What is the most likely cause of hyperprolactinaemia in a patient showing symptoms of galactorrhoea, decreased libido, and amenorrhoea?

Hyperprolactinaemia is characterized by elevated levels of prolactin in the blood, which can lead to symptoms such as galactorrhoea (inappropriate lactation), decreased libido, and amenorrhoea (absence of menstruation). One of the most common causes of hyperprolactinaemia, especially in the context of the symptoms described, is a prolactin-secreting pituitary tumor, also known as a prolactinoma.

Prolactinomas are benign tumors of the pituitary gland that produce excess prolactin, leading to the notable clinical manifestations of galactorrhoea due to increased prolactin levels, as well as menstrual irregularities and decreased libido due to the hormone's effects on the reproductive axis.

While hypothyroidism can also lead to elevated prolactin levels due to increased thyrotropin-releasing hormone (TRH), which stimulates prolactin secretion, it is less commonly the sole cause of the specific symptoms when a prolactin-secreting tumor is more likely, especially given the presentation of galactorrhoea. Stress can lead to temporary elevations in prolactin but is not typically the primary or most significant cause. Lastly, pregnancy is a