Hyperprolactinemia: This is the most common cause of galactorrhea, often due to a prolactinoma, which is a benign tumor of the pituitary gland. Elevated prolactin levels can lead to milk production not associated with childbirth or nursing.

Hypothyroidism: Can cause an increase in TRH (thyrotropin-releasing hormone), which in turn can stimulate prolactin release, leading to galactorrhea.
Medications: Certain drugs such as antipsychotics, metoclopramide, and some antidepressants can cause hyperprolactinemia as a side effect.
Idiopathic: In some cases, no specific cause for galactorrhea can be found, and it is considered idiopathic.

Pituitary Macroadenoma: Although less common than microadenomas (prolactinomas), macroadenomas can cause significant symptoms due to their size and potential to compress surrounding structures, including the optic nerves.
Cranial Lesions: Other lesions in the brain, such as craniopharyngiomas or meningiomas, can affect the pituitary stalk and lead to hyperprolactinemia.
Hypothalamic Disease: Diseases affecting the hypothalamus can disrupt normal pituitary function, leading to hyperprolactinemia.

Acromegaly: Caused by excess growth hormone, often due to a pituitary adenoma, which can co-secrete prolactin.
Tuberous Sclerosis: A genetic disorder that can cause a variety of tumors, including those in the pituitary gland, leading to hyperprolactinemia.
Neurofibromatosis: Type 1 neurofibromatosis can be associated with optic gliomas and other central nervous system tumors that might indirectly cause galactorrhea through effects on the hypothalamic-pituitary axis.