It has been postulated that dopamine, the metabolite of L-dopa, may play a role in the regulation of secretion of anterior pituitary hormones. However, many investigators reported that L-dopa had no effect on circulating TSH levels in humans and experimental animals. In attempt to study hypothalamo-pituitary system in thyrotoxic patients, the effect of L-dopa on serum TSH levels was tested.
A single oral administration of L-dopa failed to show discernible changes of serum TSH in euthyroid subjects and in untreated thyrotoxic patients. Serum TSH response to synthetic TRH was also negligible in thyrotoxic patients, while a significant increase in serum TSH was observed in euthyroid subjects. Exaggerated TSH response to TRH, associated with elevated basal TSH levels, was shown in patients with hypothyroid state during antithyroid drug therapy for hyperthyroidism. All patients with elevated basal TSH levels responded to L-dopa with 40% drop from the basal level. A similar phenomenon was also observed with a drip infusion of L-dopa.
These observations suggest that hypothalamo-pituitary system in thyrotoxic patients is maintained intact and provide evidence that dopamine affect TSH secretory mechanism, particularly in a situation in which TSH secretion is stimulated.
