Margaret A.WinkerMD, Senior EditorIndividualAuthorPhil B.FontanarosaMD, Senior EditorIndividualAuthor
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.
Schneyer CR. Latent Infection of Cells in HIV Disease. JAMA. 1998;279(10):750. doi:10-1001/pubs.JAMA-ISSN-0098-7484-279-10-jac80000