Diagnosis and drug therapy of prolactinoma

Abstract

A prolactin-secreting pituitary tumour is the most frequent cause of hyperprolactinaemia that commonly occurs in clinical practice. Prolactinomas occur more frequently in women than in men and may differ in size, invasive growth and secretory activity. At presentation, macroadenomas are more frequently diagnosed in men. Specific immunohistochemical stains are necessary to prove the presence of prolactin in the tumour cells. The main investigations in the diagnosis of a prolactin-secreting adenoma are hormonal and radiological. As prolactin is a pulsatile hormone, it is a general rule to obtain several blood samples by taking a single sample on 3 separate days or 3 sequential samples (every 30 minutes) in restful conditions. Prolactin levels of 100 to 200 micrograms/L are commonly considered diagnostic for the presence of a prolactinoma; however, prolactinoma cannot be excluded in the presence of lower levels, and prolactin levels > 100 micrograms/L are present in some patients with idiopathic hyperprolactinaemia. Several dynamic function tests have been proposed to differentiate idiopathic from tumorous hyperprolactinaemia. Although they could be used for group discrimination, these tests cannot be used for individual patients. To differentiate between a prolactinoma and a pseudoprolactinoma, thyrotrophin response to a dopamine receptor antagonist may be used, as only prolactinomas may have an increased response. A short course of dopaminergic drugs may also be of some help, as in macroprolactinomas only a shrinkage may be observed. After hyperprolactinaemia is confirmed, imaging with computerised tomography (CT) and magnetic resonance imaging (MRI) are necessary to define the presence of a lesion compatible with a pituitary tumour. There is now a general agreement that medical therapy is of first choice in patients with prolactinomas. Bromocriptine, the most common drug used in this condition, is a semisynthetic ergot alkaloid that directly stimulates specific pituitary cell membrane dopamine D2 receptors and inhibits prolactin synthesis and secretion. In most patients, a reduction or normalisation of prolactin levels is usually observed, together with the disappearance or improvement of clinical symptoms. The sensitivity to bromocriptine is variable and patients may need different dose of the drug. Bromocriptine is also able to shrink the tumour in most patients; however, a few reports of disease progression during therapy have been described. The need for close follow-up, including prolactin levels and CT or MRI studies, is therefore emphasised. Bromocriptine is conventionally given in 2 or 3 daily doses; however, a single evening dose has been shown to be equally effective. Bromocriptine is usually well tolerated by the majority of patients; some adverse effects (nausea, vomiting, postural hypotension) may be initially present, but they usually wear off in time. To prevent such adverse effects it is advisable to start treatment with a low dose during the evening meal and gradually increase the dose over days or weeks. A few patients are unable to tolerate oral bromocriptine, so different formulations of bromocriptine or alternative dopamine agonist drugs (lisuride, terguride, metergoline, dihydroergocryptine, quinagolide, cabergoline, pergolide) have been proposed. Of particular clinical relevance because of their good tolerability and sustained activity are cabergoline and quinagolide. Particular attention should be paid to pregnancy in prolactinoma patients, as tumour enlargement has been reported. As the risk for this occurrence is low in patients with microprolactinoma, there is a general agreement that the drug can be stopped once pregnancy is diagnosed. In patients with macroprolactinoma the risk of tumour enlargement is higher. Therefore, primary therapy with bromocriptine until the tumour has shrank is suggested before pregnancy is attempted. Bromocriptine should be stopped as soon as pregnancy is confirmed, but re