Background: Levothyroxine (L-T4) absorption varies between individuals, and can be affected by various concomitantly administered drugs. Case reports have indicated an association between cotreatment with ciprofloxacin or rifampin and hypothyroidism in patients on a stable L-T4 dose. Methods: The effects of two antibiotics on T4 absorption were prospectively assessed in a double-blind, randomized, crossover fashion. Eight healthy volunteers received 1000 μg L-T4 combined with placebo, ciprofloxacin 750 mg, or rifampin 600 mg as single doses. We measured total plasma thyroxine (T4) concentrations over a 6-hour period after dosing using liquid chromatography-tandem mass spectrometry. For each study arm, areas under the T4 plasma concentration-time curve (T4 AUCs) were compared. Results: Coadministration of ciprofloxacin significantly decreased the T4 AUC by 39% (p=0.035), while, surprisingly, rifampin significantly increased T4 AUC by 25% (p=0.003). Conclusion: Intestinal absorption of L-T4 is differentially affected by acute coadministration of ciprofloxacin or rifampin. Mechanistic studies focused on intestinal and possibly hepatic thyroid hormone transporters are required to explain the observed drug interactions with L-T4. © Mary Ann Liebert, Inc.
CITATION STYLE
Goldberg, A. S., Tirona, R. G., Asher, L. J., Kim, R. B., & Van Uum, S. H. M. (2013). Ciprofloxacin and rifampin have opposite effects on levothyroxine absorption. Thyroid, 23(11), 1374–1378. https://doi.org/10.1089/thy.2013.0014
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