Bioavailability of two L-thyroxine formulations after oral administration to healthy dogs.

Objective To describe the pharmacokinetics of two veterinary formulations of L-thyroxine available in Australia. Methods A two-phase randomised, crossover, open-label trial followed by a third-phase parallel-dosing trial was conducted in 11 healthy dogs with an investigative oral L-thyroxine liquid formulation and a reference tablet formulation. Blood sampling was done at defined intervals and serum total L-thyroxine concentrations were measured by radioimmunoassay. The post-dose concentrations were plotted as a function of time for each period and the relative bioavailability of the two formulations were compared using a general linear model with factors for dog, phase, sequence and formulation. Results Following oral administration of the reference tablet at the dose of 100 μg/kg, a maximum plasma concentration of approximately 96.2 nmol/L (baseline endogenous corrected) was reached within 3.77 h. For the investigative liquid preparation at a dose of 50 μg/kg, the maximum plasma concentration was 60.1 nmol/L (baseline endogenous corrected), which was reached within 3.59 h. Conclusion The geometric mean of the relative bioavailability for the liquid/tablet product was 1.1, which suggests that the relative bioavailability of thyroxine following administration of tablet or liquid formulation is similar.