Pituitary thyroid interrelationship in rats exposed to different environmental temperatures

The present work intended to show interrelationships between triiodothyronine (T₃), thyroxine (T₄), thyroid stimulating hormone (TSH) and cyclic adenosine 3'5' monophosphate (cyclic AMP) in both the pituitary and the thyroid. Experiments were carried out on animals exposed to 0 and 37°C for 4 days and to 34°C for 21 days. Control animals were maintained at 22°C. T₄ serum levels of 2.6, 4.0, 1.1 and 0.42 μg/100 ml serum, and T₃ levels of 95, 135, 65 and 36 ng/100 ml serum were recorded at 22, 0, 34 and 37°C, respectively. The observed serum TSH levels were 265, 192, 237 and 182 μg/100 ml serum in rats exposed to 22, 0, 34 and 37°C, respectively. Pituitary TSH content was similar at 22, 0 and 34°C whereas the content in the 37°C exposed rats was twice that of controls. Thyroid cyclic AMP levels were not significantly different among the various experimental groups. The content of cyclic AMP in the pituitary was higher at 34°C by 125% and 37°C by 240%, and lower at 0°C by 56% with regard to the control. These results suggest that the 0 and 34°C exposed rats reach a new steady state in the control of thyroid hormones secretion rate. However, at 37°C, the TSH thyroxine interrelationships seem to break down.