A 28-year-old woman with galactorrhea and secondary amenorrhea has serum prolactin of 180 ng/mL. MRI shows a 6 mm microadenoma. Which hypothalamic mechanism is MOST directly responsible for the elevated prolactin?
- A Increased thyrotropin-releasing hormone secretion stimulating lactotrophs
- B Reduced dopamine (prolactin-inhibiting factor) reaching anterior pituitary via portal blood ✓
- C Autonomous pituitary lactotroph secretion independent of hypothalamic input
- D Elevated estrogen causing portal blood dopamine degradation
Explanation
Dopamine secreted by tuberoinfundibular neurons normally exerts tonic inhibition of prolactin release via D2 receptors on lactotrophs. A prolactinoma disrupts portal delivery of dopamine to surrounding normal lactotrophs, but the dominant mechanism for hyperprolactinemia from a microadenoma is the reduced dopamine suppression (stalk effect in larger tumors also contributes). TRH does stimulate prolactin but is not the primary driver here. Option C is partly true for the adenoma cells themselves but does not explain the neuroendocrine mechanism asked. Estrogen upregulates prolactin gene expression but does not degrade portal dopamine.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.