Background. By definition, a microadenoma (seen in the image below) is a tumor less than 10 mm in diameter. Pituitary adenomas may secrete hormones, but most are clinically inactive.Many pituitary lesions are discovered while investigating other neurologic problems; these lesions are called incidentalomas.RI showing a nonenhancing area in the pituitary consistent with a microadenoma in a patient with hyperprolactinemia. The microadenoma may be discovered during an investigation for the cause of a clinically diagnosed hypersecretory syndrome of hyperprolactinemia, acromegaly, or Cushing syndrome.