Evidence for endogenous dopaminergic regulation of thyrotropin (TSH) secretion in man

In order to clarify the dopaminergic influence on TSH secretion in normal subjects, the effects of a dopaminergic agent, L-dopa, and an anti-dopaminergic agent, sulpiride, on TSH secretion were studied in 28 normal subjects. Improving a commercially available radioimmunoassay (RIA) kit, we developed a high sensitivity RIA method for human TSH (assay sensitivity 0.15 microU/ml; 50% B/Bo value 3.47 microU/ml). Plasma TSH levels decreased significantly after an oral administration of dopamine agonist, L-dopa, and the mean percent decrease of plasma TSH was significantly greater in male than in female. Dopamine antagonist, sulpiride, induced a significant increase in plasma TSH in female, but a slight elevation in male. The mean percent increase in plasma TSH with sulpiride was significantly greater in female than in male. These results suggest that dopamine exerts physiologically tonic inhibition on TSH secretion and the dopaminergic tone on TSH secretion is higher in female than in male.