PHOENIX — Infants of women with Graves' disease are rarely born with suppressed thyroid function, but their mothers should continue taking lower doses of thyroid medication during pregnancy, according to data presented at the annual meeting of the American Thyroid Association.
Maternal free thyroxine (FT4) levels just above normal (at least 1.9 ng/dL) were associated with normal FT4 levels in the newborn, Dr. Naoko Momotani of the Tokyo Health Service Association in Tokyo.
This study of 249 pregnant Graves' disease patients is the first to show that a mother's thyroid hormone level is linked to her newborn's health, said Dr. Momotani.
Graves' disease can cause underactivity of the thyroid in the developing fetus. When a pregnant woman with Graves' disease takes antithyroid medication, the TSH receptor antibodies are transferred to the fetus. “But the drug doses that are ideal for the mother might be too much for the fetus,” she said.
The women in the study took antithyroid drugs throughout pregnancy. The highest reported maternal FT4 level was 4.1 ng/dL. Overall, 41 fetuses had elevated TSH, but none had a visible goiter at birth. There were no cases of below-normal fetal FT4 levels and only one case of elevated TSH in a fetus among women whose FT4 levels were greater than 1.9 ng/dL (that is, higher than the upper normal range of 1.2–1.9 ng/dL). By contrast, 102 mothers had normal free T4 levels (0.6–1.2 ng/dL) at the time of delivery, and 23 of their infants had low FT4 and/or high TSH levels at birth. Only 1 of these 23 infants had an elevated TSH level when the infants were screened for congenital hypothyroidism.
One infant had both suppressed TSH and normal free T4 levels at birth, which suggested central hypothyroidism, and the mother's FT4 in this case was 2.1 ng/dL.