Secondary Resistance to dopamine agonist after thirteen months of successful treatment in a 42 years old man.

Prolactinomas are the most common hormone-secreting pituitary tumors and represents approximately 40% of all pituitary adenomas. Pharmacology therapy with dopamine agonists is the mainstay treatment for prolactinomas. Usually, patients respond to these drugs with decreased level of the serum prolactin and with time, by tumor shrinkage. Less than 10% of patients with prolactinomas exhibit resistance to the action of dopamine agonists, as defined by the lack of normalization of the serum prolactin levels despite long- term treatment at high doses of these drugs. However secondary resistance to dopamine agonists therapy has also been described in patients who were initially responsive to treatment, either with Cabergoline or Bromocriptine but later develop dopamine agonist resistance, with elevated prolactin levels and sometimes an enlarging tumor volume several years afterwards. We report a case of a male patient with prolactinoma who developed drug resistance 13 months after initial dopamine agonist therapy.