The effect of amprolium and of payzone on the functional state of the thyroid in growing chickens.

Amprolium fed to growing chickens in a concentration of 125 p.p.m. in a commercial diet was accompanied by a small but significant decrease in thyroid activity compared with the results for untreated chickens, judged by serum PBI values. The epithelium : colloid relation was lowered. This was presumed to be a consequence of the antagonistic effect of amprolium on thiamine. Payzone feeding, 10 p.p.m. in commercial feed, caused a more pronounced retardation of thyroid activity, judged by the same criteria. A possible connection between the depressed thyroid function and the well-established improvement in weight gain and feed conversion in connection with payzone feeding is discussed.