Levothyroxine Calcium carbonate

Cholestyramine, calcium carbonate, sucralfate, aluminum hydroxide, ferrous sulfate, soybean formula, and dietary fiber supplements may impair the absorption of levothyroxine from the GI tract. Drugs that increase nondeiodinative T4 clearance include rifampin, carbamazepine, and possibly phenytoin. Amiodarone may block the conversion of T4 to T3. 

A 49-year-old woman, taking levothyroxine 150 micro-grams/day and calcium carbonate (three tablets daily) for prevention of osteoporosis, developed symptoms of hypothyroidism and had a raised serum TSH concentration (22 mU/1). She was advised to continue taking the same dose of levothyroxine but to separate her medications. Repeat biochemical testing 8 months later showed a normal serum TSH (3.3 mU/1). 

Csako G, McGriff NJ, Rotman-Pikielny P, Sarlis NJ, Pucino F. Exaggerated levothyroxine malabsorption due to calcium carbonate supplementation in gastrointestinal disorders. Ann Pharmacother 2001 35(12) 1578-83. 

Singh N, Singh PN, Hershman JM. Effect of calcium carbonate on the absorption of levothyroxine. JAMA 20CH>,283(21) 2822-5. 

Patients with hypothyroidism who are taking levothyroxine may become hypothyroid if given drugs that decrease T4 absorption (cholestyramine, iron salts, calcium carbonate, aluminum hydroxide, sucralfate, raloxifene, omeprazole, dietary soy and fiber), increase its clearance (phenytoin, car-bamazepine, phenobarbital, rifampin), or decrease T4 to T3 conversion (amiodarone, glucocorticoids, propranolol). 

Twenty patients with hypothyroidism were given levothyroxine, to which calcium carbonate 1.2 g daily was then added for 3 months. While taking the calcium carbonate their mean free thyroxine levels fell from 16.7 to... 

In vitro studies indicate that levothyroxine is adsorbed onto calcium carbonate when the pH is low (as in the stomach), which would reduce the amount available for absorption. ... 

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