35-S-antithyroid drug concentration and organic binding of iodine in the human thyroid

35S-methimazole (MMI), 35S-carbimazole or 35S-propylthiouracil (PTU) were given orally to fifty-five patients at various times up to 12 h before surgical thyroidectomy. The amount of 35S radioactivity and labelled drug in thyroid and plasma samples was measured. Intrathyroidal inhibition or organic binding of iodine by MMI, carbimazole and PTU was measured after intravenous administration of 131I, 132I or 125I-iodide. After administration of 35S-carbimazole or 35S-MMI the thyroid to serum (T/S) ratio of 35S radioactivity was greater in thyrotoxic glands than in non-toxic adenoma tissue. 35S-MMI was found in thyroid and plasma samples after administration of 35S-carbimazole. The T/S 35S-MMI was greater than 1 in most but not all patients. 35S radioactivity was also concentrated in the thyroid after administration of 35S-PTU. In thyrotoxic glands there was an 80% inhibition of iodine organification in patients receiving MMI and 60% for those receiving PTU. It is suggested that carbimazole and MMI can be given once or twice daily in some patients but PTU would be less suitable for this dose schedule.