This study showed that the use calcium carbonate (e.g., Tums, Rolaids, Os-Cal, etc.), may interfere with the use of synthetic thyroid hormone, levothyroxine (Synthroid). It is proposed that the calcium carbonate binds to thyroxine (T4), especially in the acidic environment of the stomach, and reduces it's bioavailability. This interaction is particularly important because concurrent treatment with both of these drugs is common, particularly in postmenopausal women. Patients who took 1200 mg of elemental calcium as calcium carbonate daily along with their thyroid hormones, for 3 months, were found to have significant decreases in their free T4 and total T4 levels, as well as increases in their TSH levels.

Thyroid hormone is absorbed mostly in the upper portion of the small intestine. Thus, adsorption at a stomach pH would only partially explain the effect of calcium on the thyroid function studies. There may be other mechanisms operating in the small intestine. Nonetheless, the effect of acidity on the binding of calcium and thyroid hormone may explain why only 13 of 20 patients had a decreased free T4 level and only 4 of 20 patients had a thyrotropin level above the normal range while taking calcium. It is possible that these patients may have increased acidity in the stomach compared with the others. Alternatively, the patients who did not exhibit the effect of calcium on their thyroid function tests may have had relative lack of stomach acid.

JAMA. 2000; 283: 2822-2825.