Dynamic thyroid testing, thyroid histology and thyroxine replacement therapy in Basenji dogs

Abstract

Objective: To monitor the long-term health status of a cohort of client-owned Basenji dogs over several years; to compare the accuracy of various laboratory methods to assess thyroid status; and to document any adverse effects of routinely recommended doses of thyroxine for confirmed or presumptive hypothyroidism. Methods: Health outcomes were followed by 1-8 years of clinical surveillance using serum total thyroid hormone (TT4) and canine thyroid-stimulating hormone (cTSH) concentrations, and either dynamic thyroid testing using recombinant human (rh) TSH, and/or thyroid histopathology at necropsy. The long-term accuracy of the various diagnostic interpretative criteria for rhTSH stimulation was analysed. The clinical history and serial concentrations of TT4 and cTSH in dogs suspected of exhibiting adverse clinical responses to standard thyroxine supplementation were reviewed. Results: When rhTSH interpretative criteria were compared with thyroid histology at necropsy, Larsson's equation correctly identified normal Basenji dogs from dogs with hypothyroidism and extensive thyroid pathology, yet provided an equivocal result for dogs that were not hypothyroid. Despite sequential low TT4 concentration measurements over several years of clinical surveillance, these 'equivocal' dogs had unremarkable thyroid histology (consistent with breed and age) at necropsy. Thyroxine supplementation at recommended dose rates resulted in clinical signs of thyrotoxicosis in six elderly Basenji dogs. Conclusion: Age and breed may account for dose-related adverse responses in Basenji dogs dosed with thyroxine at recommended doses. We recommend that when conducting therapeutic thyroid supplementation trials in this breed, the drug be administered in gradually increasing doses to avoid dose-related side effects. In a breed with a lower than 'normal' reference interval for serum TT4 concentration, attempts to achieve the post-pill TT4 concentration achieved in normal dogs may induce signs of thyrotoxicosis. Serial cTSH concentration measurements are helpful when adjusting thyroxine supplementation in Basenji dogs. Larsson's equation can be of assistance in interpreting dynamic thyroid testing using rhTSH in this breed. The overall prevalence of hypothyroidism in the Basenji breed appears to be low.9 page(s