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Background: To control hyperthyroidism due to Graves' disease, antithyroid drugs should be administered. Several studies have shown that exposure to methimazole (MMI) during the first trimester of pregnancy increases the incidence of specific congenital anomalies that are collectively referred to as MMI embryopathy. Congenital anomalies associated with exposure to propylthiouracil (PTU) have also recently been reported., Methods: This study investigated whether substituting potassium iodide (KI) for MMI in the first trimester would result in a lower incidence of major congenital anomalies than continuing treatment with MMI alone. The cases of 283 women with Graves' disease (GD) were reviewed whose treatment was switched from MMI to KI in the first trimester (iodine group), as well as the cases of 1333 patients treated with MMI alone (MMI group) for comparison. Another major outcome of interest was the incidence of neonatal thyroid dysfunction. The subjects of the analysis of major congenital anomalies and neonatal thyroid dysfunction were live-born infants., Results: The incidence of major anomalies was 4/260 (1.53%) in the iodine group, which was significantly lower than the incidence of 47/1134 (4.14%) in the MMI group. Two neonates in the iodine group had anomalies consistent with MMI embryopathy (0.8%), as opposed to 18 neonates in the MMI group (1.6%). None of the neonates exposed to KI had thyroid dysfunction or goiter., Conclusions: Substituting KI for MMI as a means of controlling hyperthyroidism in GD patients during the first trimester may reduce the incidence of congenital anomalies, at least in iodine-sufficient regions.

Gestational transient thyrotoxicosis (GTT) is defined as transient thyrotoxicosis caused by the stimulating effect of human chorionic gonadotropin (hCG) during pregnancy. We attempted to identify the serum hCG level that causes GTT, and we compared the serum hCG levels and thyroid hormone levels of GTT patients according to whether they had a background of thyroid disease. We also evaluated serum hCG as a parameter for differentiating between active Graves' disease (GD) and GTT. We reviewed the 135 cases of pregnant women who came to our hospital to be evaluated for thyrotoxicosis during their 7th to 14th week of pregnancy, and their serum hCG level was measured at that time. Among the 135 pregnant women with thyrotoxicosis; 103 of the women had GTT, and the other 32 women had active GD. There were no correlations between their serum hCG levels and free thyroid hormone levels. There were no significant differences in thyroid hormone levels or hCG levels among the GTT groups with different thyroid disease backgrounds; i.e., the GTT group without thyroid disease, GTT group with chronic thyroiditis, GTT group with non-functioning thyroid nodules, and GTT group with GD in remission. The serum hCG level of the GTT group was significantly higher than in the active GD group, but it was not a good parameter for differentiating between the two groups. The FT3/FT4 ratio of the active GD was significantly higher than in GTT group, and was a better parameter for differentiation.