Postoperative evaluation of dopaminergic tone in prolactinoma patients. II. Plasma thyrotropin response to metoclopramide.

In 24 patients who had a PRL-secreting pituitary adenoma, diagnosed by pituitary dynamic function tests and CT scan, and confirmed at surgery, the TSH response to a dopamine (DA)-antagonist drug, metoclopramide (MCP), was studied pre- and postoperatively to elucidate whether altered DA tone was present and related to hyperprolactinemia. Preoperatively, a TSH response to MCP occurred in 18 patients. Plasma TSH levels did not increase after MCP in 5 patients who had a macroprolactinoma and in 1 patient with a microprolactinoma located just beneath the diaphragm of the sella turcica. Postoperatively, in all patients who had a prolonged clinical remission and normalization of PRL dynamic tests TSH did not respond to MCP (9 of 24 patients). In 4 patients who had normal or borderline PRL levels in the immediate postoperative period, the TSH response to MCP disappeared, but became evident later together with progressive elevation of PRL levels. TSH increases after MCP occurred in all patients who had abnormal PRL levels after surgery, except in 2 patients with a macroprolactinoma infiltrating the neighboring structures. In conclusion, these results confirm the existence of increased DA tone in patients with a prolactinoma. However, the presence of an increased TSH response to DA antagonist drugs could be masked in patients who had large tumors or tumors located just beneath the sellar diaphragm. The TSH test after MCP administration can readily detect increased DA tone in the postoperative period even when PRL levels remain slightly elevated or borderline.