Inhibition of new thyroid hormone synthesis in hyperthyroidism

Dr Leonard Wartofsky is a professor of medicine at Georgetown University Hospital, chairman emeritus of the Department of Medicine at the Washington Hospital Center, and past president of the American Thyroid Association and the Endocrine Society.

What are the treatment options for inhibiting the synthesis and secretion of new hormones in severe hyperthyroidism? How much time is needed to see clinical outcomes?

Leonard Wartofsky, MD, MPH: The inhibition of new thyroid hormone synthesis is by the typical use of antithyroid drugs propylthiouracil (PTU) or methimazole (INN thiamazole), in the United States called Tapazole. These agents inhibit organification of iodine, coupling of iodotyrosines, and the ultimate synthesis of thyroid hormone. The release of preformed hormone that has already been synthesized and is stored in the gland is important because that will continue on, even though the patient is taking PTU or methimazole.

If you fully block new synthesis with PTU or with methimazole, it can still take 6 to 8 weeks for blood levels to begin to drop. So by inhibiting release with another agent, one can accelerate that reduction in thyroid hormone levels in the blood. The agent we use is either iodine itself—Lugol solution or saturated solution of potassium iodide (SSKI)—or lithium carbonate.

Lithium carbonate is given in doses of 300 mg 3 or 4 times a day with careful monitoring of blood lithium levels to keep them in the range of 1 mEq/L and not higher than 1.2 mEq/L. The iodine is given as 3 to 5 drops 3 times a day. This is a pharmacologic excess of iodine that is quite effective and one would see a result of a falling thyroid hormone level within 3 or 4 days rather than 6 or 8 weeks.