The pituitary gland is situated beneath the hypothalamus of the brain, in a depression, called pituitary fossa or sella turcica. The human pituitary comprises anterior and posterior lobes. The anterior pituitary consists of distinct endocrine cell types which produce and secrete the various hormones. The posterior pituitary is formed by the axons and terminals of magnocellular neurosecretory neurons originating in the hypothalamus. Pituicytes (a type of glial cells) surround and support the terminals. The posterior pituitary hormones are synthesized in the hypothalamus, packed into granules, transported down the axons, and released, by exocytosis, into the systemic veins. Anterior Pituitary produces:
- Thyroid Stimulating Hormone (TSH)
- Adreno corticotropic hormone (ACTH)
- Gonadotrophins i.e. Leutinizing hormone, prolactin and growth hormone.
Posterior Pituitary produces:
- Antidiuretic hormone or vasopressin
The pituitary gland is composed almost entirely of cells which make hormones. Thus, a benign tumour of this gland- an adenoma- will often make the same hormone as its cell of origin, but the production of the hormone will not be under the control of the hypothalamus and will usually be produced in excess. Thus, very small tumours in the pituitary, only a few millimeters in diameter, can have extensive effects on the rest of the body. Pituitary adenomas arise predominantly in the anterior pituitary which constitutes about 80% of the pituitary volume. Before the development of immunohistochemistry, the cells in the pituitary were labeled according to their staining properties with various dyes, but this produced confusing names that were not obviously linked to their function (e.g. chromophobe adenoma). These terms may appear in some literature but it is better to refer to the cells by their products (e.g. growth hormones). The effects of pituitary adenomas will be specific to the hormones which they produce.
- Adenomas of growth hormone (GH) producing cells:
- Pituitary gigantism if occurring before puberty when the epiphyseal plates are still open and long bones can grow.
- Acromegaly if after puberty, with disproportionate growth of the bones in the jaw, hands and feet.
- Adenomas of adrenocorticotrophic hormone(ACTH) producing cells
- Cushing’s Syndrome
- Adenomas of prolactin producing cells
- Galactorrhea, amenorrhea, loss of libido, infertility.
- Adenomas of thyroid stimulating hormone (TSH) producing cells
- A rare (1%) cause of hyperthyroidism.
Adenomas of the other hormone producing cells such as follicle stimulating hormone and leutinizing hormone producing cells in the anterior pituitary can occur, but they are much less common than those listed above. Sometimes pituitary adenomas do not produce hormones but they expand within the confined space of the sella turcica and cause pressure atrophy of the remaining pituitary with resultant deficiencies of all the pituitary hormones. This leads to end endocrine organ deficiencies such as hypothyroidism, hypoadrenalism, etc. Since the pituitary gland is confined in the pituitary fossa, with the only space available for expansion being superior to this, then structures above it may be compressed by pituitary adenomas. The optic chiasma lies immediately above the pituitary fossa, so a pituitary adenoma may cause pressure atrophy on this with a resultant defect in the lateral fields of vision- a bitemporal hemianopia.