A growth or cyst arising from the EPITHELIUM, a lining layer of cells on the inside of organs. Although BENIGN, adenomas can, as they enlarge, press on adjacent tissue such as nerves or, in the case of an adenoma of the PITUITARY GLAND, on brain tissue, causing symptoms. Where adenomas arise in ENDOCRINE GLANDS, such as the adrenals, pancreas, pituitary and thyroid, they can provoke excessive production of the hormone normally produced by the gland. If an adenoma is causing or is likely to cause symptoms it is usually surgically removed (see also TUMOUR).