Cortisol

Definition

Cortisol is a glucocorticoid steroid hormone synthesized in the zona fasciculata of the adrenal cortex from cholesterol. It circulates bound largely to corticosteroid-binding globulin (CBG), with a small free fraction that produces the physiological effects.

Cortisol acts through the glucocorticoid receptor (GR), which is present in nearly every cell in the body. Its effects are broad and coordinated:

• Metabolic: mobilizes glucose (gluconeogenesis in liver), mobilizes fatty acids and amino acids, raises blood glucose
• Cardiovascular: sensitizes vascular tissue to catecholamines, maintaining blood pressure under stress
• Immune: suppresses inflammatory cytokine production and immune cell activity in the acute phase; contributes to glucocorticoid resistance and paradoxical inflammation with chronic exposure
• Cognitive: influences memory encoding and retrieval; acute cortisol enhances emotional memory formation while high sustained levels can impair hippocampal function
• Behavioral: affects motivation, arousal, and reward processing

Under negative feedback from cortisol, the hypothalamus and pituitary reduce CRH and ACTH release, closing the loop.

What people often confuse this with

• "Stress hormone" as a moral category: cortisol is not inherently bad. It is one of the most important adaptive hormones in the body. What matters is pattern (rhythm), context (acute vs chronic), and level (physiologic vs pathological). The popular framing of cortisol as universally toxic is inaccurate.
• Adrenaline (epinephrine): a different hormone from a different part of the adrenal (the medulla, not the cortex), operating on a much faster timescale (seconds vs minutes) as part of the sympathetic nervous system rather than the HPA axis.
• The HPA axis itself: cortisol is the output. The axis is the regulatory circuit that produces cortisol.
• Adrenal fatigue: not a recognized medical condition. The specific term is HPA dysregulation, and even that requires care in how it's used clinically.
• Cortisone: an inactive metabolite of cortisol, converted to cortisol by 11-beta-hydroxysteroid dehydrogenase type 1 in target tissues.

What research says

• Cortisol rhythms are highly conserved and are one of the more robust biomarkers of circadian function
• Depression is associated with cortisol elevation in a substantial subset of patients but the pattern is heterogeneous
• PTSD shows low basal cortisol with heightened stress reactivity
• Chronic stress can produce hyper- or hypocortisolism depending on stressor duration and individual factors
• Cortisol responsivity to standardized psychological stressors (Trier Social Stress Test) is a widely-used research tool
• Early-life adversity produces long-term changes in HPA reactivity measurable in adulthood
• Cortisol crosses the placenta with implications for fetal HPA development in maternal chronic stress

The diurnal rhythm

In healthy adults, cortisol follows a stereotyped daily pattern:

• Cortisol awakening response (CAR): cortisol rises sharply in the first 30 to 45 minutes after waking, roughly doubling the pre-waking level. This is a distinct phenomenon that reflects the anticipatory demand of starting the day.
• Daytime decline: cortisol falls gradually through the day, punctuated by responses to meals and stressors.
• Evening nadir: reaches its lowest point around midnight.
• Overnight rise: begins increasing again several hours before waking, driven by circadian output from the suprachiasmatic nucleus.

Disruptions to this rhythm show up in shift work, chronic stress, depression, PTSD, and various medical conditions. The rhythm itself, not just the total amount, matters for physiological function.

Key features

• glucocorticoid steroid released by the adrenal cortex under ACTH signal from the pituitary
• follows a diurnal rhythm with a cortisol awakening response and evening nadir
• acute release is protective and adaptive
• chronic elevation is associated with depression, cardiometabolic disease, and immune dysregulation
• chronic blunting (low reactivity) is associated with PTSD and some chronic pain conditions
• measurable in blood, saliva (unbound fraction), and hair (long-term average exposure)
• pharmaceutical hydrocortisone is used clinically for adrenal insufficiency, autoimmune disease, and severe inflammation

Where it fits

Cortisol sits at the intersection of stress research, endocrinology, and psychiatry. It matters clinically in several ways:

• Depression research: elevated cortisol is a common (though not universal) finding in major depressive disorder, particularly the melancholic subtype. Historically the dexamethasone suppression test (DST) was used to identify feedback failure but was retired from routine clinical use due to poor specificity.
• PTSD research: the classic finding is paradoxically low basal cortisol with heightened reactivity to trauma reminders, a distinct pattern from depression.
• Cushing syndrome: pathological cortisol excess (from tumor, exogenous steroids, or ectopic ACTH) produces central obesity, hypertension, insulin resistance, mood changes, and physical stigmata.
• Addison disease: pathological cortisol deficiency (autoimmune adrenal destruction being the most common cause in developed countries) produces fatigue, hypotension, salt craving, and can present as a life-threatening crisis.
• Allostatic load research: chronic cortisol elevation is one measurable marker of the cumulative physiological wear from chronic stress.

Measurement

Cortisol can be measured in several ways, each with tradeoffs:

• Blood (serum) cortisol: the traditional approach; measures total cortisol at a point in time; requires venipuncture
• Salivary cortisol: measures the free (biologically active) fraction; non-invasive; commonly used in research for CAR measurement
• Urinary free cortisol: measures 24-hour cortisol production; used in Cushing syndrome workup
• Hair cortisol: measures long-term cortisol exposure over months; useful for chronic-stress research
• Late-night salivary cortisol: sensitive for Cushing syndrome screening (loss of the normal nadir)

For research, multiple samples across the day are typically needed to characterize the rhythm rather than a single spot measurement.