Mild Hyperthyroidism May Be Best Bet in Graves' Pregnancy


PHOENIX — Infants are rarely born with suppressed thyroid function if their mothers have Graves' disease but continue to take lower doses of thyroid medication during pregnancy, based on data from 249 pregnant Graves' disease patients.

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 said at the annual meeting of the American Thyroid Association.

Although previous research has suggested that a mother's thyroid hormone level is linked to her newborn's health, this study is the first to show such a relationship, said Dr. Momotani of the Tokyo Health Service Association in Tokyo.

Graves' disease involves overactivity of the entire thyroid gland, which 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, which prevents fetal hypothyroidism.

“But the drug doses that are ideal for the mother might be too much for the fetus,” Dr. Momotani said.

Keeping pregnant Graves' disease patients in a mild hyperthyroid state may be a noninvasive way to care for these women and prevent thyroid problems in 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, a total of 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. But 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.

It is important to remember that the range of FT4 values in women with Graves' disease varies, and some infants may have initial suppressed thyroid function, but most of these infants do well with close follow-up, Dr. Momotani said.

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