PRL and non-functioning pituitary adenoma: NFPA represents a heterogeneous group of adenomas and may be classified as gonadotropinomas, silent tumors that express only one pituitary hormone [adrenocorticotropin hormone (ACTH); thyroid-stimulating hormone (TSH), prolactin and growth hormone (GH)], multiple pituitary hormones (silent adenoma subtype 3) or no hormone (null cell) based on hormone expression on immunohistochemical examination.