ObjectiveTo determine the time it takes for prolactin (PRL) levels to increase after initiation of therapy with haloperidol(a neuroleptic medication) and the pattern and extent of the increase.MethodsSeventeen individuals scheduled to be given treatment with neuroleptic drugs were enrolled. Baseline blood specimens were collected for PRL and TSH levels. Follow-up samples for PRL determinations were obtained every 3 days for 18 days after initiation of haloperidol therapy.ResultsThere were 17 subjects: 14 women and three men. Two of the subjects were found to have hyperprolactinemia due to hypothyroidism and were excluded from the basic study. The 15 remaining subjects all had normal baseline PRL levels and normal TSH values. The PRL levels in all 15 showed a similar pattern: a rapid increase in PRL levels in the first 6–9 days, followed by a plateau that remained, with minor fluctuations, throughout the study. The highest meanpeak level of PRL was 37.4 ng/mL and the maximum was 77 ng/mL. The two individuals with hypothyroidism had some-what elevated baseline PRL levels; although they showed the same initial pattern of increase after haloperidol administration, their PRL levels reached values well above 100 ng/mL.ConclusionThere is a distinct pattern of response of PRL to haloperidol. The PRL level increases for 6–9 days, then plateaus, the peak being between 30 and 50 ng/mL, and always remains below 77 ng/mL. In our study, levels that continued to increase, or increased above 77 ng/mL, indicated the presence of hypothyroidism. The patterns and levels of the increase were uninfluenced by the therapeutic dose of the medication given. Given that the patients in our study with elevated levels of TSH had such high levels of PRL, all patients should have TSH determinations at the initiation of therapy with neuroleptic medications such as haloperidol.