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To the Editor.—I would like to report an interaction between 2 commonly prescribed drugs, levothyroxine sodium and calcium carbonate, which reduces levothyroxine efficacy. This inhibition may be reversed by separating the administration of these 2 medications.

In a recent evaluation of 3 women with thyroid cancer who were receiving levothyroxine to suppress serum thyroid-stimulating hormone (TSH) levels, it was noted that simultaneous ingestion of some calcium formulations reduced levothyroxine effectiveness (Table 1). For example, patient 1 took levothyroxine (125 µg/d) and had a baseline serum TSH concentration of 0.08 mU/L (normal, 0.5-4.0 mU/L). She subsequently began to take calcium carbonate (in the form of Tums) for prevention of osteoporosis, often taking it together with levothyroxine. She experienced fatigue and a 4.5-kg weight gain over the next 5 months, and her serum TSH level was found to have risen to 13.3 mU/L. She then stopped taking calcium carbonate . . . [Full Text of this Article]

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THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES

Effect of Calcium Carbonate on the Absorption of Levothyroxine
Singh et al.
JAMA 2000;283:2822-2825.
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Calcium Carbonate-Induced Hypothyroidism
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ANN INTERN MED 2000;132:595-595.
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