Primary tumours of the pituitary gland are, with rare exceptions, adenomas arising from the adenohypophysial cells and they account for about 10% of clinically treated intracranial tumours. They can be characterized histologically by immunocytochemical staining and they often result in the excessive secretion of one or more hormones, giving rise to particular clinical disorders. It also appears that small, clinically unrecognized pituitary tumours are very common, since the incidence of solitary adenomas in unselected postmortem examinations is as great as 27%, of which 41% stain for prolactin (Costello 1936, Burrow et al. 1981).
The largest group of clinically hypersecretory adenomas are prolactin-secreting tumours, many of which are microadenomas (i.e. <10 mm when they are detected), followed by somatotrophic and then corticotrophic tumours. Gonadotrophic tumours with hypersecretion which is clinically apparent are uncommon, and thyrotrophic adenomas are rare (Smallridge 1987), amounting to 0.2% in one large surgical series (Wilson 1984).