**3. Classification of pituitary adenomas**

Multiple classification systems have been adopted to classify pituitary adenomas. They can be classified either based on functioning status (i.e. secretory and nonsecretory) or on the size of the adenoma (i.e. >1 cm in diameter macroadenoma, <1 cm diameter microadenoma). Other classification systems include pathological findings under light microscopy with hematoxylin and eosin stains (basophilic, acidophilic, or chromophobic), or growth characteristics found on imaging studies (e.g. modified Hardy's classification for suprasellar extension and Knosp classification for parasellar/cavernous sinus extension).

The functional classification is the most widely used. It classifies pituitary adenomas on the hormone secretion status and the resultant endocrinologic manifestation. Functioning adenomas may secrete PRL, GH, ACTH, TSH, or FSH/ LH, and patients usually present with endocrinologic manifestations of endogenous effect of the hyper-secreted hormone. Non-functioning adenomas usually present with mass effect on surrounding neuronal or vascular structures or with pituitary dysfunction due to compression on normal glandular tissue.
