The study found that women with an untreated underactive thyroid condition during pregnancy are nearly four times more likely to have children with lower IQ scores. The researchers indicated that approximately 1 out of every 50 women have hypothyroidism during pregnancy. Other experts, however, believe this number may actually be far larger, and that a larger percentage of the population is undiagnosed or undertreated.
The study showed that 19 percent of the children born to mothers with untreated thyroid deficiency had IQ scores of 85 or lower. This was compared to a reduced IQ level of only 5 percent of those born to mothers without such thyroid problems. According to James E. Haddow, MD, lead study author, the range below an 85 IQ level can mean significant impairment for children. According to Haddow:
The children whose scores are in this range may face life-long developmental challenges. It might be possible to prevent these problems through the early diagnosis and treatment of thyroid disease in their mothers.The study found that the 62 children whose mothers were hypothyroid during pregnancy performed less well on all the various intelligence and IQ tests used for measurement. The children of the 48 women who were not treated for thyroid disease during the pregnancy had an average I.Q. score that was 7 points lower than the children in the control group, with 19 percent scoring 85 or less.
Interestingly, the researchers also found that the mothers who were subsequently discovered to be hypothyroid had gone an average of five years before their doctors diagnosed the thyroid disorder. A few of the women were not diagnosed until 10 years later. This is an ongoing problem for the entire population, and some estimates find that there are as many as 13 million people with hypothyroidism in the U.S., the majority of them women, and as many as half undiagnosed. (See HELP! My TSH Is "Normal" But I Think I'm Hypothyroid.)
The NEJM mentioned that screening of pregnancy mothers for hypothyroidism might be recommended. Typically, women who have undergone a procedure to remove or ablate the thyroid, or who have a diagnosed autoimmune hypothyroidism are aware of their hypothyroidism, and should practice particular care in preparation for pregnancy and in management of thyroid levels during pregnancy. The greatest danger, however, is in the many women hypothyroidism who are undiagnosed, who have normal thyroid levels with elevated antibodies, or who have untreated subclinical hypothyroidism -- all due to chronic autoimmune thyroiditis (Hashimoto's Thyroiditis).
Another issue is the concern over iodine intake. In a NEJM editorial accompanying the research findings, Dr. Robert Utiger said:
Despite the presumption that hypothyroidism in most pregnant women is caused by chronic autoimmune thyroiditis, which cannot be prevented, the difference among countries suggests another possible explanation -- iodine deficiency, which is preventable. . . It is likely that both chronic autoimmune thyroiditis and iodine deficiency contribute to the occurrence of hypothyroidism in pregnant women in many countries.Source:
Haddow, James E. M.D., et. al. "Maternal Thyroid Deficiency during Pregnancy and Subsequent Neuropsychological Development of the Child," New England Journal of Medicine, Volume 341:549-555, August 19, 1999 Number 8, Online