Pituitary adenomas are benign, glandular tumors of the pituitary gland. They are pretty common tumors (they account for about 10% of all intracranial neoplasms). Most are clinically silent for years, until they get big enough to cause endocrine abnormalities or mass effects.
Endocrine abnormalities occur when pituitary adenomas secrete hormones. For some reason, pituitary adenomas occur almost exclusively in the anterior pituitary – so the hormones they may secrete include prolactin, growth hormone, thyroid-stimulating hormone, follicle-stimulating hormone, luteinizing hormone, and adrenocorticotropic hormone. Most pituitary adenomas come to clinical attention this way (with symptoms of hyperpituitarism). The converse is also true: most patients with hyperpituitarism have a pituitary adenoma.
Some pituitary adenomas do not secrete hormones at all, though (or secrete them so poorly that no significant endocrine symptoms are produced). These adenomas come to clinical attention when they reach a size that causes mass effects in the brain. Such effects may include symptoms of increased intracranial pressure (nausea, vomiting and headache).
The most characteristic mass effect symptom, though, is loss of vision in the lateral visual fields (called “bilateral hemianopsia”). The pituitary sits in its little sella turcica surrounded on both sides by the optic nerves. A growing pituitary adenoma, like the massive one in the image above, will compress the medial portions of the optic nerves first – which, as you may recall from neuroanatomy, supply the lateral visual fields. So the first thing to go in the patient’s vision will be lateral vision. If the adenoma continues to grow, the visual loss will expand to include the medial (nasal) visual fields.
Microscopically, pituitary adenomas are usually composed of sheets or cords of uniform, polygonal cells. Most are composed of a single cell type (acidophil, basophil, or chromophobe). To find out what hormones (if any) the adenoma is producing, you need to do special immunohistochemical stains. Some adenomas can be pleomorphic and have increased mitotic activity, which is annoying to the pathologist, since that makes them look more like malignant tumors.
The most common hormone produced by pituitary adenomas is prolactin. Prolactin-producing adenomas usually secrete prolactin efficiently enough to cause symptoms early on (especially in women, who notice menstrual cycle changes, like amenorrhea). Other symptoms of prolactin excess include loss of libido and infertility.
The least common type of pituitary adenoma is a TSH-producing adenoma (these account for only about 1% of all pituitary adenomas). Other types of pituitary adenomas include GH-producing adenomas (which cause gigantism if they occur prior to puberty or acromegaly if they arise in the post-pubertal period), ACTH-producing adenomas (which cause Cushing disease), and FSH- and LH-producing adenomas (which usually secrete hormone at such low levels that they come to attention because of mass effects, rather than endocrine symptoms).
So, if you have a patient with hyperpituitarism or with bilateral hemianopsia, the first thing you should consider is a pituitary adenoma. Imaging studies will give you an idea of what’s going on in the pituitary – but of course, pathologic examination of the tissue is definitive.