Thyroxine and triiodothyronine in blood after ingestion of iodinated casein by chicks

White Leghorn cockerels were fed a broiler ration containing 0 (controls), 0.02 or 0.04% Protamone (iodinated casein) either continuously or for 10-14 days after which the 0.02% Protamone ration was switched to control or vice versa. Body growth rate was temporarily suppressed during the first 14 days of feeding 0.02% Protamone. Thyroid weight was promptly depressed during feeding of Protamone and increased more slowly when Protamone was removed from the feed. Only a fleeting increase in oxygen consumption rate was detectable in birds fed either 0.02 or 0.04% Protamone. Serum T4 and T3 promptly rose when Protamone was fed; continued feeding resulted in compensatory adjustments in secretion and perhaps peripheral metabolism of iodohormones within 2-3 days so that a level close to 200 ng. T3 or 2.5 mug T4/100 ml. serum was subsequently maintained.