Pituitary adenoma can be broken down - “adeno” refers to a gland and “oma” refers to a tumor, so pituitary adenoma is a tumor that develops in the hormone-producing cells of the pituitary gland.
Normally, the pituitary is a pea-sized gland, hanging by a stalk from the base of the brain.
It sits just behind the eyes near the optic chiasm, which is where the optic nerves cross.
The anterior pituitary, which is the front of the pituitary gland, contains a few different types of cells, each of which secretes a different hormone.
The largest group of cells are the somatotropes which secrete growth hormone, or GH for short, which goes on to promote tissue and organ growth.
The second largest cell group are the corticotrophs which secrete adrenocorticotropic hormone, or ACTH for short.
ACTH stimulates the adrenal glands to secrete cortisol, a hormone that controls the stress response and metabolic regulation.
A smaller cell group are the lactotrophs which secrete prolactin.
Prolactin stimulates breast milk production, and also inhibits ovulation, which is when an egg cell is released from the ovary, and inhibits spermatogenesis, which is the development of sperm cells.
There are also thyrotrophs which are cells that secrete thyroid stimulating hormone, or TSH which goes on to stimulate the thyroid gland.
And finally, there are also gonadotrophs which secrete two gonadotropic hormones - luteinizing hormone, or LH, and follicle-stimulating hormone, or FSH, both of which go on to stimulate the ovaries or testes.
In pituitary adenomas, one of these cells mutates and becomes neoplastic, meaning that it starts dividing uncontrollably and over time it forms a tumor.
But these cells don’t invade neighboring tissues, so this is considered a benign tumor rather than a malignant one.
Pituitary adenomas can be classified by their size, adenomas smaller than 1cm are called microadenomas, and those larger than 1cm are called macroadenomas.
Macroadenomas are more likely to compress surrounding structures like the meninges, which is the protective layer overlying the brain that typically causes pain when it’s stretched.
Macroadenomas can also compress optic nerves as they cross at the optic chiasm.
That can affect a person’s ability to view things that are in the temporal visual field of both eyes, also called “bitemporal hemianopia”.
Finally, the compression can also affect other healthy pituitary cells and interfere with their ability to make hormones.
Pituitary adenomas that secrete hormones are called functional adenomas, whereas those that don’t are called non-functional adenomas.
Functional pituitary adenomas are divided into a few different types depending on the cells that they arise from and the hormone these cells produce.
The most common type of pituitary adenoma is a prolactinoma which arises from lactotrophs that make prolactin.
In women, excess prolactin causes amenorrhea, which is when there is loss of menstrual bleeding and galactorrhea, which is a milky nipple discharge.
In men, excess prolactin causes a low libido - a low sex drive and gynecomastia or breast enlargement.
The second most common type of pituitary adenoma arises from somatotropes that make growth hormone.