What Are Anti-thyroid Drugs?

by Elaine Moore

Anti-thyroid drugs (ATDs) are compounds that interfere with the body’s production of thyroid hormone, therby reducing symptoms of hyperthyrodism. ATDs were discovered accidentally in the mid-1940’s when thiocyanate compounds used for heart disease were found to cause hypothyroidism. This led to the development of a number of compounds specifically tailored to reduce thyroid hormone production.

The rate of remission using ATDs has much to do with following specific protocols designed to determine certain changes indicative of remission. These protocols include Doppler sonography to measure thyroid volume, the RAI-U tests, and tests to measure thyroid antiboides. When thyroid volume, RAI-U,or antibody titers show a significant decrease, patient's are achieving remission, and when these levels fall into the normal range, remission is achieved. Following this protocol, patients in one study had no relapses.

In Europe and Japan, where these protocols are routinely followed remission rates are high, nearing 95%, whereas in the United States, patients are frequently taken off ATDs before remission is achieved. Thus, remission rates in the United States are typically about 40% and patients are more likely to have relapses.

According to a number of studies, the high iodine diet of individuals in the United States also accounts for the lower remission rate seen here. Iodine is a known trigger for autoimmune thyroid disease. Furthermore, ATDs work by inhibiting iodine absorption in the gut. With a high iodine diet, higher doses of ATDs are needed. And the higher the dose, the greater incidence of side effects, including rashes, hives, agranulocytosis (abnormal decrease in segmented neutrophilic white blood cells) and liver disease.

The antithyroid drugs most frequently used today are chemicals known as thioureylenes, which belong to the thionamide family. Thioureylene compounds include propylthiouracil (PTU) and methimazole (Tapazole).

In Great Britain and Europe, carbimazole, a derivative of methimazole is most often used. The active ingredient in both compounds is the same. Other ATDs include aniline derivatives such as sulfonamides and polyhdric phenols such as resorcinol. Other compounds with antithyroid properties include lithium salts, high concentrations of saturated potassium iodine, thiouracil derivatives, oral imaging contrast dyes, some anticonvulsant drugs and iodide transport (ionic) inhibitors such as perchlorate.

Initially, patients using antithyroid drugs are started out on relatively high doses. However, before the maximum drug effects can be seen, the normal stores of thyroid hormone in the thyroid gland must be used up. This takes 4-6 weeks depending on which drug is used. Before that, effects associated with reduced thyroid hormone levels are usually noticed. These include reduced nervousness and palpitations, increased strength, weight gain and reduction in goiter. However, if the patient becomes hypothyroid quickly, hypothyroidism may cause an increase in goiter size. Blood levels of FT4 and FT3 can be used to determine the cause of goiter. Since it takes many weeks to months for TSH to reflect changes in thyroid hormone levels, the TSH test is not reliable for patients using antithyroid drug therapy. ♦

© 11 Mar 2006 Copyrighted by Elaine Moore