A prolactinoma is a benign (non-cancerous) adenoma (tumour) of the pituitary gland that excessively produces a hormone called prolactin. Tumours under 10mm in diameter are called microprolactinoma and tumors above 10mm in diameter are referred to as macroprolactinoma. A prolactinoma is the most common type of pituitary tumour. Symptoms are related to, too much prolactin in the blood or pressure from the tumour on surrounding tissue. Higher prolactin levels are found in pregnant women as prolactin stimulates the mammary glands to produce milk. After childbirth, the mother's prolactin levels fall, unless she breast feeds. Then, prolactin levels rise to maintain milk production. It's not known what causes prolactinomas, but they are the commonest type of hormone producing, pituitary tumour. Prolactinomas come in various sizes, but the majority are less than 10mm in diameter and in many cases don't show any symptoms. The symptoms that do occur depend on the sex of the patient and the size of the tumour. With larger tumours, patients may have pressure symptoms such as headaches or visual problems (this is because the nerves to the eyes pass over the top of the pituitary gland). In a small number of patients, an enlarged pituitary may cause pressure on these nerves and produce visual disturbance. Prolactinomas can usually be treated effectively with medication, rather than surgery. Blood tests and an MRI or CT scan are used to diagnose prolactinomas. Throughout treatment an Endocrinologist will monitor the patients progress.