Evaluation of Levothyroxine Bioavailability after Oral Administration of a Fixed Combination of Soy Isoflavones in Post-menopausal Female Volunteers.

Background: Post-menopausal women under treatment with levothyroxine for their medical conditions may take concomitantly dietary supplements containing soy isoflavones in combination to treat their post-menopausal symptoms. The aim of this study was to investigate the effect of a fixed combination of soy isoflavones on the oral bioavailability of levothyroxine in post-menopausal female volunteers. Methods: 12 healthy post-menopausal female, who were on stable oral levothyroxine as replacement/supplementation therapy for hypothyroidism, received a single recommended oral dose of a food supplement containing 60 mg of soy isoflavones (>19% genistin and daidzin) concomitantly with (test) and 6 h later (reference) the administration of levothyroxine in a randomized, open label, crossover fashion. Plasma concentrations of levothyroxine and soy isoflavones (daidzin, daidzein, genistin, genistein, S-equol) were determined by LC-MS/MS. Pharmacokinetic (PK) parameters were determined by non-compartmental analysis. No effect of soy isoflavones was assumed if the 90% confidence intervals (CIs) for the estimated ratio test/reference was included in the acceptance limits 0.80–1.25 for PK parameters C max and AUC t . Results: The test/reference ratios C max and AUC t of levothyroxine were very close to unity (1.02 and 0.99, respectively) and the corresponding 90% CIs (0.99–1.04 and 0.88–1.12, respectively) fell entirely within the acceptance bioequivalence limits. Conclusion: The combination of soy isoflavones used in the present investigation does not affect the rate and extent of levothyroxine absorption when administered concomitantly in post-menopausal women.