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To describe the pharmacokinetics of two veterinary formulations of L-thyroxine available in Australia. 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. 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. 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. © 2013 The Authors. Australian Veterinary Journal © 2013 Australian Veterinary Association.


C Simpson, J L Devi, T Whittem. Bioavailability of two L-thyroxine formulations after oral administration to healthy dogs. Australian veterinary journal. 2013 Mar;91(3):83-8

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PMID: 23438458

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