The HPA axis operates through a neuroendocrine cascade: hypothalamic paraventricular nucleus (PVN) neurons secrete CRH and AVP into the hypophyseal portal circulation, stimulating anterior pituitary corticotrophs via CRH-R1 and V1b receptors respectively. ACTH, cleaved from pro-opiomelanocortin (POMC), drives adrenocortical steroidogenesis through the cholesterol side-chain cleavage enzyme and subsequent zona fasciculata enzymes (17-alpha-hydroxylase, 11-beta-hydroxylase) to produce cortisol. Cortisol acts through genomic mechanisms (glucocorticoid receptor-mediated transcription) and non-genomic pathways (rapid membrane effects). Negative feedback occurs at both hypothalamic (suppressing CRH/AVP) and pituitary (suppressing POMC/ACTH) levels. The SAM axis provides rapid catecholaminergic responses through preganglionic sympathetic fibers activating adrenal medullary chromaffin cells. Chronic HPA dysregulation manifests as either hypercortisolism (Cushing syndrome) or hypocortisolism (adrenal insufficiency), each with distinct diagnostic algorithms and management strategies. Modern stress science recognizes allostatic load as the cumulative physiologic toll of chronic stress, contributing to cardiometabolic disease, immune dysfunction, neurodegeneration, and accelerated cellular aging. The clinician must order and interpret dynamic endocrine testing, prescribe replacement or suppressive therapy, manage complications, and integrate stress reduction into comprehensive treatment plans.