Unit Converter
Prolactin (PRL)
(Pituitary Hormone for Lactation - Key Marker in Galactorrhea, Infertility, Pituitary Adenomas & Hypothyroidism)
Synonyms
- Prolactin
- PRL
- Lactotropic hormone
- Luteotropic hormone
- Lactotropin
- Pituitary prolactin
- hPRL (human prolactin)
Units of Measurement
- µIU/mL
- mIU/L
- µg/L
- ng/mL
- ng/dL
- ng/100 mL
- ng%
Unit Relationships
ng/mL ↔ mIU/L
Assay-specific, but for most modern assays:
1 ng/mL≈21.2 mIU/L1\ \text{ng/mL} \approx 21.2\ \text{mIU/L}1 ng/mL≈21.2 mIU/L 1 mIU/L≈0.047 ng/mL1\ \text{mIU/L} \approx 0.047\ \text{ng/mL}1 mIU/L≈0.047 ng/mL
µg/L
1 µg/L=1 ng/mL1\ \text{µg/L} = 1\ \text{ng/mL}1 µg/L=1 ng/mL
ng/dL / ng%
1 ng/mL=100 ng/dL1\ \text{ng/mL} = 100\ \text{ng/dL}1 ng/mL=100 ng/dL \text{ng%} = \text{ng/dL}
µIU/mL
Equivalent numerically to mIU/L depending on assay calibration.
Description
Prolactin is a peptide hormone synthesized and secreted by lactotroph cells in the anterior pituitary.
Functions:
- Stimulates milk production postpartum
- Suppresses ovulation by inhibiting GnRH
- Modulates immune function and reproductive behavior
Prolactin secretion is tonically inhibited by dopamine.
Any condition reducing dopamine tone → higher PRL.
Physiological Role
- Initiates and maintains lactation
- Inhibits hypothalamic GnRH → ↓ LH/FSH
- Natural contraception during breastfeeding
- Supports breast development
- Mild immunomodulatory functions
Levels rise:
- During sleep
- Stress
- Pregnancy
- Nipple stimulation
- Hypoglycemia
- Chest wall stimulation
Clinical Significance
HIGH Prolactin (Hyperprolactinemia)
(Most important clinical use)
Major Causes
1. Prolactinoma (Pituitary Adenoma)
- Microadenoma (<10 mm)
- Macroadenoma (>10 mm)
Often causes PRL >200–250 ng/mL.
2. Medications
That block dopamine:
- Antipsychotics (risperidone, haloperidol)
- Antiemetics (metoclopramide, domperidone)
- Antidepressants
- Verapamil
- Methyldopa
- Opiates
3. Hypothyroidism
↑ TRH → ↑ PRL.
4. Pregnancy & Lactation
Normal physiological elevations - often high (100–400 ng/mL).
5. Chest Wall Trauma / Burns / Surgery
6. Chronic Renal Failure
Reduced PRL clearance.
7. Macroprolactinemia
Biologically inactive PRL - causes falsely high results.
Symptoms of Hyperprolactinemia
Women
- Amenorrhea
- Oligomenorrhea
- Galactorrhea
- Infertility
- Low libido
- Hot flashes
Men
- Low testosterone
- Decreased libido
- Erectile dysfunction
- Infertility
- Gynecomastia (rare)
Tumor Mass-Effect Symptoms
If macroadenoma:
- Vision defects (bitemporal hemianopia)
- Headache
LOW Prolactin (Rare)
Causes:
- Pituitary insufficiency (Sheehan syndrome, panhypopituitarism)
- Drugs: dopamine, bromocriptine, cabergoline
- Autoimmune hypophysitis
Usually asymptomatic, except failure of lactation postpartum.
Reference Intervals
(Tietz 8E + Mayo + ARUP + Endocrine Society)
Women
- Non-pregnant: 4.8 – 23.3 ng/mL
- Pregnant:
- 1st trimester: 10 – 44 ng/mL
- 2nd trimester: 17 – 114 ng/mL
- 3rd trimester: 34 – 386 ng/mL
- 1st trimester: 10 – 44 ng/mL
Men
- 3.0 – 15.2 ng/mL
Postmenopausal
- 2 – 20 ng/mL
Critical Values
- >200–250 ng/mL strongly suggests a prolactinoma
- >500 ng/mL typically indicates a macroadenoma
Diagnostic Uses
1. Evaluation ofAmenorrhea & Oligomenorrhea
Most common use in young women.
2. Infertility Workup
Hyperprolactinemia suppresses ovulation and spermatogenesis.
3. Galactorrhea Evaluation
4. Pituitary Tumor Diagnosis
PRL helps distinguish:
- Prolactinoma
- Non-functioning pituitary adenoma with stalk compression
5. Monitoring Prolactinoma Treatment
- Dopamine agonists (cabergoline, bromocriptine)
- Tumor shrinkage correlates with PRL fall
6. Early Pregnancy Assessment
Physiological rise confirms hormonal activity.
7. Hypothyroidism Screening
Elevated PRL may be an early clue.
Analytical Notes
- Measure mid-morning (avoid early AM surge).
- Avoid breast stimulation and sexual activity before test.
- High-dose hook effect can falsely lower PRL in very large tumors → request dilution.
- Always check macroprolactin if elevation unexplained.
Clinical Pearls
- PRL >200 ng/mL → prolactinoma likely.
- If PRL is only mildly elevated (20–60 ng/mL), rule out:
- Hypothyroidism
- Medications
- Stress
- Pregnancy
- Hypothyroidism
- Repeat test before imaging to confirm persistent elevation.
- Macroprolactin causes high PRL but no symptoms - PEG precipitation test helps.
- Cabergoline is more effective and better tolerated than bromocriptine.
Interesting Fact
Prolactin has over 300 documented biological functions, more than any other pituitary hormone - earning it the nickname “the multitasking hormone.”
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Pituitary Hormones
- Endocrine Society Clinical Practice Guideline - Hyperprolactinemia
- ACOG Reproductive Endocrinology Guidelines
- Mayo Clinic Laboratories - Prolactin
- ARUP Consult - Pituitary Hormone Evaluation
- MedlinePlus / NIH - Prolactin Test