Thyrotropin alpha- and beta-subunit responses to thyrotropin-releasing hormone and domperidone in normal subjects and in patients with microprolactinomas

Microprolactinoma is a particular pathological situation characterized by the presence of increased hypothalamic dopaminergic tone reactive to tumoral hyperprolactinemia. Since dopamine (DA) is a physiological regulating factor of the secretion of thyroid-stimulating hormone (TSH), we investigated the responses of serum TSH (holo-TSH) and its subunits (alpha-subunit: alpha-sub, and TSH-beta) to thyrotropin-releasing hormone (TRH) and domperidone (DOM; an antidopaminergic drug acting outside the blood-brain barrier) in 36 euthyroid subjects (20 controls and 16 patients with microprolactinoma) in order to evaluate the possible in vivo effects of DA excess on TSH subunit secretion. No significant difference in serum TSH increase after TRH (200 micrograms i.v.) was observed between patients with microprolactinoma and controls (TSH net incremental area under the curve, nAUC: 146 +/- 9, mean +/- SE, and 143 +/- 7.7 micrograms/l/60 min, respectively), while serum alpha-sub and TSH-beta responses were markedly reduced in patients with microprolactinoma as compared to those found in normals (alpha-sub nAUC: 3.0 +/- 0.5 vs. 19.8 +/- 2.2 micrograms/l/60 min, p less than 0.001; TSH-beta nAUC: 5.0 +/- 0.8 vs. 9.5 +/- 0.9 micrograms/l/60 min, p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)