The effects of dopaminergic blockade on serum TSH and prolactin levels in thyrotoxicosis.

In the first part of this study, we have demonstrated that, in 7 patients with untreated thyrotoxicosis, a 7 day regime of the long acting dopamine antagonist metoclopramide (10 mg orally 8 hourly) produces more adequate dopaminergic blockade at pituitary level than a single oral 10 mg dose of the compound as assessed by serum prolactin responses. Subsequently, we have employed this protracted oral metoclopramide regime to evaluate the contribution of dopaminergic tone to the abnormal TSH and prolactin responsiveness of thyrotoxicosis. Serum TSH and prolactin responses to iv TRH (200 micrograms) were measured in 10 untreated thyrotoxic patients before and after a 7 day period of metoclopramide 10 mg orally 8 hourly. Ten euthyroid individuals were studied in similar fashion, their serum samples being analysed for prolactin levels alone, thus providing a control group for prolactin responsiveness to TRH, before and after metoclopramide. In the thyrotoxic patients basal TSH levels did not change as a consequence of metoclopramide therapy and the TSH response to TRH remained flat. Basal prolactin levels were similar in thyrotoxic and euthyroid individuals and the increase in prolactin, seen in both groups after metoclopramide, was smaller in the thyrotoxic group than in the euthyroid group. Prolactin responsiveness to TRH was significantly impaired in the thyrotoxic subjects as compared to euthyroid subjects. After metoclopramide there was a significant decline in prolactin responsiveness in the euthyroid group, and a similar, though insignificant, trend in the thyrotoxic patients. We conclude that in thyrotoxicosis dopaminergic tone plays no major part in the suppression of TSH levels, nor in the impaired prolactin responsiveness to TRH.