ABSTRACT
Effects of long term administration of iopanoic acid (IOP), a potent inhibitor of peripheral conversion of thyroxine (T4) into triiodothyronine (T3), on body weight and gonad development in intact and in thyroidectomized (Thx) redheaded bunting that received replacement therapy with T4 were studied. IOP prevented the premigratory increase in body weight observed in intact bunting (during March/April). In contrast to the Thx birds receiving T4 only, IOP administration in combination with T4 caused a significant decrease in body weight of Thx birds. The gonad development in intact and Thx birds that received IOP was significantly inhibited. Results suggest that IOP through an effective inhibition of peripheral T4-monodeiodination may prevent the spring premigratory fattening. Emphasis is given for an important role of T3 in the physiological preparations associated with migration.