When both the fetus and mother have thyroid deficiency, it can be expected that neuropsychological development will be compromised, but it is not clear whether similar problems occur when only the mother is hypothyroid. This study reviewed thyrotropin (TSH) values in stored sera from >25,000 pregnant women. Forty-seven had serum TSH values at or above the 99.7th percentile for all pregnant women, and 15 others had levels between the 98th and 99.6th percentiles as well as a low thyroxine level (below 99.7 nmol/liter). The hypothyroid women were matched with 124 women whose TSH values were normal, and their children underwent 15 tests when they were 7 to 9 years of age. These tests evaluated intelligence, language function, attention, reading ability, visuomotor function, and performance at school.
None of the infants in this study had congenital hypothyroidism. The hypothyroid and normothyroid women were comparable demographically and in their pregnancies. Case children performed less well than control children on all of the neuropsychological tests. Full-scale Wechsler IQ scores were an average of 4 points lower in the children of hypothyroid mothers than in the comparison group; 15 and 5 percent, respectively, had scores of 85 or below. IQ scores of the children of the 48 women not treated for hypothyroidism during pregnancy were an average of 7 points lower than those of control children, and 19 percent had scores of 85 or less. When examined 11 years after delivery, 64 percent of untreated women and 4 percent of control women had confirmed hypothyroidism.
These findings suggest that it may be worthwhile to routinely screen for hypothyroidism in early pregnancy. Testing at the first prenatal visit and prompt follow-up of those with positive results would permit timely treatment and could prevent adverse neuropsychological effects during early child development.
N Engl J Med 1999;341:549-555