ABSTRACT
The effect of some environmental factors namely: temperature and hydration on thyroid activity was studied in an experimental model using albino rat. The parameters used for the assessment of such an activity were: protein bound iodine [PBI], free thyroxine [T[4]] and total cholesterol in serum. The levels of PBI and T[4] in serum were higher in winter than in summer and this demonstrates clearly the role of the thethyroid gland in calorigenesia. PBI and T[4] levels were also higher with hydration. Cholesterol levels were higher in summer than in winter and on dehydration than hydration. The increase in T[4] was accompanied by a decrease in cholesterol levels in serum and vice versa. T[4] seems to play a role in sterol removal from blood. This investigation demonstrates the importance of environmental factors in evaluating thyroid activity and the present data may be useful as a standard for further studies on this animal