Effect of prednisone on thyroid gland morphology and plasma thyroxine and triiodothyronine concentrations in the dog.

The effects of multiple IM injections of prednisone (2.2 mg/kg) on thyroid morphology and on plasma thyroxine (T4) and triiodothyronine (T3) concentrations in dogs were determined, and the effects of a single prednisone injection on the same circulating iodothyronine concentrations were assessed. Plasma T4 and T3 concentrations decreased significantly after 3 injections of prednisone were made. However, with a single prednisone injection, there was no significant difference in plasma T4 values between control and treated animals. Starting at 8 hours after a single prednisone injection and continuing through day 2, plasma T3 values were significantly lowered. Prednisone treatment also resulted in significantly more thyroid cytoplasmic colloid droplets per follicular cell (3.19 vs 0.57 in treated and control dogs). Lysosomal hydrolysis of colloid appeared to be inhibited by glucocorticoids in dogs, resulting in an alteration of normal thyroid gland functioning.