Prolactin

Prolactin is a key biological component secreted from the pituitary gland that regulates milk production. An increase in its level in the blood may lead to menstrual irregularities, ovulation problems and infertility. Clinical evaluation determines whether the level is of physiological or pathological origin.

The clinical approach explaining the causes of elevated prolactin examines pituitary adenoma, thyroid disorders and drug effects under separate headings. Each factor affects hormone levels differently and differential diagnosis requires detailed laboratory analysis.

The diagnostic methods used in prolactin measurement are standardized by controlling fasting status and reducing stress factors. Repeated measurements are important in eliminating transient elevations and confirming persistent disorders.

Prolactin treatment strategies include dose adjustment of dopamine agonists, management of underlying diseases and regular monitoring of hormone levels. These approaches both provide symptom control and contribute to the rebalancing of reproductive functions.

What Is Prolactin?

Prolactin is a hormone secreted from the pituitary gland that regulates various hormonal functions, primarily milk production. In women, it initiates and maintains milk production after birth, while in men it also affects hormonal balance. High prolactin levels may lead to problems such as menstrual irregularities, infertility, loss of sexual desire and nipple discharge. It is evaluated by measuring its level in the blood.

How Does Our Body Keep Prolactin Levels Under Control?

The control mechanism of prolactin is quite different and unique compared to other pituitary hormones. While most hormones are produced under the influence of “accelerating” (releasing) signals from the hypothalamus in the brain, prolactin is, on the contrary, kept under control by a constant and powerful “brake” system.

The main component of this brake system is a brain chemical called dopamine. Nerve cells in the hypothalamus continuously send dopamine to the pituitary gland. Dopamine binds to special receptors (D2 receptors) on the surface of prolactin-producing cells in the pituitary and strongly suppresses both the production and secretion of prolactin.

Problems related to high prolactin (hyperprolactinemia) are fundamentally based on impairment of this brake system. Any condition that disrupts dopamine production, blocks its transport to the pituitary (for example, a mass compressing the pituitary stalk) or blocks the effect of dopamine at the cellular level (for example, some psychiatric drugs) will impair this brake. When the brake system is deactivated, prolactin-producing cells start to work uncontrollably and blood prolactin levels rise.

What Are the Natural Stimuli That Increase Prolactin?

Although the dominant control mechanism is this brake system, certain conditions can temporarily override it and naturally increase prolactin secretion.

The main physiological (natural) stimuli that increase prolactin secretion are:

• Breastfeeding (nipple stimulation)
• Pregnancy (due to high estrogen)
• Stress (emotional or physical)
• Intense exercise
• Sleep
• Sexual intercourse
• TRH (thyrotropin-releasing hormone)

The connection with TRH is particularly important in clinical practice. When the thyroid gland underfunctions (hypothyroidism), the body increases TRH production to stimulate the thyroid. However, this increased TRH also stimulates prolactin cells as a side effect and raises prolactin levels.

What Are the Functions of Prolactin Beyond Breastfeeding?

Although its name is associated with “milk” (pro-lactation), this role represents only a small part of prolactin’s biological activities. Prolactin is a multifunctional hormone with profound effects on reproduction, metabolism, the immune system and even behavior. It is known to have more than 300 different functions in the body.

In terms of reproduction, prolactin is not only a hormone that causes problems when it is high. On the contrary, physiologic levels of prolactin are absolutely essential for female fertility.

This requirement has been clearly demonstrated in experimental animal models lacking the prolactin gene (PRL-/-), which are completely infertile. The reason is that prolactin plays a vital role in maintaining the corpus luteum, which forms after ovulation and produces progesterone, a hormone that is critical for the continuation of pregnancy.

This reveals a crucial distinction in clinical practice: pathologically and chronically elevated prolactin levels are anti-fertility, whereas physiologic, rhythmic prolactin secretion is fundamentally pro-fertility.

What Are the Causes of High Prolactin (Hyperprolactinemia)?

Hyperprolactinemia, defined as blood prolactin levels above the reference range, is a common endocrine disorder. To make an accurate diagnosis, a wide range of possible causes must be considered.

The causes of elevated prolactin can be grouped into three main categories.

Physiological (Natural) Causes:

• Pregnancy
• Breastfeeding
• Stress (including blood-draw stress)
• Intense exercise
• Sleep

Pathological (Disease) Causes:

• Prolactinoma (a benign prolactin-secreting pituitary tumor)
• Other pituitary masses (do not secrete prolactin but disrupt the dopamine brake)
• Primary hypothyroidism (underactive thyroid)
• Chronic kidney failure
• Chronic liver failure (cirrhosis)
• Polycystic ovary syndrome (PCOS)
• Chest wall disorders (herpes zoster, surgical scars, trauma)

Pharmacological (Drug-Related) Causes:

• Antipsychotic drugs (e.g., risperidone, haloperidol)
• Anti-nausea drugs (especially metoclopramide)
• Certain antidepressants
• Certain antihypertensives (methyldopa)
• Opioids (strong painkillers)
• Estrogen-containing medications

How Is High Prolactin Diagnosed Step by Step?

When we are faced with suspected high prolactin, we follow a logical, stepwise approach to accurately identify the underlying cause.

There is an algorithm used to reach the correct diagnosis:

• Step 1: Initial Blood Test. Prolactin level is measured in the blood at a time away from acute stress (it can be done at any time of day).
• Step 2: Confirmation and Clinical History. If elevation is confirmed, a detailed medical history is taken. In particular, pregnancy, breastfeeding, thyroid-related symptoms (fatigue, feeling cold, weight gain) and all medications used (prescribed or over the counter) are reviewed.
• Step 3: Secondary Laboratory Tests. To rule out other common systemic causes, TSH (thyroid test) is always ordered. Kidney and liver function tests are evaluated if necessary.
• Step 4: Pituitary Imaging. If physiological causes, medication use and conditions such as hypothyroidism are excluded, the next step is imaging of the pituitary gland. The gold standard for this is contrast-enhanced pituitary MRI (magnetic resonance imaging). MRI clearly shows whether there is a prolactinoma or another mass compressing the stalk.
• Step 5: Special Tests. In some confusing situations, special tests (PEG precipitation or dilution studies) may be requested to rule out misleading laboratory results such as macroprolactin or the “hook effect”.

What Does the Blood Prolactin Value Tell Us?

The degree of elevation of the prolactin level can provide valuable clues about the underlying cause.

• Normal Level: Generally below 25 µg/L (ng/mL) in non-pregnant women.
• Mild to Moderate Elevation (25–200 µg/L): Diagnostically the most challenging range. It can be caused by almost any factor, including stress, medications, hypothyroidism, PCOS, “stalk effect” or a small microadenoma (prolactinoma).
• Marked Elevation (>200–250 µg/L): These levels very strongly suggest a prolactin-secreting pituitary tumor (prolactinoma).
• Very High Levels (>500 µg/L): Such high levels almost certainly indicate a large tumor (macroadenoma, >1 cm).

A very critical clinical distinction exists here: normally, there is a correlation between tumor size and prolactin level (large tumor = very high prolactin).

If an MRI shows a large mass (e.g., 2 cm), but the prolactin level is only moderately elevated, such as 80 µg/L, this discrepancy suggests that the mass is not a prolactinoma but most likely a non-functioning adenoma that does not produce prolactin and increases prolactin indirectly by compressing the pituitary stalk and disrupting the dopamine brake (“stalk effect”).

This distinction is vital because their treatments are completely different: a true prolactinoma is primarily treated with medication, whereas a large, non-functioning adenoma with a stalk effect generally requires surgical removal.

Can Prolactin Tests Give Misleading Results?

Yes, there are two important laboratory conditions that, if not recognized, can lead to misdiagnosis and unnecessary treatments: macroprolactinemia and the hook effect.

• Macroprolactinemia (Big Prolactin)

In this case, a large portion of the prolactin circulating in the blood is in the form of a large, biologically inactive complex called “macroprolactin” (often bound to an antibody). Because this large molecule is cleared slowly from the body, it accumulates in the blood. Standard laboratory assays measure this inactive molecule as “prolactin,” so the prolactin level appears elevated on the lab report.

However, because this molecule cannot bind to receptors and exert effects at the tissue level, the person does not show typical symptoms of high prolactin (such as menstrual irregularities or milk production). This condition should be considered especially in patients with elevated prolactin levels but completely regular periods or no symptoms at all.

Macroprolactinemia is a benign condition and does not require treatment. It is detected by a simple laboratory test called PEG precipitation. Failure to recognize this condition may lead to unnecessary MRI scans and years of unnecessary medication.

• Hook Effect

This is essentially the opposite problem of macroprolactinemia: extremely high prolactin levels appearing “normal” or only mildly elevated in the laboratory result.

This is a test artifact and most often occurs in patients with giant prolactinomas. The prolactin concentration in the blood is so extremely high (in the thousands) that it saturates the assay system and interferes with proper measurement. As a result, the device reports a value that is much lower than the true level.

The hook effect is suspected when MRI shows a very large pituitary tumor, but the prolactin level is unexpectedly low relative to the size of the tumor. The way to overcome this problem is to repeat the assay in the laboratory after diluting the blood sample (for example, 1:100). This reveals the true level and prevents the patient from being mistakenly diagnosed with a “non-functioning adenoma” and sent for surgery, instead allowing proper diagnosis as a giant prolactinoma and initiation of medical therapy.

How Does High Prolactin Cause Infertility in Women?

High prolactin is one of the most important and treatable causes of female infertility. Approximately one-third of women with amenorrhea (absence of periods) have hyperprolactinemia as an underlying cause. High prolactin undermines fertility in three different ways:

• Central (Brain) Effect: Suppression of Ovulation

This is the most fundamental and powerful effect. Chronically elevated prolactin levels disrupt the normal pulsatile secretion of GnRH, the main reproductive command hormone in the hypothalamus. When the GnRH rhythm is disturbed, secretion of FSH and LH from the pituitary is also suppressed. Without adequate FSH and LH, the ovaries cannot develop follicles properly and ovulation does not occur. This leads to menstrual irregularities, anovulation and, in severe cases, complete absence of periods (amenorrhea).

• Direct Ovarian Effect: Impairment of Egg Quality

In addition to central suppression, there is evidence that pathologically elevated prolactin levels may have direct adverse effects on the ovary. It is thought that high prolactin may impair the normal function of the cells surrounding the egg, interfering with follicular development and reducing oocyte competence (the ability to be fertilized and to form a healthy embryo).

• Uterine (Endometrial) Effect: Impaired Implantation

A healthy embryo alone is not enough for a successful pregnancy; a receptive uterine lining (endometrium) is also needed. High prolactin may impair implantation by both suppressing ovarian estrogen production (leading to inadequate endometrial thickening) and directly disrupting the process of decidualization, in which the endometrium becomes receptive to the embryo.

How Does High Prolactin Affect Male Infertility?

Although high prolactin is less common in men than in women, it is an important and treatable cause of infertility and sexual dysfunction. The pathophysiology is quite similar to that in women: central suppression of the reproductive axis.

• Elevated prolactin levels suppress GnRH secretion from the hypothalamus in men as well. This leads to decreased LH and FSH secretion from the pituitary.
• Low LH: Fails to adequately stimulate Leydig cells in the testes, leading to decreased testosterone production (secondary hypogonadism).
• Low FSH: Fails to adequately stimulate Sertoli cells, resulting in impaired spermatogenesis.

The fact that this condition is treatable is extremely important. When prolactin levels are normalized with medication, the suppression of GnRH is lifted, testosterone and sperm production resume, and fertility can be restored.

What Symptoms and Sperm Problems Does High Prolactin Cause in Men?

Hormonal disturbances caused by high prolactin in men manifest with both clinical symptoms and impaired semen parameters.

The clinical picture is largely due to low testosterone (hypogonadism). The most common complaints are:

• Decreased sexual desire (low libido)
• Erectile dysfunction
• Infertility
• Low energy and fatigue
• Reduced muscle mass
• Breast enlargement (gynecomastia) (less common)
• Milk discharge from the breast (galactorrhea) (very rare)

If the underlying cause is a macroadenoma (large tumor), neurological symptoms may also occur:

• Chronic headaches
• Visual disturbances (especially bitemporal visual field defects due to pressure on the optic chiasm)

The adverse effects on semen parameters include:

• Oligospermia (reduced sperm count)
• Azoospermia (no sperm in the ejaculate)
• Asthenospermia (reduced sperm motility)
• Teratospermia (abnormal sperm morphology)

How Is High Prolactin Treated in Those Who Desire Fertility?

When high prolactin is detected in a patient who wishes to have children, our main goals are to normalize prolactin levels, restore normal reproductive function (ovulation or sperm production) and, if a prolactinoma is present, shrink the tumor and relieve any mass effects.

In almost all symptomatic cases of elevated prolactin, including prolactinoma, the first-line treatment is medical therapy. Medications called dopamine agonists are used for this purpose.

These drugs bind to dopamine receptors in the pituitary and mimic the effect of the body’s own dopamine (the brake system). Thus, they send a “stop” signal to prolactin-producing cells. In 80–90% of patients they are highly effective in normalizing prolactin levels and restoring reproductive function. In addition to suppressing secretion, they also cause prolactinoma cells to shrink, resulting in a marked reduction in tumor size.

Which Drugs Are Used in Prolactin Treatment?

There are two main drugs most commonly used in clinical practice: cabergoline and bromocriptine.

Cabergoline:
A newer, more potent drug that is more effective in normalizing prolactin levels.

• More successful in reducing tumor size.
• Has a long duration of action, so taking it once or twice a week is sufficient.
• Side effects such as nausea and dizziness are less common than with bromocriptine.

Because of these advantages, it is generally the first-choice medication today.

Bromocriptine:

• An older drug, but still effective.
• Shorter duration of action, so it needs to be taken two or three times a day.

It may cause side effects such as nausea and dizziness more frequently.

Its most important advantage is that there is decades of safety data for its use in pregnancy. Therefore, it remains an important option in patients who are planning pregnancy or who need to continue treatment during pregnancy.

How Is Drug- or Thyroid-Related High Prolactin Managed?

If high prolactin is caused by a medication (for example, a psychiatric drug), the ideal management is to discuss the situation with the prescribing physician and, if possible, discontinue the drug or switch to an alternative medication with a lower risk of raising prolactin. If the drug must be used and cannot be changed, a low dose of a dopamine agonist (such as cabergoline) may be added.

If the elevation is due to primary hypothyroidism, dopamine agonists are definitely not the treatment. The treatment is to replace the missing thyroid hormone (levothyroxine). When thyroid function normalizes, excessive TRH stimulation resolves and prolactin levels spontaneously return to normal.

When Are Surgery or Radiotherapy Needed for Prolactin Disorders?

Given how effective medical therapy is, surgery and radiotherapy are considered only in very specific circumstances.

Surgery (usually via a transsphenoidal approach through the nose) is required in the following situations:

• Resistance to medical therapy (prolactin levels do not fall)
• Severe side effects leading to intolerance of medication
• Rapidly progressive visual field loss despite medical treatment
• Pituitary apoplexy (sudden hemorrhage into the tumor)
• Large non-functioning masses causing a stalk effect and elevated prolactin

Radiotherapy is a third-line option and is reserved for aggressive tumors that do not respond to medication or surgery.

Why Is Prolactin Important Before IVF?

In patients with chronic, clinically significant hyperprolactinemia who are planning IVF treatment, it is standard practice to normalize prolactin levels before starting treatment. Dopamine agonists are used to restore prolactin to normal and, once the hormonal environment is corrected, ovarian stimulation is begun.

However, it is crucial to distinguish chronic, pathological hyperprolactinemia from the transient prolactin elevation that occurs during IVF treatment.

In IVF cycles, we use high-dose injections to stimulate the ovaries. These injections cause the growth of many follicles. All of these growing follicles produce high amounts of estrogen. Estrogen is a strong stimulator of prolactin secretion.

As a result, transient, physiological increases in prolactin levels during IVF cycles due to rising estrogen levels are very common and expected. This is a completely different phenomenon from chronic prolactin disease prior to treatment.

Does Rising Prolactin During IVF Affect Success?

The significance of prolactin levels during IVF cycles is still a complex and much-debated topic in medicine. Although we clearly know that chronic elevation is harmful, it remains uncertain whether transient elevations during treatment are harmful or perhaps even beneficial.

This brings to mind the concept of the “Goldilocks hypothesis” or “U-shaped curve”: pathologically very low or chronically very high prolactin levels are detrimental, but maintaining prolactin within a certain physiologic range during follicular development may be “just right” and associated with the best outcomes.

Some studies suggest that prolactin in follicular fluid may play a positive role. Higher prolactin concentrations in follicular fluid have been reported to be associated with better oocyte quality and maturation.

However, the evidence is inconsistent. Some studies have found no relationship between transient prolactin elevations and IVF success, while others have suggested that extremely high prolactin levels on trigger day (e.g., >90 ng/mL) may reduce implantation rates.

This complex evidence suggests that the clinician’s goal is not to “zero out” prolactin, but rather to optimize its beneficial physiological effects.

Is There a Special Use of Prolactin Drugs in IVF Protocols?

Routinely suppressing the transient, physiological increase in prolactin caused by rising estrogen during ovarian stimulation in IVF is controversial and not standard practice.

However, a much more innovative and targeted use of dopamine agonists has been described in a specific group of patients known as “poor responders” who have had previous unsuccessful IVF attempts.

This strategy is called the “Bromocriptine Rebound Protocol”. The aim of this protocol is not to treat high prolactin per se, but to “reset” the prolactin signaling system.

The protocol is as follows:

• Bromocriptine is started in the second half (luteal phase) of the cycle preceding ovarian stimulation.
• The drug is abruptly stopped about 7 days before starting stimulation injections.

The underlying hypothesis is that in some poor responders, the cells surrounding the oocyte may have become desensitized to the beneficial effects of prolactin on maturation. Administering bromocriptine beforehand “silences” these receptors, and when the drug is abruptly discontinued, a rebound effect makes the cells hypersensitive to prolactin. As a result, they respond more strongly and physiologically to the natural prolactin increase during stimulation. In the study describing this method, patients were reported to achieve higher rates of good-quality embryos and higher live birth rates.

Can Patients with Prolactinoma (Tumor) Become Pregnant and How Is Pregnancy Managed?

Yes, patients with a prolactinoma diagnosis can safely become pregnant. However, the process requires careful planning and a multidisciplinary approach, depending on tumor size (micro or macro). The main concern is that rising estrogen levels during pregnancy may stimulate tumor growth.

Pre-Pregnancy Preparation:

• First, prolactin levels are normalized with a dopamine agonist (usually cabergoline) and ovulation is restored.
• A baseline pituitary MRI is obtained before pregnancy to document tumor size and its relationship to the optic pathways.
• If a macroadenoma (>10 mm) is present, the aim is to reduce tumor size with medical therapy before allowing pregnancy, thereby creating a “safety margin” even if some growth occurs during pregnancy.

Management During Pregnancy:

• Once pregnancy is confirmed, management varies according to initial tumor size.
• Microadenoma (<10 mm):
• The dopamine agonist (cabergoline or bromocriptine) is usually discontinued immediately.
• The risk of clinically significant tumor growth during pregnancy is very low (<5%).
• Routine MRI or visual field testing is not required.
• The patient is monitored clinically for symptoms such as severe headache or visual disturbances.

If symptoms occur, MRI is performed and bromocriptine, which is considered safe in pregnancy, may be restarted.

Breastfeeding is generally considered safe.

• Macroadenoma (>10 mm):
• The risk of symptomatic enlargement is higher (20–30%).

Whether to discontinue or continue medication is decided on an individual basis. Depending on tumor location and pre-pregnancy shrinkage, some patients may continue bromocriptine during pregnancy.

Close monitoring is essential. Formal visual field testing every trimester is recommended.

If symptoms or visual field changes occur, MRI is obtained and bromocriptine is started or the dose is increased.

Breastfeeding is generally safe.

An important note: measuring prolactin levels during pregnancy is meaningless. In normal pregnancy, prolactin levels rise physiologically to very high levels, and this increase is not correlated with tumor growth.

What Should Be Done When High Prolactin Is Found Incidentally?

If high prolactin is detected incidentally in a patient (for example, during a routine test while menstrual cycles are regular or for another reason) and the patient has no symptoms, the first and most crucial step is to rule out macroprolactinemia.

If PEG precipitation reveals that the elevation is due to inactive macroprolactin, the finding has no clinical significance and does not require treatment or further work-up (such as MRI).

If the incidental elevation represents true hyperprolactinemia (for example, due to an incidental microadenoma), but the patient is asymptomatic and does not desire fertility, simple observation (follow-up without treatment) is a valid option.

How Does Stress-Related High Prolactin Affect IVF?

Being diagnosed with infertility and undergoing a challenging treatment process such as IVF is, in itself, a major psychological and physical stressor. Therefore, transient stress-related elevations of prolactin are very common during treatment.

The clinical significance of this stress-related prolactin elevation for IVF outcomes remains one of the most debated topics in the medical literature. Some studies have suggested that high stress levels may negatively affect fertilization rates or embryo quality, whereas other well-designed studies have found no significant correlation between psychological stress levels before or during IVF and final pregnancy outcomes.

Our approach as clinicians should be to distinguish acute, physiologic stress responses from chronic, pathological hyperprolactinemia that chronically suppresses the reproductive system. While the latter absolutely requires medical treatment to achieve fertility, reflexively treating every mild prolactin elevation in a stressed IVF patient with dopamine agonists is likely unnecessary and represents over-medicalization of a normal physiological response.

A more rational strategy is to acknowledge the emotional burden of treatment and recommend stress-management techniques such as meditation, yoga or counseling to help reduce anxiety. Even if their direct impact on pregnancy rates is unproven, these interventions undoubtedly improve overall well-being and quality of life during this difficult process.

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