Background: Recent literature describing the effect of gastric acid suppression on levothyroxine absorption has been inconsistent. Also, ezetimibe, a lipid-lowering compound that inhibits intestinal absorption of cholesterol, may interfere with levothyroxine absorption. The objective of this study was to measure changes in levothyroxine absorption before and after famotidine, esomeprazole or single-dose ezetimibe."
Methods: We conducted levothyroxine absorption testing on 30 healthy volunteers, excluding those with thyroid disease. Subjects were randomized to receive one of three regimens: 1 week of either famotidine or esomeprazole, or a single dose of ezetimibe administered simultaneously with levothyroxine (n = 10 in each group). Baseline levothyroxine absorption testing was performed on all subjects using 600 mcg of Synthroid with thyroid hormone levels checked at 0, 2, 4, 6, and 8 hours after administration, and then repeated 6 weeks later, after administration of one of the three study drug regimens. The area under the curve (AUC) over 8 hours for serum thyroxine (T4), triiodothyronine (T3), and free T4 index, and the mean peak hormone levels achieved during levothyroxine absorption testing at baseline and following administration of one of the three study medications were compared using paired t-tests.
Results: Peak mean hormone levels and AUCs of T4, T3, and free T4 index during absorption testing before and after each of three study medications did not differ. Results for differences before and after study medication did not reach significance even when using the subtractive correction method of AUC calculation.
Conclusions: No differences were noted in levothyroxine absorption after gastric acid suppression with 1 week of famotidine or esomeprazole. A simultaneously administered dose of ezetimibe did not significantly change levothyroxine absorption.