Macroprolactin Test: Causes, Symptoms & Norms

What Is Macroprolactin?

In my clinic, a woman with scanty periods and unexpected breast milk production often raises the question of prolactin excess. However, the standard prolactin test can sometimes fool us — the elevation may be due to a harmless variant called macroprolactin.

Macroprolactin is a large, biologically inactive complex formed when prolactin binds to antibodies (usually IgG) in the bloodstream. Because it is cleared more slowly than normal prolactin, it accumulates and artificially inflates the measured prolactin level. This condition, known as macroprolactinaemia, is not a disease per se but explains many cases of apparently high prolactin without symptoms.

Why Measure Macroprolactin?

The macroprolactin test (often reported as a percentage or absolute concentration) helps distinguish between true hyperprolactinaemia — which may be caused by a pituitary tumour, hypothyroidism, or medications — and macroprolactinaemia, which is benign and usually needs no treatment.

In my practice, I routinely request this test when a patient has elevated prolactin but no typical symptoms (e.g., galactorrhoea, irregular cycles, low libido) and no visible pituitary adenoma on imaging.

Reference Ranges for Macroprolactin

Macroprolactin is typically expressed as a percentage of total prolactin. Most laboratories use a cutoff of ≥60% to indicate significant macroprolactinaemia. Absolute levels (in mIU/L) are sometimes reported but are less standardised. The table below shows typical serum prolactin reference ranges to help interpret total prolactin results before macroprolactin testing.

Note: Always use your laboratory’s specific reference intervals. Macroprolactin >60% of total prolactin suggests macroprolactinaemia regardless of total level.

How Is the Macroprolactin Test Done?

The test is a simple blood draw, usually performed on the same sample used for prolactin. After the initial prolactin result is high, the lab adds polyethylene glycol (PEG) precipitation to remove macroprolactin. The remaining prolactin (monomeric prolactin) is measured. The percentage of macroprolactin is then calculated.

Common Symptoms and Clinical Scenarios

Macroprolactin Symptoms vs. True Hyperprolactinaemia

Most people with macroprolactinaemia have no symptoms of prolactin excess. However, some may experience mild, non‑specific issues. In contrast, true hyperprolactinaemia typically causes:

• Galactorrhoea (milk production unrelated to breastfeeding)
• Menstrual irregularities or amenorrhea
• Erectile dysfunction and low libido in men
• Infertility
• Headaches or visual field defects (if a large tumour is present)

Is High Macroprolactin Dangerous?

By itself, macroprolactin is not dangerous. It does not stimulate breast tissue or suppress the reproductive axis. However, failing to identify macroprolactinaemia can lead to unnecessary worry and inappropriate treatments — such as dopamine agonist therapy — which carry side effects.

Macroprolactin During Pregnancy

Pregnancy naturally raises prolactin. Macroprolactinaemia is usually incidental and does not harm the mother or fetus. I reassure my pregnant patients that macroprolactin does not require monitoring or treatment during pregnancy.

Causes of Macroprolactinaemia

Most cases are idiopathic — meaning no underlying cause is found. The body simply produces anti‑prolactin autoantibodies. Other causes include:

• Autoimmune conditions (e.g., systemic lupus erythematosus)
• Certain medications (rarely)
• Other chronic inflammatory states

Treatment and Management

If macroprolactinaemia is confirmed (and true hyperprolactinaemia excluded), no treatment is required. I advise patients that this is a benign laboratory finding. Annual monitoring of prolactin levels is not necessary unless symptoms develop. If symptoms like headache or visual changes appear, repeat imaging rather than routine blood tests is appropriate.