ProlactinPRL
The pituitary hormone behind milk production, which also suppresses sex hormones when high.
Prolactin is made by the pituitary gland. Its best-known role is stimulating milk production after birth, but it is present in everyone and influences the reproductive axis.
It matters because a raised level suppresses the signals that drive testosterone and oestrogen production, so it is checked when periods stop, when libido or erectile function change, or when fertility is being assessed. Causes range from certain medications to a benign pituitary tumour.
In practice prolactin rises with stress, sleep, exercise and even the stress of a blood draw, so a mildly raised result is usually repeated on a rested morning sample before it is taken seriously.