Interaction between levothyroxine and calcium carbonate.

Levothyroxine (L-T) is one of the most commonly prescribed medications. It is given as either physiologic replacement therapy in patients with hypothyroidism or as interventional therapy to suppress thyroid-stimulating hormone (TSH) secretion in patients with nodular thyroid disease or thyroid cancer. Patients treated with L-T4 suppressive therapy (a state known as exogenous subclinical hyperthyroidism) might be at increased risk for osteoporosis. Because of this, patients are encouraged to take a calcium supplement, such as calcium carbonate.1 Calcium supplementation is especially important for preventing or treating osteoporosis in postmenopausal women. However, several conditions or drugs might alter L-T4 requirements for both replacement and interventional therapy.2 We report a case of clinically significant interaction between L-T4 and calcium carbonate.