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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


Copyright 1998 American Medical Association. All Rights Reserved. Applicable FARS/DFARS Restrictions Apply to Government Use.1998

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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