Prolactin is a hormone produced primarily by the pituitary gland, a small organ located at the base of the brain. Although historically recognized for stimulating milk production, prolactin is present in both men and women and has functions extending far beyond reproduction. Measuring the level of this hormone in the blood, known as a serum prolactin test, is a common diagnostic tool. This test helps medical professionals investigate various health issues, providing insight into the complex workings of the endocrine system.
What Prolactin Does and How It Is Measured
Prolactin’s most well-known role is the development of mammary glands and the initiation of milk production after childbirth. It also plays a role in regulating the reproductive system in both sexes, influencing ovarian and testicular function. Furthermore, prolactin affects immune modulation and is involved in aspects of metabolism, including glucose homeostasis and fat metabolism.
The pituitary gland’s release of prolactin is primarily controlled by the inhibitory neurotransmitter dopamine, which suppresses its secretion. A serum prolactin test is typically ordered when a patient presents with symptoms such as unexplained infertility, irregular menstrual cycles, or spontaneous milky nipple discharge (galactorrhea). To obtain an accurate reading, the blood sample is often drawn three to four hours after waking, as prolactin levels naturally fluctuate and are highest shortly after sleep.
Preparation for the test usually involves avoiding intense exercise, fasting, or high stress, as these factors can temporarily elevate levels. Normal ranges differ significantly: non-pregnant women generally have levels below 25 nanograms per milliliter (ng/mL), while men typically have levels below 20 ng/mL. Levels exceeding these upper limits indicate hyperprolactinemia.
Symptoms and Causes of Elevated Prolactin
Hyperprolactinemia disrupts the balance of sex hormones, leading to various symptoms. In women, high prolactin often interferes with the menstrual cycle, causing irregular periods (oligomenorrhea) or absence of periods (amenorrhea). This hormonal imbalance can also result in infertility, decreased libido, and vaginal dryness. The most common symptom is galactorrhea, the production of breast milk outside of pregnancy or breastfeeding.
Men experiencing hyperprolactinemia often present with symptoms related to suppressed testosterone production. These include erectile dysfunction, decreased libido, and infertility. Over time, men may also observe physical changes such as enlarged breast tissue (gynecomastia) and a reduction in body hair and muscle mass. In both sexes, chronic hyperprolactinemia may contribute to bone density loss.
The causes of elevated prolactin are broadly categorized into physiological, pharmacological, and pathological factors. Physiological elevations are temporary and can be triggered by stress, intense exercise, sleep, or chest wall irritation (e.g., a shingles rash or injury). Pharmacological causes include certain prescription medications, notably antipsychotics, specific antidepressants, some high blood pressure medications, and opioids. These drugs interfere with dopamine’s inhibitory action, causing prolactin levels to rise.
The most common pathological cause is a prolactinoma, a noncancerous, prolactin-producing tumor of the pituitary gland. These tumors are classified by size as microprolactinomas (smaller than one centimeter) or macroprolactinomas (larger than one centimeter). Other pathological conditions leading to high prolactin include chronic kidney disease, liver disease, and primary hypothyroidism. Large tumors near the pituitary, even if not producing prolactin, can also compress the pituitary stalk, disrupting the flow of inhibitory dopamine and causing secondary hyperprolactinemia.
Managing and Treating High Prolactin
Treatment for hyperprolactinemia is tailored to the underlying cause and the severity of symptoms. If medication is the cause, the first step is often adjusting the dosage or switching to an alternative drug, though this requires careful consideration of the original condition being treated. If the elevation is due to a systemic condition like hypothyroidism, treating that primary disorder with hormone replacement therapy can often normalize prolactin levels.
For hyperprolactinemia caused by a prolactinoma, medical therapy with dopamine agonists is the standard first-line treatment. These medications mimic dopamine’s action, effectively suppressing prolactin production and often causing the tumor to shrink significantly. Cabergoline and bromocriptine are the two most common agents used, with cabergoline often favored due to its higher efficacy and better tolerability.
Prolactin levels typically return to normal within weeks of starting dopamine agonists, resolving symptoms and restoring reproductive function. Surgery is usually reserved as a second-line option for patients who cannot tolerate medication side effects or whose tumors do not respond to medical therapy. This trans-sphenoidal surgery involves removing the tumor through the nasal passages. Routine monitoring is required to track prolactin levels, assess tumor size, and evaluate for complications like bone loss.