Prolactin

A pituitary hormone whose main job is milk production, and which is also released by stress.

A persistently raised value suppresses the reproductive hormones, so it is checked when periods stop or libido and testosterone fall without explanation.

The draw itself can raise it: a stressful venepuncture, exercise, a meal or recent nipple stimulation are all enough, so a mildly high result is normally repeated on a rested sample. Some antipsychotics and anti-nausea drugs raise it markedly.

Worth discussing a raised value that repeats on a calm, rested sample.