Interference with the absorption of levothyroxine

Levothyroxine sodium is commonly prescribed for physiologic replacement of hypothyroidism and suppression of thyroid neoplasia. The absorption of levothyroxine (L-T4) is approximately 80% after oral administration. Gastrointestinal diseases may cause malabsorption, and certain drugs have been shown to interfere with the absorption of L-T4. These include ferrous sulfate, sucralfate, bile acid sequestrants used to treat hypercholesterolemia, aluminum hydroxide given as an antacid, and calcium carbonate. Various methods have been used to study the effects of these medications on the absorption of levothyroxine.