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Prolactin (PRL) and thyrotropin (TSH) serum levels were determined in a group of young female heroin addicts treated in hospital with methadone in tapered doses. At admission, from 24 to 3 h after the last dose of 'street' heroin, basal PRL values were 21 +/- 2.6 ng/ml, i.e. significantly higher (P less than 0.001) than in controls: 9.8 +/- 0.7; there was no significant differences of basal TSH values between heroin addicts and normal subjects: 2.5 +/- 0.2 vs. 2.2 +/- 0.1 mu units/ml, respectively. The day after admission, when methadone treatment was begun, the PRL and TSH pituitary response to thyrotropin releasing hormone (TRH) 200 micrograms i.v. in 12 patients was studied. The PRL response was significantly reduced (P less than 0.001) in heroin addicts vs. controls, the mean values of highest percent increments over basal were 368 +/- 41.8 and 847 +/- 80.7, respectively. The TSH response in the two groups was similar, the mean highest per cent increment over basal 393 +/- 53.4 in heroin addicts vs. 367 +/- 39.2 in controls. The increase of the PRL basal concentration and its decreased pituitary response following TRH were probably related to a change in the tuberoinfundibolar dopaminergic system (TIDA) because of a chronic intake of opiates whereas the impaired function of the dopaminergic system did not alter the thyrotropin pituitary secretion.