Abstract
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 Cmax and AUCt.
Results: The test/reference ratios Cmax and AUCt 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.
Key words
interactions - food supplement - pharmacokinetics - clinical study - climacteric syndrome
