Bioequivalence study of levothyroxine tablets compared to reference tablets and an oral solution

The study was designed to evaluate the bioequivalence of three levothyroxine sodium (CAS 51-48-9) formulations, i.e. a test and a reference tablet and an oral solution. A bioequivalence study was carried out in 25 healthy volunteers, who were administered a single dose of 600 microg levothyroxine in the form of the test formulation (levothyroxine sodium tablets 200 microg; Eferox), the originator product, and an oral solution. The trial was performed in one study center according to an open, randomized, three-way cross-over design with wash-out periods of 35 days between administration. Blood samples were taken up to 48 h post dose, the plasma was separated and the concentrations of levothyroxine and triiodothyronine were determined by radioimmunoassay with I125 labeling method. The levothyroxine mean Cmax were 112.0+/-17.3 ng/ml, 113.4+/-18.5 ng/ ml and 111.3+/-15.1 ng/ml, while the mean AUC0-24 were 2263.7+/-332.8 ng x h/ ml, 2307.3+/-351.3 ng x h/ml and 2286.1+/-331.0 ng x h/ml for the test and reference tablets as well as for the oral solution, respectively. No significant differences were found of principal pharmacokinetic parameters between the studied formulations. The 90%-confidence interval for the primary target parameters, intra-individual ratios of AUC0-24 and Cmax of levothyroxine were within the acceptance ranges for bioequivalence trials, i.e. AUC0-24 0.954-1.016 and 0.966-1.011 as well as Cmax 0.948-1.027 and 0.968-1.032 for test tablets versus reference tablets and the oral solution, respectively. Similar results were observed for triiodothyronine. In the light of the present study it can be concluded that the levothyroxine test tablet is bioequivalent to the reference formulation in respect of extent and rate of absorption. The results of the present trial confirm the findings of a previous study, performed under steady-state conditions with Eferox tablets 100 microg in patients without thyroid function.