A prolactinoma is a tumor (adenoma) of the pituitary gland that produces the hormone prolactin. It is the most common type of functioning pituitary tumor. Symptoms of prolactinoma are due to abnormally high levels of prolactin in the blood (hyperprolactinemia), or due to pressure of the tumor on surrounding brain tissue and/or the optic nerves. Based on its size, a prolactinoma may be classified as a microprolactinoma (<10mm diameter) or a macroprolactinoma (>10mm diameter).

The most common symptoms at the time of diagnosis often differ between males and females. Females tend to experience more symptoms related directly to abnormal high prolactin levels, such as inappropriate lactation (occurring outside of nursing or recent childbirth) or an absence of menstruation. By contrast, males are more likely to initially present with symptoms of the tumor compressing their brain tissue, such as headaches and vision changes, as well as exhibit fertility issues and hormonal disruptions such as a loss of libido.

The symptoms due to a prolactinoma are broadly divided into those that are caused by increased prolactin levels and those caused directly by the mass itself. Those that are caused by increased prolactin levels are:

Those that are caused directly by the mass itself are:

A prolactinoma can cause infertility, temporary or permanent, in both male and female patients. It is one of the most common causes of infertility in women, with some sources estimating as much as 20% of female infertility being caused by hyperprolactinemia. It represents a smaller proportion of male infertility. However, among people who have prolactinomas, males are more likely to experience infertility, as their tumors are often more aggressive and more frequently cause hypogonadism.

Prolactinoma is most frequently a sporadic tumor, occurring in people without any concrete risk factors or family members with similar conditions. In a minority of cases, genetics predispose the patient to having one or more pituitary gland tumors during their lifetime. Most cases of prolactinoma have no known causes or risk factors.

Though most pituitary tumors are sporadic, some genetic syndromes include increased risk for pituitary adenomas including Multiple endocrine neoplasia type 1 (caused by a mutation in the MEN1 gene), multiple endocrine neoplasia type 4 (MEN4 gene), Carney complex, and Familial isolated pituitary adenoma (FIPA). Despite their frequent association with genetic syndromes that cause multiple cancers in affected body tissues, the large majority of prolactinomas are monoclonal in origin (originating from a single cell developing a random mutation), even in cases where the tumor begins producing multiple distinct hormones aside from prolactin.

A micro-prolactinoma usually does not grow to become a macro-prolactinoma, and also does not often become metastatic. As such, it is considered a benign tumor, even if it causes symptoms due to excess prolactin. However, it is still recommended that patients with known microprolactinomas should receive an MRI and visual field assessment every 6 to 12 months, to detect unexpected progression and expansion of the tumor before it becomes an emergency.