Prolactin: When to Order This Test

What Prolactin Is and Why It Matters

Prolactin is a 199-amino-acid polypeptide hormone secreted by lactotroph cells in the anterior pituitary gland. Its primary physiological role is stimulating mammary gland development and milk production after childbirth, but prolactin receptors exist in nearly every tissue in the body, including the liver, ovaries, testes, and immune cells [1]. The hormone operates under tonic inhibitory control by hypothalamic dopamine; any process that disrupts dopamine signaling can raise prolactin levels.

Outside of lactation, prolactin suppresses gonadotropin-releasing hormone (GnRH) pulsatility. This is why elevated prolactin commonly leads to secondary hypogonadism. Men may experience low testosterone, reduced libido, and erectile dysfunction. Women may develop oligomenorrhea, amenorrhea, or anovulatory infertility. A 2019 retrospective analysis of 2,601 men referred for testosterone evaluation found that 11% had prolactin levels above the upper reference limit, and those with prolactin above 50 ng/mL were significantly more likely to harbor a pituitary adenoma [2].

The test itself is straightforward. A single venous blood draw is all that is required.

Clinical Indications: When to Order

The Endocrine Society's 2011 Clinical Practice Guideline on the Diagnosis and Treatment of Hyperprolactinemia recommends checking serum prolactin in any patient presenting with the following: galactorrhea (spontaneous or expressible nipple discharge), menstrual irregularity without other explanation, infertility in women with oligomenorrhea or amenorrhea, decreased libido or erectile dysfunction in men, or suspected pituitary mass on imaging [3]. The guideline states: "We recommend measuring serum PRL levels in patients presenting with symptoms of hyperprolactinemia or with a sellar mass" [3].

Prolactin testing is also warranted during the workup of secondary hypogonadism in men. The American Urological Association (AUA) 2018 guideline on testosterone deficiency recommends that "prolactin should be obtained in men with low or low-normal gonadotropins and low testosterone" [4]. A prolactin level above 150 to 200 ng/mL in this context strongly suggests a prolactin-secreting pituitary adenoma and should prompt MRI of the sella turcica.

Testing is not recommended as a routine screening tool in asymptomatic patients. The USPSTF has no recommendation for prolactin screening in the general population. Order it when clinical suspicion exists. Do not add it to a wellness panel.

How to Prepare Patients and Collect the Sample

Prolactin secretion follows a circadian rhythm, peaking during sleep and reaching its nadir in late morning [5]. The standard recommendation is a fasting morning draw, ideally between 8 AM and 10 AM, with the patient seated and resting for 15 to 30 minutes prior to venipuncture. Stress, nipple stimulation, vigorous exercise, and a high-protein meal within two hours of the draw can all transiently raise prolactin by 20% to 50% [5].

A single mildly elevated result (25 to 40 ng/mL) should be confirmed with a repeat draw before launching an extensive workup. One study found that 33% of mildly elevated prolactin results normalized on repeat testing when proper collection protocols were followed [6]. If the repeat value remains elevated, the next step is to evaluate for medication effects, hypothyroidism, renal failure, and pituitary pathology.

The assay is a standard immunoassay available at all commercial labs. No special tubes are needed. Just a serum separator (SST) or plain red-top tube.

Normal Prolactin Ranges by Sex and Reproductive Status

Reference ranges vary by assay and laboratory, but commonly cited values from the Endocrine Society and major reference labs align closely [3][7]:

• Non-pregnant women: 2 to 29 ng/mL (some labs use an upper limit of 25 ng/mL)
• Pregnant women: levels rise progressively, reaching 100 to 200 ng/mL by the third trimester
• Postpartum/breastfeeding women: 50 to 200+ ng/mL, depending on nursing frequency
• Men: 2 to 18 ng/mL (some labs extend to 20 ng/mL)
• Children (prepubertal): typically <10 ng/mL

Prolactin is released in a pulsatile fashion, and a single value represents a snapshot. The American Association of Clinical Endocrinologists (AACE) recommends using the same laboratory and the same assay for serial monitoring to avoid inter-assay variability that can be as high as 20% [8].

What High Prolactin (Hyperprolactinemia) Means

Hyperprolactinemia is defined as a sustained prolactin level above the upper limit of the reference range. It is one of the most common pituitary abnormalities, affecting an estimated 0.4% of the general population and up to 9% of women with amenorrhea [3]. The magnitude of the elevation often points to the cause.

Mild elevation (25 to 100 ng/mL): Drug-induced hyperprolactinemia is the most common etiology. Antipsychotics (risperidone, haloperidol) are the most frequent offenders, raising prolactin in 40% to 90% of patients on typical antipsychotics [9]. Metoclopramide, domperidone, and SSRIs also contribute. Hypothyroidism (via TRH stimulation of lactotrophs), renal failure (reduced prolactin clearance), and chest wall irritation round out this range.

Moderate elevation (100 to 200 ng/mL): This zone overlaps between large drug effects and small prolactinomas (microprolactinomas <10 mm). Stalk effect from a non-functioning pituitary adenoma compressing the infundibulum can also produce levels in this range by blocking dopamine delivery [3].

Marked elevation (above 200 ng/mL): A prolactin level exceeding 200 ng/mL is highly suggestive of a macroprolactinoma. In a series of 455 patients with prolactinomas, Colao et al. reported that macroprolactinomas (≥10 mm) had a median prolactin of 640 ng/mL, while microprolactinomas had a median of 66 ng/mL [10].

One clinical pitfall is the "hook effect," in which very high prolactin concentrations (sometimes exceeding 10 to 000 ng/mL) saturate the assay antibodies and produce a falsely normal or mildly elevated result. When a large pituitary mass is present but the prolactin is unexpectedly low, request serial dilution of the sample to unmask the true value [3].

What Low Prolactin Means

Low prolactin receives less clinical attention than high prolactin, but values consistently below 2 to 3 ng/mL may indicate hypopituitarism. Sheehan syndrome (postpartum pituitary necrosis), pituitary apoplexy, or infiltrative diseases like hemochromatosis and sarcoidosis can destroy lactotroph cells and produce prolactin deficiency [11].

A low prolactin level also raises the possibility of excessive dopamine agonist therapy. Patients on cabergoline or bromocriptine for a known prolactinoma should have doses titrated to maintain prolactin in the normal range, not suppressed below it. There is limited evidence that very low prolactin may associate with metabolic syndrome features, although this remains an area of active investigation [12].

In practical terms, if a patient has a prolactin below the lower limit, evaluate the rest of the pituitary axis (ACTH, TSH, LH, FSH, GH/IGF-1) and consider MRI if other deficiencies are present.

How to Lower Prolactin: Treatment of Hyperprolactinemia

The first step is always to exclude physiological and pharmacological causes. Stop or switch the offending medication when clinically safe. Correct hypothyroidism with levothyroxine if TSH is elevated.

For prolactinomas, dopamine agonist therapy is first-line. Cabergoline is preferred over bromocriptine based on a Cochrane systematic review of 2,092 patients across five randomized controlled trials, which found cabergoline normalized prolactin in 85% of patients compared with 59% for bromocriptine, with fewer gastrointestinal side effects [13]. Standard dosing for cabergoline starts at 0.25 mg twice weekly and is titrated every four weeks based on prolactin response, up to 1 to 2 mg twice weekly.

Dr. Shlomo Melmed, a pituitary specialist at Cedars-Sinai and co-author of the Endocrine Society guideline, has written: "Cabergoline is the treatment of choice for most prolactinomas, including macroprolactinomas, because of its superior efficacy, tolerability, and convenient dosing schedule" [14].

Surgery (transsphenoidal adenomectomy) is reserved for patients who are intolerant of or resistant to dopamine agonists, those with CSF leak from tumor apoplexy, or those who prefer definitive treatment. Surgical cure rates are 80% to 90% for microprolactinomas and 30% to 40% for macroprolactinomas in experienced neurosurgical centers [3].

How to Raise Prolactin

In most clinical contexts, raising prolactin is not a therapeutic goal. However, there is one specific scenario. Patients who struggle with lactation after delivery sometimes have insufficient prolactin response. Domperidone (10 mg three times daily) is used off-label in many countries outside the United States to augment milk supply by blocking peripheral dopamine D2 receptors and raising prolactin [15]. The FDA has not approved domperidone for this indication in the U.S. due to cardiac QT-prolongation concerns at higher doses.

Metoclopramide (10 mg three times daily before meals) is an alternative galactogogue that raises prolactin through the same D2-blockade mechanism. A randomized trial of 80 mothers with insufficient lactation showed that metoclopramide increased milk volume by 66% compared to placebo at 10 days [16]. Side effects include drowsiness and, rarely, tardive dyskinesia with prolonged use.

Beyond lactation support, there is no established clinical reason to pharmacologically raise prolactin. If prolactin is low due to hypopituitarism, the treatment target is the downstream deficiency (estrogen, testosterone, cortisol, thyroid hormone), not the prolactin level itself.

Monitoring Prolactin Over Time

After initiating treatment for hyperprolactinemia, the Endocrine Society recommends rechecking prolactin at one month and then every three to six months until stable [3]. For patients on cabergoline for a prolactinoma, MRI of the pituitary should be performed six months after starting therapy and then annually for macroprolactinomas. Microprolactinomas can be imaged less frequently if prolactin normalizes.

Dose reduction or discontinuation of cabergoline may be attempted after at least two years of therapy if prolactin has been normal and the tumor has decreased by ≥50% on MRI. A meta-analysis of 19 studies (N=743 patients) found that 36% of patients with microprolactinomas remained in remission after cabergoline withdrawal, with recurrence most likely within the first 12 months [17].

For drug-induced hyperprolactinemia, monitoring depends on whether symptoms are present. Asymptomatic mild elevation in a patient who requires an antipsychotic does not mandate prolactin-lowering therapy. Symptomatic patients (galactorrhea, bone loss from hypogonadism) should have the antipsychotic switched to a prolactin-sparing agent like aripiprazole or quetiapine when psychiatrically feasible, per the AACE 2020 position statement [8].

Special Populations: Pregnancy, Pediatrics, and Macroprolactin

Pregnancy: Prolactin rises physiologically throughout pregnancy, so the test has limited interpretive value after the first trimester. Women with known microprolactinomas can safely discontinue cabergoline once pregnancy is confirmed, as the risk of clinically significant tumor growth is only 2.6% [3]. Macroprolactinomas carry a 21% growth risk during pregnancy, and these patients require close clinical monitoring with visual field testing each trimester.

Pediatrics: Prolactinomas in children and adolescents are rare but tend to be more aggressive. A large pediatric series reported that 80% of prolactinomas diagnosed before age 18 were macroprolactinomas, compared with 40% in adults [18]. Cabergoline remains first-line in this age group.

Macroprolactin: Some patients have elevated total prolactin due to macroprolactin, an IgG-bound form with limited biological activity. Macroprolactin can account for up to 25% of cases referred for hyperprolactinemia workup [19]. If the patient has elevated prolactin but no symptoms, request macroprolactin screening (polyethylene glycol precipitation) before proceeding with MRI or treatment.

Clinical Decision Points: Putting It All Together

The prolactin test is inexpensive, widely available, and yields high diagnostic value when ordered for the right indications. Order it when symptoms or lab findings suggest pituitary dysfunction or secondary hypogonadism. Do not order it as part of a routine wellness panel. If the first result is mildly elevated, repeat it under proper conditions before pursuing imaging.

Patients with prolactin above 100 ng/mL and no obvious drug cause need pituitary MRI. Patients with prolactin above 200 ng/mL almost certainly have a macroprolactinoma. Patients with confirmed prolactinomas should start cabergoline 0.25 mg twice weekly and have prolactin rechecked in four weeks.