Influence of acetylsalicylic acid and ketoprofen on canine thyroid function tests

Many factors including drugs can influence thyroid function in humans, rats and dogs. Studies in humans report significant effects of non-steroidal anti-inflammatory agents (NSAIDs) on thyroid function tests, which can lead to misinterpretation of the results and inappropriate therapeutic decisions. As NSAIDs are used more and more frequently in dogs, it is important to know to what extent they can influence results. Eighteen spayed female beagle dogs were randomly assigned to three treatment sequences in a 3 x 3 crossover study design with treatments consisting of acetylsalicylic acid (ASA) (25 mg/kg BW q 12 h), ketoprofen (Keto) (1 mg/kg BW q 24 h) or placebo administered for a 1-week period with a 3-week washout period between treatment periods. Blood samples for determination of total thyroxine (TT4), free thyroxine (FT4), total triiodothyronine (TT3), thyrotropin (TSH), reverse triiodothyronine (rT3), Keto and ASA concentrations were taken during each treatment period on days 0, 1, 3 and 7. During the washout period samples were taken weekly. A significant decrease in TT4 was observed as soon as 24 h after ASA administration, whereas the decrease in TT3 was less pronounced and differed significantly from the placebo only after 1 week of administration. No significant effects were found for free T4 and TSH with ASA administration. No significant effects on thyroid results were found following Keto administration. The results indicate that TT4 can be markedly decreased by ASA therapy and until the results of further studies are available, thyroid function test results should be interpreted cautiously in dogs on NSAIDs therapy.