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From The JAMA Web Sites
March 11, 1998

Latent Infection of Cells in HIV Disease

Author Affiliations

Margaret A.WinkerMD, Senior EditorIndividualAuthorPhil B.FontanarosaMD, Senior EditorIndividualAuthor

JAMA. 1998;279(10):750. doi:10-1001/pubs.JAMA-ISSN-0098-7484-279-10-jac80000

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 but maintained her levothyroxine regimen. Three weeks later her serum TSH level had declined to 0.68 mU/L.

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