patients and their offspring delay in diagnosis and treatment," said Richard A. Dickey, president of the American Association of Clinical Endocrinologists. "This is of concern. We should not be depriving children of intellectual capacity."
Hypothyroidism--low levels of thyroid hormone--affects about 1 in every 100 women of childbearing age. Unless thyroid deficiency is detected and treated in an expectant mother, her fetus is deprived of thyroid hormone until the second trimester, when the thyroid gland in the fetus fully develops.
In mid- to late fetal development and during the first few months of life, thyroid deficiency can cause mental retardation and neurological deficits. For that reason, newborns are routinely screened for thyroid deficiency at birth.
But whether thyroid hormone is needed during the first trimester of life has been less certain. Now a new study, published last month in the New England Journal of Medicine, suggests that thyroid hormone plays an important role in intellectual and motor development early in pregnancy. The study found that infants born to mothers with undetected hypothyroidism had significantly lower IQ levels by ages 7 to 9.
Paul Ladenson, director of the Division of Endocrinology and Metabolism at the Johns Hopkins Medical Institutions in Baltimore, said the findings are "the most convincing to date" that children born to women with undetected hypothyroidism are at risk for future neurological problems.
The multi-center study, led by James E. Haddow at the Foundation for Blood Research in Scarborough, Maine, measured thyroid hormone levels in blood samples collected from 25,216 pregnant women. Participants in the study had blood drawn between January 1987 and March 1990. Samples were frozen at the time and analyzed by the researchers in 1996 and 1997.
The team of scientists, which also included researchers from Dartmouth Medical School in Hanover, N.H., and from Children's Hospital and Beth Israel Deaconess Medical Center in Boston, then located 62 of the women whose blood samples showed low levels of thyroid hormone and 124 women with normal thyroid levels for follow-up study. Their children, who by then ranged in age from 7 to 9 years old, also underwent 15 tests designed to measure intelligence, attention, language, reading ability, school performance and visual-motor performance.
Thyroid deficiency was undetected at the time of pregnancy in 48 of the 62 women with low thyroid levels. The IQ scores of children born to these women were on average seven points lower than those of youngsters born to women with normal thyroid levels. Nearly one in every five of these children had IQ levels of 85 or lower, leading the research team to conclude that "undiagnosed hypothyroidism in pregnant women may adversely affect their fetuses." An IQ score of 100 is considered average; a score of 85 or lower, according to Haddow, is linked with learning problems.
Based on the findings, Haddow and his colleagues suggested that screening all pregnant women for hypothyroidism with a simple blood test "may be worthwhile, even when the degree of deficiency is mild and does not cause immediate clinical manifestations in the woman."
But such screening remains controversial, partly because of its cost and the question of whether it identifies the real problem. The American Association of Clinical Endocrinologists (AACE) and the American College of Endocrinology called for all women planning to get pregnant to be screened before they conceive a child. But the AACE also said that once a woman is pregnant, routine screening "should be left to the judgment of the physician in consultation with the patient."
For now, the Endocrine Society, a group of more than 10,000 scientists and physicians in 80 countries, recommends that thyroid testing be limited to women "with a personal or family history of thyroid disease or symptoms suggestive of hypothyroidism." Such women should be screened when they are "planning a pregnancy or as soon as possible after conception."
Blood tests are the "best way to make such a diagnosis, since the signs of hypothyroidism can be masked by pregnancy," said J. Larry Jameson, president of the Endocrine Society. Symptoms are nonspecific and include fatigue that sufferers often attribute to simply being pregnant.
Testing, which measures levels of thyroid-stimulating hormone, costs about $25 per person, but many health insurance plans don't cover routine screening.
"Unfortunately, the reality is that nobody wants to pay for it," said Ladenson of Johns Hopkins.
For treatment, doctors prescribe daily doses of thyroxine--the chemical substance that the thyroid gland no longer produces adequately in people with hypothyroidism. In pregnant women, thyroxine crosses the placenta, providing the substance to the fetus for normal brain development, and studies show that it "is very safe to take during pregnancy," Ladenson said.
However, in an editorial accompanying the study, Robert D. Utiger, a physician who is an editor at the New England Journal of Medicine, advised that other strategies to reduce hypothyroidism in pregnancy should be tried before turning to universal screening.
The presumption is that thyroid deficiency in most American women is caused by an autoimmune condition, Utiger said. In other countries, evidence suggests that iodine deficiency can also play an important role. Utiger noted that in Belgium, nationwide iodine supplementation "improved thyroid function in both pregnant women and their infants."
In the United States, iodine intake has been thought to be sufficient, "but has declined substantially in the past 15 years," Utiger wrote. He speculated that this is due to changes in Americans' diet and in animal feed. "Before routine screening for hypothyroidism in pregnant women is introduced," he said, "efforts should be made to increase dietary iodine intake, by ensuring, for example, that all prenatal vitamins--and, indeed, all vitamin products--contain iodine."
Iodine intake can also be increased by using iodized salt or by fortifying other foods with the mineral, he said. "The beneficiaries would be not only pregnant women and their offspring, but everyone," Utiger concluded.
More information on hypothyroidism and other thyroid diseases is available on the World Wide Web from:
* The Endocrine Society,
* The American Association of Clinical Endocrinologists, www.aace.com.
THYROID AT A GLANCE
DESCRIPTION: A small butterfly-shaped gland that sits at the front of the neck just in front of the windpipe, or trachea.
FUNCTION: Produces three important hormones -- thyroxine, triiodothyronine and calcitonin -- which travel through the bloodstream. Thyroxine and triiodothyronine, which contain iodine, regulate the body's metabolism, physical growth and mental development. Calcitonin helps regulate the amount of calcium in the body. The pituitary gland and the hypothalamus help control the thyroid gland.
DISORDERS: About 20 million Americans have problems caused by thyroid disorders. Overproduction, called hyperthyroidism, makes the body use energy faster than it should, causing fatigue, anxiety, palpitations, sweating, weight loss, diarrhea and a fast heart rate. A lack of sufficient production, called hypothyroidism, makes the body use energy slower than it should, causing fatigue, memory problems, dry skin, hair loss, weight gain, constipation and a slow heart rate. The thyroid can also become enlarged -- it is then called a goiter -- or inflamed, a condition called thyroiditis. Sometimes it also develops lumps.
Source: The Thyroid Society for Education and Research; The American Medical Association.