Carbidopa plus L-dopa pretreatment inhibits the prolactin (PRL) response to thyrotropin-releasing hormone and thus cannot distinguish central from pituitary sites of prolactin stimulation.

Contrary to a previous report, pretreatment of normal men with carbidopa plus L-dopa (Sinemet 25/250) markedly inhibited the PRL response to TRH, a stimulus that acts directly on the pituitary. Thus, the results of carbidopa/L-dopa testing cannot be used to determine whether agents that stimulate PRL secretion act on the pituitary or at a higher central nervous system level.