Ciprofloxacin and rifampin have opposite effects on levothyroxine absorption

Program: Abstracts - Orals, Featured Poster Presentations, and Posters
Session: SUN 414-431-HPT Axis Biology
Sunday, June 16, 2013: 1:45 PM-3:45 PM
Expo Halls ABC (Moscone Center)

Poster Board SUN-420
Alyse Goldberg*1, Rommel Tirona2, Linda Asher3, Richard Kim2 and Stan Van Uum3
1Western University, ON, Canada, 2Western University, 3Western University, London, ON, Canada
Background: Levothyroxine (T4) absorption varies between individuals, and can be affected by various concomitantly administered drugs. Case reports have indicated that treatment with ciprofloxacin or rifampin can interact with T4 treatment in hypothyroid patients.  

Purpose: We hypothesize that T4 transporting proteins expressed in the gut are the mechanistic site for inhibition of thyroid hormone absorption by concomitantly administered drugs. Thus, the goal of this study was to formally determine the acute effect of co-administration of ciprofloxacin on T4 absorption and compare that with a well-known OATP inhibitor (rifampin) to better elucidate the potential relevance of intestinal transporters to T4 absorption.

Methods: The effects of that two antibiotics on T4 absorption were prospectively assessed in double blind-randomized, crossover fashion. Eight healthy volunteers received 1000 microgram T4 combined with placebo, ciprofloxacin 750 mg or rifampin 600 mg as single doses. We measured total plasma T4 concentrations over a 6 hour period after dosing using liquid chromatography-tandem mass spectrometry (LC-MS/MS). For each study arm, T4 areas under the plasma concentration-time curve (AUC) were compared.

Results: Co-administration 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 T4 is differentially affected by acute co-administration of ciprofloxacin or rifampin.  Mechanistic studies focused on intestinal, and possibly hepatic thyroid hormone transporters are required to explain the observed drug interactions with T4.

Nothing to Disclose: AG, RT, LA, RK, SV

*Please take note of The Endocrine Society's News Embargo Policy at

Sources of Research Support: This study was funded by the Lawson Health Research Institute.