The effect of insulin administration on the hypothalamic-pituitary thyroid axis in the rat (author's transl)

Abstract

The effect of insulin administration on the hypothalamic-pituitary thyroid axis in the rat was studied. Regular insulin (1.0U/100g of body weight) was injected into male Wistar rats weighing 200g After decapitation, blood was collected at various intervals. TRH contents in the hypothalamus (H), and TRH, TSH, T4 and T3 levels in serum were measured by each specific radioimmunoassay. Blood glucose levels were measured by the autoanalyzer method.Before the insulin injection, the TRH contents in H, TRH, TSH, T4 and T3 in serum were 4.2ng, 6pg/ml, 262ng/ml, 5.5μg/dl and 55ng/dl respectively with blood glucose 101mg/ dl. After the insulin injection, the TRH contents in H started to fall at 5min, reached its nadir at 20min, at which the level was 2.5ng, and then returned to initial levels. The TRH levels in serum rose to 47pg/ml at 5min and then returned to initial levels after the insulin injection. The TSH levels in serum increased at 15min, with a peak value (900ng/ml) at 30min after the insulin injection. The T3 levels in serum started to rise at 45min and reached 120ng/dl after the insulin injection. The T4 levels in serum did not change during the observation period.Blood glucose levels decreased rapidly, reached their nadir at 15min and then gradually returned to initial levels after the insulin injection. Pretreatment of 1.0g of glucose did not affect the TRH increase in serum after the insulin injection. Pretreatment of T4, T3 or dexamethasone suppressed the TRH increase in serum after the insulin injection.These data suggest that insulin administration might stimulate TRH release from the hypothalamus and that T4, T3 or dexamethasone might inhibit TRH release in rats.