Cortisol is the main glucocorticoid hormone in humans, synthesized in the zona fasciculata of the adrenal cortex from cholesterol. It is famous as "the stress hormone," which is a true but shallow label. Cortisol runs a sharp daily rhythm that anchors wakefulness, regulates glucose and blood pressure, suppresses inflammation, and permits the body to survive sustained demand — until the same loops, held open too long, begin to corrode everything they touch. ## At a glance ## What it does Cortisol's day job is fuel mobilization and stress preparedness. It raises blood glucose by stimulating hepatic gluconeogenesis and inhibiting peripheral glucose uptake; it breaks down protein in muscle and bone to supply amino acids; it mobilizes fatty acids from adipose tissue. This energy push is why cortisol peaks early in the morning — it is the metabolic signal that says "you are about to start moving, here is the fuel." It also sustains blood pressure (by sensitizing vessels to catecholamines), suppresses the immune system (by damping inflammatory cytokines and T-cell responses), supports cognition and alertness, and shuts down reproductive and growth axes when other systems need the energy. The suppression is not a bug — when you need to run from a predator, spending resources on libido and bone-building would be a waste. The morning spike has a distinct subcomponent called the cortisol awakening response (CAR) — a further 50-60% rise above baseline in the 30-45 minutes after waking. The CAR appears to prime alertness and readiness for the day. It is blunted in chronic fatigue, some depressions, burnout syndromes, and adrenal insufficiency. ## How it works Cortisol lives inside the HPA axis. The hypothalamus releases corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) in response to circadian signals, physical stressors (injury, hypoglycemia, exercise), or psychological stress processed through the amygdala. CRH hits the anterior pituitary and triggers release of adrenocorticotropic hormone (ACTH). ACTH travels to the adrenal cortex and drives cortisol synthesis from cholesterol through pregnenolone and progesterone intermediates. Cortisol then feeds back at both the hypothalamus and pituitary to shut down its own upstream signals. Once released, cortisol circulates about 90% bound to corticosteroid-binding globulin (CBG) and to a lesser extent albumin. Only the free fraction is biologically active. It diffuses into target cells (it is lipid-soluble), binds the glucocorticoid receptor (GR) in the cytoplasm, and the complex translocates to the nucleus to alter transcription of hundreds of target genes. Some effects (like vascular tone support) happen faster through non-genomic membrane mechanisms. Tissue sensitivity to cortisol is not uniform. The enzyme 11-beta-HSD1 in liver and fat activates cortisone to cortisol locally, amplifying glucocorticoid effects in those tissues. 11-beta-HSD2 in kidney does the opposite, protecting the mineralocorticoid receptor from being hit by cortisol. Licorice contains glycyrrhizin, which inhibits 11-beta-HSD2 and can cause cortisol to spill onto the mineralocorticoid receptor, producing hypertension and low potassium. ## Levels & ranges Cortisol's reference range depends entirely on when you measure. Morning serum cortisol (8 a.m.) in healthy adults typically runs 10-20 µg/dL; by late evening it drops below 5 µg/dL. Because of this rhythm, a single isolated serum cortisol is almost useless without context. Salivary cortisol measures the free (biologically active) fraction and is useful for diagnosing Cushing's syndrome via a midnight or late-night sample that should normally be suppressed. The ACTH stimulation test — administer synthetic ACTH (cosyntropin) and measure cortisol 30 and 60 minutes later — evaluates whether the adrenal cortex can respond when asked. A healthy response pushes cortisol above about 18-20 µg/dL. Failure to rise suggests primary adrenal insufficiency or long-standing secondary (the adrenals have atrophied from chronic low ACTH). The 1 mg overnight dexamethasone suppression test probes the other direction: give a synthetic glucocorticoid that should suppress morning cortisol below about 1.8 µg/dL. Failure to suppress suggests Cushing's syndrome. 24-hour urine free cortisol is the other classic screen for excess — sustained elevation above the reference range (roughly 45 µg/24h depending on assay) suggests true hypercortisolism. Saliva-based "cortisol rhythm" panels sold direct-to-consumer sometimes overstate their clinical usefulness. They are excellent for the specific question of endogenous Cushing's screening and can be informative in insomnia or unusual fatigue patterns. They are not a diagnostic tool for "adrenal fatigue" — a condition that does not exist in mainstream endocrinology. ## When it goes wrong Deficiency (adrenal insufficiency) splits into primary (Addison's disease — the adrenals themselves fail, usually autoimmune in developed countries, tuberculous historically) and secondary (the pituitary fails to produce ACTH, often from chronic exogenous steroid use). Primary Addison's presents with fatigue, weight loss, skin hyperpigmentation (melanocyte-stimulating hormone rises alongside ACTH), low sodium, high potassium, low blood pressure, and in crisis a life-threatening picture of shock. Treatment is lifelong glucocorticoid and mineralocorticoid replacement. The most common practical cortisol deficiency scenario is iatrogenic HPA suppression from chronic steroid use. Patients on prednisone above physiologic doses for more than a few weeks have a suppressed HPA axis and cannot make enough cortisol if steroids are abruptly stopped or if they hit a major stressor. Adrenal crisis from sudden steroid withdrawal or unrecognized suppression under surgical stress is a medical emergency. Excess (hypercortisolism, Cushing's syndrome) causes central obesity with thin extremities, moon facies, purple abdominal striae, thin skin and easy bruising, hypertension, hyperglycemia, osteoporosis, proximal muscle weakness, psychiatric changes, and infertility. Cushing's disease specifically refers to pituitary ACTH-secreting adenoma; Cushing's syndrome is the broader umbrella including iatrogenic (prescribed steroids — by far the most common cause), adrenal adenoma, and ectopic ACTH from tumors. Chronic mild cortisol elevation from ongoing psychological stress does not usually cross into Cushing's territory but is still harmful. It drives visceral fat accumulation, insulin resistance, sleep fragmentation, mood instability, immune changes, and bone loss over years. "Stress kills" is a cliché; the cortisol pathway is one of the real mechanisms behind it. ## Interactions Circadian misalignment wrecks cortisol rhythm. Shift workers and chronic jet-laggers show flattened daily curves that correlate with metabolic disease and mood problems. Poor sleep elevates late-night cortisol; a single night of severe sleep deprivation blunts the next morning's CAR. Caffeine acutely raises cortisol, though regular coffee drinkers develop some tolerance. Alcohol raises cortisol acutely and disrupts the rhythm chronically. Exercise acutely raises cortisol (proportional to intensity and duration); regular moderate training improves the daily curve and stress response, but overtraining flattens it. Psychological stress drives CRH output, and how much cortisol rises is modulated by subjective control, social support, and predictability — the same "objective" stressor produces very different cortisol responses in different people. Medication interactions are important. Exogenous glucocorticoids (prednisone, dexamethasone) suppress the HPA axis via negative feedback; chronic use requires tapering and stress-dose coverage. Ketoconazole, metyrapone, and mifepristone are used pharmacologically to block cortisol synthesis or action in Cushing's. Estrogen raises CBG, which raises total serum cortisol without raising free cortisol — a common source of falsely high cortisol labs in women on hormonal contraception or HT. ## Honest take "Adrenal fatigue" is not a real diagnosis — the hypothesis that chronic stress depletes the adrenals to subclinical failure has never held up to proper testing. What is real is HPA axis dysregulation from chronic stress, poor sleep, and misaligned circadian signals, and the best interventions for that are unglamorous: regular sleep, morning light, daily movement, and reducing actual stressors. Salivary cortisol panels from direct-to-consumer labs are often oversold as diagnostic instruments; they are useful for screening Cushing's and occasionally illuminating, but they do not justify taking adaptogens or "adrenal support" supplements. On the flip side, iatrogenic Cushing's and HPA suppression from chronic prescribed steroids are under-appreciated — anyone on systemic steroids for more than a few weeks should understand taper and stress dosing before something bad happens. ## Sources - Nieman et al., Endocrine Society Clinical Practice Guideline — diagnosis of Cushing's syndrome. - Bornstein et al., Endocrine Society Clinical Practice Guideline — diagnosis and treatment of primary adrenal insufficiency. - Dickerson & Kemeny, Psychological Bulletin — the meta-analytic review of acute psychological stressors and cortisol response. - Tsigos & Chrousos, Journal of Psychosomatic Research — the classic on HPA axis function under stress.

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