Thyroid hormone analogs and fetal goiter.

Transfer of iodothyronine across the placenta in most species occurs only with difficulty. Recently, biologically active, nonhalogenated thyroid hormone analogs have been synthesized with properties which might favor placental transfer. To test this possibility, we compared the doses of T4, T3, and thyroid hormone analogs necessary to prevent propylthiouracil-induced goiter formation in rat fetuses. T4 and T3 prevented fetal goiter, but in doses that caused maternal hyperthyroidism; in contrast, the thyroid hormone analogs prevented fetal goiter in doses that were not thyrotoxic to the mother.