Prolactin, And Why It Is Measured

Prolactin is known as the breastfeeding hormone, which understates it considerably. It appears on blood test requests for reasons that have nothing to do with feeding an infant.

The pituitary releases it under restraint

Prolactin is produced by cells in the anterior pituitary gland. Unusually among pituitary hormones, its default state is release, and the hypothalamus mainly acts to suppress it.

Dopamine is the suppressing signal, travelling from the hypothalamus to the pituitary. As long as that signal arrives, prolactin secretion is held down.

This inverted arrangement explains a great deal of the clinical picture, because anything that interrupts the dopamine signal raises prolactin whether or not the pituitary itself is abnormal.

Raised prolactin suppresses reproductive signalling

Elevated prolactin reduces the pulsatile release of the hormone that drives the reproductive axis, which in turn lowers the signals reaching the ovaries or testes.

The consequences are recognisable: irregular or absent periods, reduced fertility, low libido in either sex, and in men reduced testosterone and erectile difficulty.

This is the evolutionary logic of the breastfeeding association. High prolactin during lactation naturally suppresses fertility, and the same suppression occurs whenever the level is high for other reasons.

Why levels rise

Medications are a frequent cause. Several classes, particularly some that act on dopamine pathways, remove the inhibitory signal and allow prolactin to climb.

A benign pituitary tumour producing prolactin is another established cause, and a large one can also press on nearby structures, which is why visual field changes are asked about.

Pregnancy, an underactive thyroid, chest wall irritation and kidney disease all raise it too, which is why an isolated raised result triggers further questions rather than a conclusion.

The test is easy to disturb

Prolactin rises with stress, including the stress of the blood draw itself, and with sleep, exercise, meals and nipple stimulation. Mildly raised results are therefore common and often meaningless.

Because of this, a modestly high value is usually repeated under controlled conditions before anything is concluded from it. A single reading is a prompt to look again.

There is also a laboratory phenomenon in which prolactin bound to antibodies inflates the measured value without any biological effect, and specific testing distinguishes it.

Why this is firmly clinical territory

Interpreting a prolactin result requires knowing the medications, the symptoms, the thyroid status and often imaging. It is not a number that means anything in isolation.

Treatment, where it is needed, may involve changing a medication or addressing the underlying cause, and those are decisions with trade-offs that need a prescriber's judgement.

Anyone who has ordered their own hormone panel and found a raised prolactin should take it to a doctor rather than interpret it, precisely because the causes are so varied.