Prolactin Test Measurement
A prolactin (PRL) test measures the level of the hormone prolactin in the blood, which is primarily secreted by lactotrophs in the anterior pituitary gland and is essential for lactation, while abnormal levels can indicate conditions such as prolactinomas, medication effects, or other endocrine disorders. 1, 2
What Prolactin Is and Its Function
Prolactin is a 23 kDa single chain protein composed of 199 amino acids that is:
- Primarily synthesized and released by lactotrophs in the anterior pituitary gland
- Under inhibitory control by hypothalamic dopamine in a negative feedback manner
- Essential during pregnancy and lactation for:
- Development of mammary glands
- Milk synthesis
- Maintenance of milk secretion 2
During pregnancy, serum prolactin levels rise rapidly with an increase in the size and number of lactotrophs. During lactation, suckling induces rapid secretion of prolactin via a neuroendocrine reflex pathway. 2
Normal Reference Ranges
The Endocrine Society and American College of Endocrinology provide these reference ranges for serum prolactin levels:
| Population | Reference Range (ng/mL) |
|---|---|
| Non-pregnant females | 3.0-30.0 |
| Pregnant females | 10.0-209.0 |
| Postmenopausal females | 2.0-20.0 |
| Males | 2.0-18.0 (implied) [1] |
Clinical Significance of Abnormal Levels
Elevated Prolactin (Hyperprolactinemia)
Elevated prolactin levels may indicate:
- Pituitary tumors (prolactinomas - most common pituitary tumor)
- Medication effects (especially antipsychotics)
- Hypothyroidism
- Stress
- Pregnancy
- Nipple stimulation/chest wall trauma
Clinical presentations include:
- In women: menstrual disturbances, galactorrhea, infertility
- In men: hypogonadism, decreased libido, erectile dysfunction
- In both: visual field defects (with large tumors)
- Long-term: increased risk of osteoporosis due to hypogonadism 1, 2
Low Prolactin
Low prolactin levels are less commonly measured clinically but may be associated with:
- Pituitary insufficiency
- Some cases of polycystic ovarian syndrome (PCOS) 1
Clinical Applications of Prolactin Testing
Prolactin measurement is recommended in patients presenting with:
- Menstrual disturbances
- Galactorrhea
- Visual field loss
- Symptoms of hypogonadism
- Low testosterone levels combined with low or low/normal luteinizing hormone levels 1
Important Considerations in Prolactin Testing
Laboratory Measurement Challenges
- Considerable variability exists between routinely available prolactin immunoassays
- Different reactivity towards monomeric prolactin and macroprolactin
- Lack of commutability of the WHO 3rd International Standard between methods 2
Macroprolactinemia
- A common cause of interference in prolactin assays
- May lead to incorrect diagnosis and unnecessary investigations
- Measurement of prolactin post polyethylene glycol precipitation (PEG) is recommended when prolactin levels are above the reference interval 2
Pitfalls in Interpretation
Borderline Elevated Levels: Marginally elevated prolactin levels may not always indicate a prolactinoma. Other conditions including other types of pituitary tumors should be considered. 3, 4
False Reassurance: In patients with known risk factors (e.g., MEN1 syndrome), normal or borderline prolactin levels don't exclude the possibility of a non-prolactin-secreting pituitary tumor. 3
Timing of Sample Collection: Prolactin levels can be affected by:
- Sleep (increased during sleep)
- Stress (including venipuncture stress)
- Sexual intercourse
- Time of day (circadian variation)
- Menstrual cycle phase (higher in late follicular phase) 5
Medication Review: Always review medications, especially antipsychotics, which can cause hyperprolactinemia 1
When prolactin levels are elevated, patients should be referred to an endocrinologist for comprehensive evaluation, especially if the cause is not immediately apparent or if a pituitary tumor is suspected. 1