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Human Body Study

Prolactin Axis (Lactotropic)

The prolactin axis Hypothalamic dopamine continuously suppresses prolactin release from anterior pituitary lactotrophs (tonic inhibition, the dominant signal). TRH and estrogen are stimulatory. Prolactin acts on the mammary gland to drive milk synthesis and exerts short-loop negative feedback by stimulating hypothalamic dopamine release. Prolactin also suppresses the HPG axis, underlying lactational amenorrhea. Hypothalamus tuberoinfundibular dopamine neurons Anterior pituitary lactotrophs Mammary gland alveolar epithelium Milk synthesis · lactation + suppression of HPG axis (amenorrhea) dopamine (tonic) TRH (+) estrogen (+) (pregnancy) prolactin milk ↑ hypothalamic dopamine short loop Inverted logic: anything that blocks dopamine delivery — stalk damage, antipsychotics — RAISES prolactin. Treated with dopamine agonists (cabergoline, bromocriptine), opposite of most endocrine therapy.

Signaling chain

Section titled “Signaling chain”

Unique among the pituitary axes: prolactin is under tonic inhibition. Dopamine from the hypothalamus continuously suppresses prolactin release from anterior pituitary lactotrophs. TRH and estrogen are stimulatory. There is no dedicated peripheral target gland feeding back — so this is a “half-axis.”

Function

Section titled “Function”

Initiates and maintains lactation (milk synthesis in the mammary gland). Prolactin also suppresses the HPG axis — high prolactin inhibits GnRH, which underlies lactational amenorrhea and the infertility seen in pathologic hyperprolactinemia.

Feedback

Section titled “Feedback”

Prolactin stimulates hypothalamic dopamine release, providing short-loop negative feedback on itself. Because control is inhibitory, anything that interrupts dopamine delivery (pituitary stalk compression, dopamine-antagonist drugs) raises prolactin.

Clinical relevance

Section titled “Clinical relevance”
  • Hyperprolactinemia — prolactinoma, dopamine-antagonist drugs (antipsychotics, metoclopramide), hypothyroidism (high TRH), stalk effect. Causes galactorrhea, amenorrhea, infertility, low libido, hypogonadism.
  • Treated with dopamine agonists (cabergoline, bromocriptine) — note this is the opposite logic of most endocrine therapy.
  • Macroprolactin can cause spuriously elevated readings.

Key labs

Section titled “Key labs”

Serum prolactin (fasting, non-stressed), TSH, pregnancy test, pituitary MRI if persistently high.