Effects of oral administration of anti-inflammatory doses of prednisone on thyroid hormone response to thyrotropin-releasing hormone and thyrotropin in clinically normal dogs

Abstract

SUMMARY Prednisone was given orally to 12 dogs daily for 35 days at an anti-inflammatory dosage (1.1 mg/kg of body weight in divided dose, q 12 h) to study its effect on thyroxine (T4) and triiodothyronine (T3) metabolism. Six of these dogs were surgically thyroidectomized (THX-Pred) and maintained in euthyroid status by daily SC injections of T4 to study peripheral metabolism while receiving prednisone; 6 dogs with intact thyroid gland (Pred) were given prednisone; and 6 additional dogs were given gelatin capsule vehicle as a control group (Ctrl). Baseline T4 concentration after 4 weeks of treatment was not significantly different in dogs of the THX-Pred or Pred group (mean ± SEM, 2.58 ± 0.28 or 3.38 ± 0.58 μg/dl, respectively) vs dogs of the Ctrl group (2.12 ± 0.30 μg/dl). A supranormal response of T4 to thyrotropin was observed in dogs of the Pred group, but the T4 response to thyrotropinreleasing hormone was normal. Baseline T3 concentration in dogs of both steroid-treated groups was significantly (P< 0.05) lower after 2 and 4 weeks of prednisone administration vs pretreatment values, but normalized 2 weeks after prednisone was stopped. Free T3 (FT3) and T4 (FT4) fractions and absolute FT3 and FT4 concentrations were not altered by prednisone administration. Reverse T3 (rT3) concentration in vehicle-treated Ctrl dogs (26.6 ± 3.5 ng/dl) was not different from rT3 concentration in dogs of the THX-Pred (25.7 ± 4.3 ng/dl) and Pred (28.9 ± 3.8 ng/dl) groups after 4 weeks of medication. These data indicate that daily oral administration of such anti-inflammatory dose of prednisone for 1 month reduces baseline serum T3 concentration, does not alter serum T4 concentration, and enhances thyroidal sensitivity to thyrotropin.