Chapter 31: Hypothalamus & Pituitary Gland Regulation

Hypothalamus & Pituitary Gland Regulation distinguishes the functional anatomy of the neurohypophysis, or posterior pituitary, from the adenohypophysis, or anterior pituitary, emphasizing their distinct embryologic origins and vascular connections via the hypophyseal portal system. The synthesis and secretion of posterior pituitary hormones, arginine vasopressin (AVP) and oxytocin, are detailed, highlighting their production in hypothalamic magnocellular neurons and their respective roles in renal water conservation and reproductive reflexes like milk ejection and parturition. The text extensively covers the anterior pituitary hormones—adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), growth hormone (GH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin—describing how specific hypothalamic releasing and inhibiting factors regulate their release. Detailed mechanisms are provided for the hypothalamic-pituitary-adrenal (HPA) axis, including the processing of proopiomelanocortin (POMC) to form ACTH and the regulatory feedback loops involving cortisol and stress. Similarly, the hypothalamic-pituitary-thyroid axis is explored, explaining how thyrotropin-releasing hormone (TRH) and thyroid hormones modulate the secretion of the alpha and beta subunits of TSH. A significant portion of the chapter focuses on the somatotropic axis, examining the pulsatile nature of GH secretion during sleep, its dual regulation by growth hormone-releasing hormone (GHRH) and somatostatin, and its metabolic effects on lipolysis and insulin resistance, alongside its growth-promoting actions mediated by insulin-like growth factor 1 (IGF-1). The discussion further includes the gonadotropins essential for reproductive function and the unique inhibitory control of prolactin by dopamine. The chapter concludes by integrating these hormonal axes with energy homeostasis, specifically investigating how leptin signaling informs the hypothalamus about adipose stores to adjust metabolic and reproductive hormone output during states of energy surplus or deprivation.