Short term influence of prednisone and phenobarbital on thyroid function in euthyroid dogs

The short term effects of prednisone and phenobarbital on serum total thyro xine (tT4), free thyroxine (fT4), and thyroid stimulating hormone (TSH) wer e evaluated in euthyroid dogs. Twenty-six beagles were randomly divided int o 3 groups receiving, respectively, a placebo, prednisone (1.2 to 2 mg/kg b ody weight, per os, every 12 hours for 3 weeks), or phenobarbital (1.8 to 3 mg/kg body weight for 1 week, then 2.7 to 4.5 mg/kg body weight, per os, e very 12 hours for 2 weeks). Blood samples taken over a 6-week period were a ssayed for serum tT4, fT4, and TSH. Phenobarbital therapy in our study did not affect serum tT4, fT4, or TSH concentrations. Prednisone therapy, howev er, significantly decreased serum tT4 and fT4, but did not affect serum TSH concentrations.