Avian muscular dystrophy: serum thyroid defect and limited improvement with methimazole and propylthiouracil.

Serum concentrations of triiodothyronine (T3) and thyroxine (T4) were determined by radioimmunoassay in normal and genetically related muscular dystrophic chicks at 2 through 42 days ex ovo. There were no significant differences in T4 concentrations, but T3 concentrations were reduced about 35% below normal values in dystrophic birds at 14 to 42 days. The situation was reversed, however, on day 2, with T3 concentrations about 50% greater in dystrophic than in normal serum. Administration of T3 beginning on day 2 ex ovo did not alter phenotypic expression of dystrophic signs. Administration of the thyroid "antagonists," methimazole and propylthiouracil, however, significantly increased righting ability and reduced serum creatine kinase activity in dystrophic chicks. None of the administered substances improved the histopathology of dystrophic pectoralis major muscles. The data indicate that serum T3 concentrations may provide an early "marker" for avian dystrophy, and suggest that lowered serum T3 concentrations in older chicks may represent a compensatory response to the elevated serum T3 in newly hatched dystrophic chicks.