text: Histological classification utilizes an immunohistological characterization of the tumors in terms of their hormone production.  Historically they were classed as either basophilic, acidophilic, or chromophobic on the basis of whether or not they took up the tinctorial stains hematoxylin and eosin. This classification has fallen into disuse, in favor of a classification based on what type of hormone is secreted by the tumor. Approximately 20–25% of adenomas do not secrete any readily identifiable active hormones ('non-functioning tumors') yet they are still sometimes referred to as 'chromophobic'.
  Functional classification is based upon the tumors endocrine activity as determined by serum hormone levels and pituitary tissue cellular hormone secretion detected via immunohistochemical staining. The "Percentage of hormone production cases" values are the fractions of adenomas producing each related hormone of each tumor type as compared to all cases of pituitary tumors, and does not directly correlate to the percentages of each tumor type because of smaller or greater incidences of absence of secretion of the expected hormone. Thus, non secretive adenomas may be either null cell adenomas or a more specific adenoma that, however, remains non-secretive.
  Any type of pituitary adenocarcinoma listed in the table below may cause compressive symptoms due to local expansion in addition to the systemic effects of secreted hormones listed in the pathology column.
  Null cell adenomas by definition do not secrete hormones, but they commonly cause compressive effects on the pituitary stalk (stalk effect). This leads to decreased levels of dopamine from the hypothalamus reaching the anterior pituitary gland. Dopamine exerts an inhibitory effect on prolactin secretion. With the absence of this inhibitory effect, prolactin levels increase and are often increased in null cell adenomas. This leads to symptoms of hypogonadism.