With hypoprolactinemia, hypo- means below, -prolactin refers to the hormone produced by the pituitary gland, and -emia refers to the blood, so hypoprolactinemia means lower than normal prolactin levels in the blood.
Normally, at the base of the brain, there’s a small pea-sized gland called the pituitary gland.
The anterior pituitary - which is the front of the pituitary gland - has a number of different cells, each of which secrete a different hormone.
One group, the lactotroph cells, secrete prolactin.
In men, prolactin decreases testosterone production.
In women, during pregnancy, elevated levels of estrogen stimulate the lactotroph cells to produce large amounts of prolactin which stimulates alveolar cells in the breasts.
In response to prolactin, the alveolar cells divide and enlarge - and once a baby is born, lactogenesis starts - which means that milk is produced.
Apart from milk production, high levels of prolactin also inhibit the release of gonadotropin releasing hormone from the hypothalamus, which results in decreased luteinizing and follicle stimulating hormone levels, which in turn, decreases estrogen levels.
In women, this can stop ovulation and menstruation, which is why women typically don’t have a menstrual period while breastfeeding.
In women that are not pregnant or breastfeeding, as well as in men, prolactin levels are usually kept in check by the hypothalamus in two ways.