What is Prolactin?

Dopamine arrives continuously from the brain and holds the prolactin-producing cells in check, so anything that interrupts that supply lets prolactin rise. Most rises are passing, from sleep or stress. Some medicines block dopamine, and prolactin stays up for as long as they are taken. A prolactinoma, a benign growth of the cells that make prolactin, produces it whatever the brain is signalling.

TSH and FT4 describe thyroid activity. When the thyroid is very underactive, the brain steps up the signal driving it, and that same signal also stimulates the prolactin-producing cells, so prolactin can move with the thyroid markers. Oestradiol acts on those same cells directly, which is why prolactin shifts with pregnancy and across the menstrual cycle.

Sleep raises prolactin overnight, with the peak in the early hours. Physical or psychological stress raises it within minutes to hours.

Prolactin does not hold steady through the day, and sleep and stress both pass within hours. A yearly test is not trying to catch either. Two readings a year apart compare the resting level prolactin returns to once those influences have passed. This is Do Health's monitoring schedule, not a general recommendation for everyone.