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Beginning in early gestation, maternal thyroid hormone production normally increases by approximately 50% in response to increased levels of serum thyroxine-binding globulin (resulting from the increase in estrogen levels) and because of stimulation of thyrotropin (TSH) receptors by human chorionic gonadotropin. 1 The placenta is a rich source of the type 3 inner ring deiodinase, which enhances the degradation of thyroxine (T4) to bioinactive reverse triiodothyronine (T3). 2 Thus, thyroid hormone demand increases, which requires an adequate iodine supply that is obtained primarily from the diet and/or as supplemental iodine (Fig. 1). In addition, fetal thyroid hormone production increases during the second half of pregnancy, further contributing to increased maternal iodine requirements because iodide readily crosses the placenta.

After oral ingestion, iodide is rapidly absorbed through the stomach and …