Investigation of choice of hyper-prolactinemia-

Basal, fasting morning prolactin(PRL) levels (normally <20 mg/L) should be measured to assess hypersecretion in hyperprolactinemia. In non-pregnant and non-lactating patients, monomeric prolactin concentrations of 500-1000 mU/L are likely to be induced by stress or drugs, and a repeat measurement is indicated. Levels between 1000 and 5000 mU/L are likely to be due to either drugs, a microprolactinoma or 'disconnection' hyperprolactinaemia. Levels above 5000 mU/L are highly suggestive of a macroprolactinoma. Reference : page 791 Davidson's Principles and practice of Medicine 22nd edition