Prolactin (PRL)

SI UNITS (recommended)


* The SI unit is the recommended method of reporting clinical laboratory results

luteotropic hormone, luteotropin
Units of measurement
μIU/mL, mIU/L, μg/L, ng/mL, ng/dL, ng/100mL, ng%

Human prolactin is a polypeptide hormone of the anterior pituitary with a molecular mass of about 22,800. Prolactin secretion is controlled by the hypothalamus primarily through the release of prolactin inhibiting factor (dopamine) and prolactin releasing factor (serotonin). Thyrotropin releasing hormone (TRH) stimulates PRL secretion and is useful as a provocative test to evaluate PRL reserves and abnormal secretion of PRL by the pituitary.

Prolactin appears in serum in three different forms. The biologically and immunologically active monomeric (“little”) form predominates (approx. 80 %), 5‐20 % is present as the biologically inactive dimeric (“big”) form and 0.5‐5 % is present as the tetrameric (“big‐big”) form having low biological activity. Macroprolactin (big-big prolactin)

The target organ for prolactin is the mammary gland, the development and differentiation of which is promoted by this hormone. High concentrations of prolactin have an inhibiting action on steroidogenesis of the ovaries and on hypophyseal gonadotropin production and secretion.

As a reference range for circulating prolactin, the literature suggests concentrations up to approximately 20 ng/mL. Values are distinctly elevated at birth but decline to adult levels in less than three months.

Women are reported to have slightly higher mean levels than men, with a slight rise at puberty — apparently estrogen related — and a corresponding fall at menopause. During pregnancy, the prolactin level climbs steadily to ten or twenty times its former value, then drops back down to normal after delivery — within three weeks in nonnursing mothers. In those who breast-feed, the decline to normal is more gradual because of the prompt and dramatic surges in prolactin release induced by suckling. Women taking oral contraceptives or under estrogen treatment may have prolactin levels higher than normal.

In assessing the significance of moderate elevations, it is important to keep in mind that prolactin is a stress hormone. Not only surgery, but events no more distressing than venipuncture or a clinical interview have been reported to occasion a transient rise.

The biological half-life of PRL is approximately 20-50 minutes. Serum PRL levels during the menstrual cycle are variable and commonly exhibit slight elevations during the mid-cycle. Prolactin levels in normal individuals tend to rise in response to physiologic stimuli including: sleep, exercise, nipple stimulation, sexual intercourse, hypoglycemia, pregnancy, and surgical stress.

Moreover, the release of prolactin is inherently episodic, and day-to-day fluctuations with CVs as high as 30% have been encountered. Finally, there is a sleep-related diurnal variation: prolactin levels increase during sleep and reach their lowest a few hours after waking. The advice sometimes given to draw samples "between nine and noon" is based on the assumption that subjects observe reasonably normal waking hours.

Prolactin deficiencies in normal individuals are rare.

Hyperprolactinemia (in men and women) is the main cause of fertility disorders. The determination of prolactin is utilized in the diagnosis of anovular cycles, hyperprolactinemic amenorrhea and galactorrhea, gynecomastia and azoopermia

Pathologic causes of hyperprolactinemia include: PRL secreting pituitary adenomas (prolactinomas), functional and organic diseases of the hypothalamus, hypothyroidism, renal failure, and ectopic tumors. Elevated levels of PRL may be observed in cases of primary hypothyroidism due to an increased secretion of TRH (stimulates PRL release) accompanied by decreased serum T4 levels and increased serum thyroid stimulating hormone concentrations. Hyperprolactinemia has also been associated with the inhibition of ovarian steroidgenesis, follicle maturation, and secretion of luteinizing hormone and follicle stimulating hormone.

Various drugs have been shown to either increase or decrease PRL levels. Administration of L-dopa suppresses PRL secretion. Bromocriptine inhibits PRL secretion and has been used in the treatment of amenorrhea and galactorrhea due to hyperprolactinemia. Administration of psychotropic drugs (phenothiazines), anti-hypertensive drugs (reserpine), and TRH tend to increase PRL secretion. Estrogen therapy also tends to elevate serum PRL levels.

Reference Intervals

Method: The electrochemiluminescence immunoassay “ECLIA”



86-324 μIU/mL

4.04-15.2 ng/mL

Women (not‐pregnant)

102-496 μIU/mL

4.79-23.3 ng/mL

SI units Conversion Calculator. Convert Prolactin (PRL) level to μIU/mL, mIU/L, μg/L, ng/mL, ng/dL, ng/100mL, ng%. Clinical laboratory units online conversion from conventional or traditional units to Si units. Table of conversion factors for Prolactin (PRL) unit conversion to μIU/mL, mIU/L, μg/L, ng/mL, ng/dL, ng/100mL, ng%.