The effect of exposure to 40 degrees on the heat production and the serum concentrations of triiodothyronine, thyroxine, and corticosterone in immature domestic fowl.

Four-week-old fowl were exposed to 40 degrees for 35 days and the effects on the serum concentrations of triiodothyronine (T3), thyroxine (T4), and corticosterone were investigated. Changes in resting heat production, food intake, body temperature, body weight gain, and growth (both in terms of skeletal growth and weight of selected organs) were also measured. Body temperature was increased at 40 degrees, and body weight gain and skeletal growth were both reduced. The serum concentrations of T3 and T4 were reduced at 40 degrees. The decrease in serum T3 was more strongly correlated with the reduction in food intake than was the reduction in T4. Resting heat production was decreased by exposure to 40 degrees; the reduction in heat production was correlated with serum T3 and serum T4 concentrations. Exposure to 40 degrees had no effect on adrenal weight or on serum corticosterone concentration but weights of the bursa, spleen, and thymus glands were decreased. These results suggest that (a) T3 is the principle metabolically active thyroid hormone in the chicken and that the reduction in heat production at 40 degrees is related to the decline in T3; (b) the reduction in T3 is a consequence of the reduction in food intake; and (c) the response to a stressor need not involve increased adrenocortical activity.