Drugs affecting thyroid function

Over the years, several drugs used in the treatment of nonthyroidal conditions have been shown to affect thyroid function. As novel drugs are introduced, novel interactions are described. The aim of this review is to summarize clinically relevant thyroidal side effects of drugs used for nonthyroidal conditions. Special focus is given to recent developments and to drugs with the largest clinical relevance.

Thyrosine kinase inhibitors are novel drugs used in the treatment of several neoplasias, including thyroid cancer. Thyroidal side effects are being increasingly detected with these drugs. Some drugs in this category affect thyroid hormone metabolism and therefore only affect patients on thyroid replacement. Others affect the thyroid directly profoundly, causing primary hypothyroidism. Immune modulators used in infectious, inflammatory, and neoplastic conditions also cause hyper- and hypothyroidism, through poorly understood immune or nonimmune mechanisms. The effects of amiodarone on the thyroid have been long recognized. However, given the complexity of these effects, several areas in this field remain problematic, such as the identification of subtypes of hyperthyroidism and the best treatment strategies. Lithium also has important antithyroid effects and it is a commonly prescribed medication. Its antithyroid effects may have clinical utility in selected clinical situations. Other drugs known to affect thyroid hormone absorption, metabolism, and transport are also briefly reviewed.

Several drugs are known to alter thyroid function as a side effect of their primary pharmacological action. Some of these effects have been recognized for decades, but novel thyroid-drug interactions are being recognized as new drugs are developed. It is important for the clinician to be familiar with thyroid-drug interactions, as enhanced surveillance may be necessary in patients undergoing therapies known to affect thyroid function.