Prolactin as a marker of dopaminergic activity at different levels of thyroid function in man

To investigate the hypothesis of an altered dopaminergic activity in hypothyroidism, seven patients without thyroid tissue were studied by means of three consecutive tests: an iv bolus of TRH (200 micrograms); a continuous iv infusion (5 mg during 30 min) of metoclopramide (MCP); and a second, post-MCP, iv bolus of TRH (200 micrograms). The study was performed three times: (A) without treatment; (B) on the 15th day while on L-T4 (150 micrograms i.d.); and (C) on the 30th day with the same treatment. Each time was a different situation of thyroid function; on the basis of basal serum TSH (P less than 0.001, A vs B vs C). The response of PRL to the first (non-primed) TRH, expressed as the sum of increments in ng/ml (mean +/- SE), was significantly higher in A (659 +/- 155) than in C (185 +/- 61). Individual PRL responses correlated with circulating T3 (P less than 0.02), but not with T4. A significant increase of PRL occurred after MCP in the three situations, but there were no differences among them. Likewise, the responses to the second (MCP-primed) TRH showed no differences. Although there was an expected high correlation (P less than 0.001) between basal TSH and circulating thyroid hormones, the maximal response of TSH to both non-primed and MCP-primed TRH was in B. After MCP, no measurable increase of TSH could be demonstrated at any of the three levels of thyroid function. These results do not support the hypothesis of an altered dopaminergic activity in hypothyroidism.(ABSTRACT TRUNCATED AT 250 WORDS)