Prolactin hormone , production , various diseases and treatment

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Prolactin: Production, Regulation, Diseases, and Treatment

1. What Is Prolactin?

Prolactin (PRL) is a 198-amino-acid single-chain polypeptide hormone with 3 internal disulfide bridges, structurally related to growth hormone. It is synthesized and secreted by lactotrophs in the anterior pituitary gland. Lactotrophs make up ~15-20% (and up to 50% during pregnancy) of anterior pituitary cells, making them the most common cell type and the most likely to give rise to pituitary tumors.

Costanzo Physiology 7th Ed., p.416
Goldman-Cecil Medicine, p.2407

2. Production and Chemistry

Prolactin is produced exclusively in the anterior pituitary lactotrophs. Stimuli increase or decrease prolactin secretion by altering transcription of the prolactin gene:

TRH (a stimulant) increases prolactin gene transcription
Dopamine (an inhibitor) decreases prolactin gene transcription

A biologically inactive variant called macroprolactin ("big prolactin" or "big big prolactin") forms when prolactin dimerizes with itself or complexes with circulating IgG. These variants are immunoreactive but biologically inactive - they can cause false elevations on assay without clinical significance.

Costanzo Physiology 7th Ed., p.416

3. Regulation of Prolactin Secretion

Prolactin regulation is unique among pituitary hormones - it is predominantly under inhibitory (tonic) control by dopamine.

Regulation of prolactin secretion - Hypothalamus, Dopamine, TRH, Anterior Pituitary, Breast axis

Fig. 9.12 from Costanzo Physiology 7th Ed. - Regulation of prolactin secretion. TRH = Thyrotropin-releasing hormone.

Inhibitory pathway (dominant)

Dopamine (prolactin-inhibiting factor, PIF) from the hypothalamus reaches the anterior pituitary via three routes:

Hypothalamic dopaminergic neurons secrete dopamine into the median eminence → hypothalamic-hypophysial portal vessels → anterior pituitary (major route)
Dopaminergic neurons of the posterior pituitary → short connecting portal veins
Non-lactotroph anterior pituitary cells secrete dopamine in a paracrine manner

Dopamine acts through D2-type receptors on lactotrophs, decreasing cAMP levels.

Stimulatory pathway

TRH (thyrotropin-releasing hormone)
VIP (vasoactive intestinal peptide)
Estrogen (stimulates prolactin gene transcription)

Negative Feedback

Prolactin itself stimulates dopamine synthesis and secretion from the hypothalamus, constituting a classic negative feedback loop.

Factors Affecting Prolactin Secretion

Stimulatory	Inhibitory
Pregnancy (estrogen)	Dopamine
Breast-feeding / suckling	Bromocriptine (dopamine agonist)
Sleep	Somatostatin
Stress	Prolactin itself (negative feedback)
TRH
Dopamine antagonists (drugs)

Costanzo Physiology 7th Ed., p.417

4. Actions of Prolactin

Breast development - at puberty, with estrogen and progesterone, stimulates proliferation and branching of mammary ducts; during pregnancy, promotes growth and development of mammary alveoli
Lactogenesis (milk production) - the primary action; stimulates synthesis of lactose, casein, and lipids in response to suckling. During pregnancy, high estrogen/progesterone inhibit lactation; their postpartum decline permits lactation
Suppression of ovulation - prolactin suppresses GnRH, and thereby LH and FSH, providing a physiologic basis for lactational amenorrhea
Maternal behavior - supports bonding and caregiving behavior
Inhibition of reproductive function - in both sexes, excess prolactin suppresses gonadotropins

Normal levels: <20 pg/mL in women; <10-15 pg/mL in men.

Goldman-Cecil Medicine, p.2407

5. Diseases Related to Prolactin

A. Hyperprolactinemia

The most clinically significant prolactin disorder. Present in <1% of the general population but up to 14% of women with abnormal menses.

Causes of Hyperprolactinemia (with prolactin ranges)

Category	Examples	Mechanism	PRL range (pg/mL)
Antiemetics	Metoclopramide, domperidone, prochlorperazine	Dopamine receptor blockade	20-70
Antipsychotics	Haloperidol, fluphenazine; paliperidone, risperidone	Dopamine receptor blockade	20-70
Cyclic antidepressants	Clomipramine	Unknown	20-70
Narcotics	Methadone, morphine	Opioid mu-receptor activation	20-70
Estrogen	Oral contraceptives	Stimulates PRL gene transcription	20-70
Stress	-	Hypothalamic	20-70
Pregnancy	-	Estrogen	20-500
Pituitary adenoma	Microprolactinoma	-	20-250
Pituitary adenoma	Macroprolactinoma	-	200->10,000
Hypothyroidism	Primary hypothyroidism	Increased TRH	Moderate; <100
Chronic renal failure / cirrhosis	-	Decreased clearance + central effects	Moderate; <100
Chest wall injury / nipple stimulation	-	Unknown	20-70
Hypothalamic lesions	Structural/infiltrative disease, non-secreting pituitary adenoma	Decreased dopamine delivery	Usually <100

Goldman-Cecil Medicine, Table 205-8

Key point: Drug causes and hypothalamic lesions cause modest hyperprolactinemia (<100 pg/mL). Prolactinomas commonly elevate levels 5 to 50 times higher (>100 pg/mL), and macroprolactinomas can reach >10,000.

Clinical Features

In premenopausal women:

Galactorrhea (milk discharge outside lactation)
Oligomenorrhea / amenorrhea
Infertility, decreased libido, vaginal dryness (from secondary estrogen deficiency)
Osteopenia (direct prolactin effect on bone + indirect via estrogen suppression)

In men (often detected late because symptoms are subtle):

Decreased libido, impotence
Reduced muscle mass, increased fat mass (from low testosterone)
Gynecomastia and rarely galactorrhea (when estrogen co-elevated, e.g. cirrhosis)
Osteopenia

In both sexes (large tumors):

Headaches
Visual field defects (bitemporal hemianopia from optic chiasm compression)

In children:

Rare (<2-3% of intracranial tumors); usually macroadenomas causing neurologic symptoms, delayed puberty, or growth retardation
Goldman-Cecil Medicine, p.2408

B. Prolactinoma

The most common cause of pathological hyperprolactinemia and the most common pituitary adenoma type.

Microprolactinoma: <10 mm; PRL 20-250 pg/mL
Macroprolactinoma: ≥10 mm; PRL 200->10,000 pg/mL; more likely to cause mass-effect symptoms
Giant prolactinoma: very large macroadenomas; rare; a 2023 systematic review of 196 cases found these require careful multimodal management

Note: When MRI shows a large pituitary mass with only modest prolactin elevation (<100), it is likely a non-secreting adenoma compressing the stalk (stalk-effect hyperprolactinemia), NOT a prolactinoma.

C. Prolactin Deficiency

Less common and clinically less significant
Can occur in hypopituitarism (with other pituitary hormone deficiencies)
Seen in hyperthyroidism (TRH suppressed → less prolactin stimulus)
Main consequence: failure of lactation in nursing mothers
No cases of isolated prolactin deficiency have been reported in men
Goldman-Cecil Medicine, p.2407

D. Macroprolactinemia

An assay artifact rather than true disease. Large prolactin aggregates (bound to IgG) register as elevated PRL but are biologically inactive. The patient has no symptoms. Diagnosed by:

Gel filtration chromatography (gold standard)
PEG (polyethylene glycol) precipitation (simpler, inexpensive alternative)

Recognition prevents unnecessary imaging and treatment.

Textbook of Family Medicine 9e, p.1006

6. Diagnosis

Serum prolactin immunoassay - straightforward; levels >100 pg/mL suggest adenoma
Important pitfall - Hook effect: Very high prolactin (>1000 pg/mL) in large macroprolactinomas can overwhelm the antibody, giving a falsely low result. Always dilute 1:100 alongside undiluted sample
Brain MRI - recommended when PRL >100 ng/mL or when symptomatic (galactorrhea, amenorrhea, visual changes)
Full pituitary hormone panel - essential when a mass is found (check TSH, cortisol, GH/IGF-1, LH, FSH, testosterone/estrogen)
Thyroid function tests - rule out hypothyroidism as a cause
Renal/liver function - rule out systemic causes

Features favoring a true prolactinoma (vs. drug-induced or stalk compression):

PRL >150 ng/mL
Loss of normal sleep-associated PRL rise
No rise in PRL in response to exogenous TRH
Textbook of Family Medicine 9e, p.1006-1007; Goldman-Cecil Medicine, p.2408

7. Treatment

Step 1: Treat reversible causes

Discontinue offending drugs (antipsychotics, metoclopramide, etc.) where safe
Treat hypothyroidism with levothyroxine
Treat renal/hepatic disease

Step 2: Medical Therapy (First-line for prolactinomas)

Dopamine agonists are the first-line treatment for prolactinomas - they reduce prolactin levels AND shrink tumor size:

Drug	Notes
Cabergoline	Preferred first-line; greater efficacy (normalizes PRL in ~90% of patients); better tolerability; twice-weekly dosing. Not preferred in pregnancy.
Bromocriptine	FDA-approved; preferred when pregnancy is desired (more safety data); daily dosing; more side effects (nausea, orthostatic hypotension)
Pergolide	Alternative ergot derivative; not FDA-approved for hyperprolactinemia

Cardiac valve caution: All ergot derivatives carry rare risk of valvular heart damage at very high doses/prolonged use. Consider yearly echocardiogram in patients with known valve disease or on doses >5 mg/week of cabergoline.

Below is a real macroprolactinoma that shrank dramatically after cabergoline (prolactin went from 8000 ng/dL → 15 ng/dL after 6 months):

MRI of a macroprolactinoma after 6 months of cabergoline therapy showing significant tumor reduction

Goldman-Cecil Medicine, Fig. 205-6

Stopping / Reducing Treatment

Dose reduction can be considered after PRL normalization for 1 year + significant tumor reduction
Withdrawal can be considered after 2 years if: normal PRL + MRI shows no tumor or >50% reduction + tumor >5 mm from optic chiasm with no cavernous sinus invasion
Monitor with MRI + PRL at 3 months, 6 months, then yearly after stopping
Macroprolactinomas are more likely to recur than microprolactinomas

Step 3: Surgery (Second-line)

Transsphenoidal resection is indicated when:

Medical treatment fails or is not tolerated
Very large tumors threatening visual pathways
Hemorrhagic infarction (pituitary apoplexy)
Patient preference

Success rates:

Microprolactinomas: 65-85% normalization; ~20% recurrence at 10 years
Macroprolactinomas: 30-40% normalization (only ~30% of macroadenomas can be completely removed)

Step 4: Radiation

Rarely used; reserved for large residual tumor post-surgery that is unresponsive to medical therapy.

Special situation: Pregnancy

Discontinue dopamine agonist once pregnancy confirmed
Monitor for visual symptoms (up to 10% of microprolactinomas and 30% of macroprolactinomas can enlarge during pregnancy)
Bromocriptine preferred over cabergoline if treatment must be restarted during pregnancy (more safety data)
Serial visual field testing each trimester for macroprolactinomas
Textbook of Family Medicine 9e, p.1007; Goldman-Cecil Medicine, p.2408-2409

Summary at a Glance

Feature	Details
Site of production	Anterior pituitary lactotrophs
Primary regulator	Dopamine (inhibitory, tonic)
Secondary stimulator	TRH, estrogen
Main physiologic role	Lactation, breast development
Key disease	Hyperprolactinemia / Prolactinoma
Most common cause of PRL >100	Prolactinoma
Most common drug cause	Antipsychotics / metoclopramide (dopamine blockade)
First-line treatment	Dopamine agonists (cabergoline > bromocriptine)
Second-line treatment	Transsphenoidal surgery

Recent evidence note: A 2024 systematic review (PMID: 38819244) highlights specific management challenges when prolactinomas coexist with psychiatric illness - dopamine agonists conflict with antipsychotic treatment, requiring careful individualization of care.

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