Cholestyramine resin, an anionic exchange resin, currently being investigated as an agent for lowering serum cholesterol levels, has been shown to interfere with the absorption of ingested thyroid hormone. A small quantity of the resin is capable of binding thyroxine under the conditions encountered in the gastrointestinal tract. Interference with thyroid hormone absorption by cholestyramine can be minimized by allowing a time interval of at least four hours between the ingestion of the two agents. Hypothyroid patients receiving cholestyramine should be examined periodically for evidence of hypothyroidism, and adjustments in dose and time interval between the two agents should be made if malabsorption of thyroid hormone is significant.
Northcutt RC, Stiel JN, Hollifield JW, Stant EG. The Influence of Cholestyramine on Thyroxine Absorption. JAMA. 1969;208(10):1857-1861. doi:10.1001/jama.1969.03160100047012