Pituitary tumour. Coloured Magnetic Resonance Imaging (MRI) scan of a sagittal section through the brain showing a large pituitary adenoma (or tumour). At centre is the tumour (blue), to the left of the brainstem (yellow) which leads into the spinal cord. The tumour has enlarged upwards causing distortion of the brain (at top); this may affect the optic nerve and cause vision defects. The pituitary gland is the master endocrine gland, secreting a number of hormones. A pituitary tumour can cause an imbalance in hormone secretion, such as found in acromegaly or Cushing's syndrome. Pituitary tumours are rare comprising about 10% of primary brain tumours, and most are benign.