The influence of thyroid disorders on the dopaminergic regulation of prolactin, thyrotropin and growth hormone

In order to clarify if hyper- and hypothyroidism change by feed-back mechanisms the dopaminergic controlled release of PRL, TSH and GH, the serum values of these hormones were measured before and following iv administration of 5 mg metoclopramide in 10 hyperthyroid, 11 euthyroid and 10 primary hypothyroid age-matched females, all consecutively investigated. The secretion pattern, as well as the quantitated response (area under the curve - AUC) of PRL were identical for the three groups, and uninfluenced by thyroid status. By contrast the TSH responses (AUC) were significantly and positively correlated to the basal TSH, suggesting that the effect of metoclopramide was dependent on the secretory capacity of the thyrotropic cells. The serum GH level was found to decrease in all three groups following metoclopramide, probably due to the inhibition of release. Stimulation of the same subjects with 200 micrograms TRH iv resulted in response curves of serum PRL and TSH, which were significantly and positively correlated to the basal serum TSH. The serum values of GH increased following TRH in the hypothyroid group, while the values of the hyperthyroid were depressed and unchanged. The present results suggest that the dopaminergic tonus on PRL, TSH and GH secretion is unaffected by thyroid feed-back mechanisms. The TRH-induced release of each of the three hormones is, however, dependent on thyroid status.