title: Bioavailability of two L-thyroxine formulations after oral administration to healthy dogs.
abstract: 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.
  
  
  CONCLUSIONS
  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.
authors: Veterinary Hospital, Faculty of Veterinary Science, The University of Melbourne, Werribee, Victoria, Australia. christopher.simpson@sarc.com.au
authors: C
authors: C
authors: Simpson
authors: J L
authors: JL
authors: Devi
authors: T
authors: T
authors: Whittem
mesh_terms: D000284:Administration, Oral; D000818:Animals; D019540:Area Under Curve; D001682:Biological Availability; D018592:Cross-Over Studies; D004285:Dogs / Q000097:blood* / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D008657:Metabolic Clearance Rate; D011897:Random Allocation; D013607:Tablets; D013974:Thyroxine / Q000008:administration & dosage* / Q000493:pharmacokinetics*
chemical_list: D013607:Tablets; D013974:Thyroxine