Information about cortical inputs, automatic function, environmental cues (light, temperature) and peripheral hormonal feedback is synthesized at the coordinating center of the endocrine system, the hypothalamus. The hypothalamus then sends signals to the pituitary to release hormones that affect the thyroid, adrenals, gonads, growth, milk production, and water balance.
Hypothalamic hormones: small peptides, non-binding protein → rapid degradation
High [ ] in pituitary-portal blood system
Low [ ] in peripheral circulation
Proximity of axis preserves the pulsatile output signals from the hypothalamic neurons.
Thyroid hormone is critical to
brain and somatic development in fetus and infants,
metabolic activity in adults, and
function of virtually every organ system.
Synthesize and secrete PTH, a principle regulator of ECF Ca2+, regulated by [Ca2+], [Mg2+], 1,25(OH)2D (active metabolite of vit D), and phosphate.
Each gland, 6-8 g, has
a cortex with 3 layers that act like independent organs (zona glomerulosa → aldosterone, fasciculata → cortisol, reticularis → androgen and estrogen precursors), and
a medulla that acts like a sympathetic ganglion to store/synthesize adrenaline and noradrenaline.
Bifunctional: sex steroid synthesis and gamete production.
Sex steroids control sexuality and affect metabolic and brain functions.
Endocrine islet β-cells produce insulin:
oppose glucose production (glycogenolysis, gluconeogenesis),
increase glucose uptake into muscle and fat.
Glucagon, epinephrine, cortisol, and GH are counterregulatory.