Model for regulation of anterior pituitary hormone secretion by three tiers of
control. Hypothalamic hormones traverse the portal system and impinge directly
upon their respective target cells. Intrapituitary cytokines and growth factors
regulate tropic cell function by paracrine (and autocrine) control. Peripheral
hormones exert negative feedback inhibition of respective pituitary trophic
hormone synthesis and secretion. Reproduced with permission from
Endocrine Reviews (2).