The diagnosis is generally entertained either on the basis of visual difficulties arising from the compression of the optic nerve by the tumor, or on the basis of manifestations of excess hormone secretion: the specifics depend on the type of hormone. Tumors which cause visual difficulty are likely to be macroadenomas greater than 10 mm in diameter; tumors less than 10 mm are microadenomas.
The most common secretory pituitary tumor is the prolactinoma, which can cause galactorrhea, hypogonadism, amenorrhea, infertility, ...
Pituitary tumors were, historically, classed as basophilic, acidophilic, or chromophobic on the basis of whether or not they took up the 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 (though tumors which do not secrete any active hormone ("non-functioning tumors") are still sometimes called "chromophobic").
At present, classification of pituitary tumors is based on plasma hormone levels or immunohistochemical staining:
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