Effects of iodine on thyroid status of fetus versus mother in treatment of Graves' disease complicated by pregnancy.

To investigate the effect of maternal iodine therapy for Graves' disease on fetal thyroid, we examined serum free T4 (FT4) and TSH levels in the fetus vs. those in the mother. Patients who were severely thyrotoxic were not included. Cord and maternal sera were tested at delivery in 35 patients with Graves' disease treated with iodine alone during pregnancy (6-40 mg daily). At the initiation of therapy, the mothers were at 11-37 weeks gestation, and FT4 levels ranged from 28.3-65.8 pmol/L. At delivery, maternal FT4 values ranged from 9.3-42.0 pmol/L, slightly above normal in 22 of the 35 mothers and normal in the other 13. Fetal FT4 levels were above the normal range occurred significantly less often than maternal levels (2 in 35; P less than 0.001), and no fetus had FT4 below normal. In the 13 mothers with normal FT4 levels, all fetal FT4 levels were normal; the fetal TSH level was above normal in 1 and normal in the remainder. A significant correlation was found between cord and maternal FT4 levels (P less than 0.05). In 12 of the 35 mothers, FT4 levels rose after a transient fall during iodine administration. The correlation of cord FT4 and maternal FT4 was closer when these 12 cases were excluded (P less than 0.001). Neither the dose of iodine nor the duration of therapy correlated with thyroid function in fetuses or mothers. Fetal TSH binding inhibitory antibody values strongly correlated with maternal TSH binding inhibitory antibody values (P less than 0.001). These findings indicate that 1) in the treatment of hyperthyroidism due to Graves' disease, iodine seldom if ever exposes the fetus to the risk of hypothyroidism; 2) the fetal thyroid is influenced by the same stimulatory and inhibitory factors as the maternal thyroid; and 3) escape from the inhibitory effects of iodine occurs less often in fetuses than in mothers, which may account at least in part for the lower thyroid status in the fetus compared to that in the mother.