Introduction
Cortisol, widely recognized as the principal stress hormone, exerts extensive influence over numerous physiological processes throughout the body. This hormone functions as the primary glucocorticoid synthesized and released by the zona fasciculata of the adrenal cortex. The hypothalamic-pituitary-adrenal (HPA) axis governs cortisol production and secretion, and disruption of this regulatory system results in cortisol excess disorders such as Cushing syndrome or deficiency states such as Addison disease (see Image. Hypothalamic-Pituitary-Adrenal Axis).
Cortisol influences metabolism, immune activity, cardiovascular tone, and the stress response by modulating glucose availability, protein catabolism, lipolysis, and inflammatory signaling. Excessive cortisol exposure, such as in Cushing syndrome, produces central obesity, muscle wasting, hypertension, and glucose intolerance. Cortisol deficiency, such as in Addison disease, causes fatigue, hypotension, weight loss, and hyperpigmentation. Understanding cortisol physiology enables clinicians to recognize deviations from normal regulation, interpret diagnostic findings accurately, and design targeted therapeutic strategies for endocrine and systemic disorders involving the HPA axis.
Cellular Level
Cortisol, a steroid hormone, is synthesized from cholesterol within the zonae fasciculata and reticularis of the adrenal cortex. Both layers produce cortisol and androgens (see Image. Steroid Hormone Synthesis Pathways from Cholesterol). The zona fasciculata constitutes approximately 75% of the adrenal cortex and primarily produces glucocorticoids. Cells in this layer, termed "clear cells," are large and lipid-rich. These columns of cells extend into the zona reticularis, where the more compact, lipid-poor cells contain lipofuscin granules. The zona reticularis primarily synthesizes androgens but also contributes to glucocorticoid production. Adrenocorticotropic hormone (ACTH) regulates both cortical zones.[1][2]
ACTH released from the anterior pituitary stimulates steroidogenesis by increasing low-density lipoprotein receptor expression and enhancing cholesterol desmolase activity, which converts cholesterol to pregnenolone—the rate-limiting step in cortisol biosynthesis.[3] Pregnenolone undergoes 17α-hydroxylation to form 17α-hydroxypregnenolone, followed by dehydrogenation via 3β-hydroxysteroid dehydrogenase to yield 17α-hydroxyprogesterone. Subsequent microsomal 21-hydroxylation produces 11-deoxycortisol, which is then converted to cortisol through 11β-hydroxylation.[4]
In healthy adults, cortisol production averages 8 to 30 mg/day.[5][6] Secretion follows a diurnal rhythm, with levels beginning to rise during the final hours of sleep, peaking near the time of awakening, and gradually declining throughout the day to reach the lowest concentration at night.[7][8]
Cortisol binds to the cytoplasmic glucocorticoid receptor after entering target cells. The activated cortisol-receptor complex translocates into the nucleus, where the receptor can function as either a homodimer or a monomer. Each receptor monomer associates with a 90-kDa heat shock protein (hsp90). In the homodimer form, the glucocorticoid receptor binds to glucocorticoid response elements in the promoter regions of target genes.[9] This process, termed "transactivation," induces the expression of anti-inflammatory proteins such as annexin I (lipocortin 1), MAPK phosphatase 1 (MKP-1), and inhibitor of κBα (IκBα).[10][11]
When acting as a monomer, the glucocorticoid receptor interacts directly with proinflammatory transcription factors, including nuclear factor-kappa B (NF-κB) and activator protein 1 (AP-1). This interaction leads to transrepression and subsequent suppression of inflammatory gene expression.[12][13][14]
Function
Carbohydrate Metabolism
Cortisol elevates blood glucose concentrations by enhancing hepatic gluconeogenesis through the activation of glucose-6-phosphatase and phosphoenolpyruvate carboxykinase, utilizing free fatty acids from lipolysis and amino acids from proteolysis as substrates.[15] Cortisol reduces glucose uptake and utilization in peripheral tissues such as skeletal muscle and adipose tissue.[16] The hormone exerts a permissive influence on glucagon and catecholamines, facilitating their hyperglycemic and insulin-antagonistic effects.[17][18] Cortisol also modulates pancreatic islet cell activity, acutely increasing insulin secretion, while sustained elevation diminishes insulin synthesis and release.[19][20][21][19] Chronic cortisol excess promotes β-cell apoptosis and contributes to hyperglucagonemia.[22][23][22]
Protein Metabolism
Cortisol induces catabolism in skeletal muscle by stimulating proteolysis and inhibiting protein synthesis. Protein degradation occurs through activation of the ubiquitin–proteasome system, driven by increased expression of atrogin 1, muscle RING-finger protein-1 (MuRF1), and other muscle-specific E3 ubiquitin ligases.[24][25] Overexpression of FOXO genes under cortisol influence accelerates muscle fiber atrophy.[26][27] Amino acids that are released serve as substrates for hepatic gluconeogenesis during stress or fasting.[28][29] Cortisol further suppresses anabolic pathways by inhibiting mTOR (mechanistic target of rapamycin) signaling and reducing insulin-like growth factor 1 activity, leading to a sustained decrease in protein synthesis.[30][31][32]
Adipose Tissue and Lipid Metabolism
Cortisol influences adipose tissue dynamics and lipid turnover through context-dependent mechanisms. The hormone promotes adipocyte differentiation and lipogenesis.[33] Cortisol also increases lipolysis under both acute and chronic stimulation. Sustained cortisol elevation leads to reduced subcutaneous fat and increased visceral adiposity, mediated by enhanced glucocorticoid receptor expression and upregulation of 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) within adipose tissue.[34][35][36] This enzyme catalyzes the conversion of inactive cortisone to active cortisol, amplifying local glucocorticoid activity in adipose depots.[37]
Skin and Connective Tissue
Cortisol suppresses fibroblast and keratinocyte proliferation and differentiation, accompanied by reduced collagen synthesis.[38] Prolonged cortisol excess produces skin and connective tissue atrophy, impaired wound healing, easy bruising, and striae formation in areas of mechanical stress.[39] Cortisol acts through the glucocorticoid receptor to inhibit epidermal cell division and DNA synthesis, downregulate type I and III collagen production in dermal fibroblasts, suppress transforming growth factor β (TGF-β) signaling, and decrease amino acid uptake. These mechanisms collectively reduce dermal thickness and compromise connective tissue strength.[40][41] Upregulation of 11β-HSD1 activity by excess cortisol further contributes to these effects, as the enzyme is highly expressed in epidermal keratinocytes and dermal fibroblasts.[42][43][44]
Calcium Homeostasis and Bone Metabolism
Prolonged cortisol elevation adversely affects bone metabolism through multiple mechanisms. The overall consequence is reduced bone formation and increased bone resorption, leading to accelerated bone loss, osteoporosis, and heightened fracture risk.[45]
Cortisol impairs osteoblast activity and promotes osteoblast apoptosis, thereby diminishing bone formation.[46][47][48] Increased osteocyte apoptosis further compromises bone microarchitecture and quality.[49][50] Osteoclast activity is enhanced through upregulation of receptor activator of nuclear factor κB ligand (RANKL) and downregulation of osteoprotegerin, resulting in greater bone resorption.[51][52]
In addition to these direct skeletal effects, cortisol excess increases urinary calcium excretion and decreases intestinal calcium absorption.[53][54] The consequent reduction in calcium availability intensifies bone demineralization and structural weakening.
Electrolyte, Fluid, and Blood Pressure Regulation
Under physiological conditions, 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2) converts cortisol to inactive cortisone, thereby preventing cortisol from activating mineralocorticoid receptors.[55][56] Excess cortisol saturates the enzymatic capacity of 11β-HSD2, allowing active cortisol to bind mineralocorticoid receptors and produce sodium retention, potassium excretion, and hypertension.[57][58] Cortisol also enhances epithelial sodium channel (ENaC) activity, further promoting sodium reabsorption.[59] In the liver, cortisol increases angiotensinogen synthesis through glucocorticoid receptor-mediated transcriptional activation, thereby augmenting the renin-angiotensin system.[60][61][62] These effects collectively elevate glomerular filtration rate through intrarenal angiotensin II activity and contribute to increased systemic blood pressure.[63][64]
Cortisol decreases vasopressin synthesis and secretion from vasopressinergic neurons in the paraventricular nucleus, leading to increased free water clearance.[65][66] The hormone also potentiates vasoconstriction by upregulating adrenergic receptor expression and enhancing vascular smooth muscle responsiveness to catecholamines, resulting in elevated vascular resistance and tone.[67] This effect is intensified by cortisol-mediated inhibition of endothelial nitric oxide and prostacyclin synthesis, both of which normally exert vasodilatory influences.[68][69]
Other Endocrine Effects
Cortisol exerts inhibitory effects on thyroid function.[70] The hormone suppresses thyrotropin-releasing hormone-induced secretion of thyroid-stimulating hormone through hypothalamic modulation.[71] Chronic cortisol excess produces decreased thyroid-stimulating hormone, thyroxine (T4), and triiodothyronine (T3) concentrations, consistent with a pattern of central hypothyroidism.[72] Cortisol also decreases peripheral conversion of T4 to T3, further lowering circulating T3 levels.[73]
Cortisol inhibits gonadotropin-releasing hormone release from the hypothalamus, resulting in reduced synthesis and secretion of follicle-stimulating hormone and luteinizing hormone, with subsequent impairment of gonadal activity.[74] Chronic cortisol elevation diminishes gametogenesis and gonadal cell viability in both men and women.[75][76][77][78]
Sustained cortisol excess also suppresses the growth hormone axis by reducing growth hormone-releasing hormone expression, increasing hypothalamic somatostatin tone, and decreasing pituitary responsiveness to growth hormone-releasing hormone.[79] The consequence is a reduction in growth hormone levels and downstream effects.[80] In children, this deficiency manifests as impaired linear growth, while in adults, it is associated with decreased insulin-like growth factor 1 concentrations.
Immunologic Effects
Cortisol plays a central role in immune regulation, particularly during acute stress responses. Glucocorticoids attenuate inflammation and are effective for short-term therapeutic use. Prolonged exposure, either from chronic pharmacologic administration exceeding prednisone-equivalent doses of 5 mg per day or from endogenous cortisol excess, produces marked immunosuppression. Both innate and adaptive immune functions are affected. Glucocorticoid receptors are expressed in nearly all cell types, mediating cortisol’s broad immunomodulatory actions, which can paradoxically promote low-grade inflammation through persistent cytokine production and immune cell infiltration.[81]
Within the innate immune system, cortisol suppresses the production of proinflammatory cytokines and interferes with pathogen recognition and response by macrophages and dendritic cells through modulation of Toll-like receptor signaling.[82][83] Cortisol reduces margination of neutrophils and eosinophils, thereby elevating circulating counts while diminishing their functional activity. The hormone also promotes apoptosis of neutrophils, eosinophils, and basophils.[84][85][86]
In the adaptive immune system, cortisol inhibits the proliferation and effector function of both T and B lymphocytes and induces lymphocyte apoptosis. Production of proinflammatory cytokines, such as interleukin 12, interferon γ, and tumor necrosis factor α, is suppressed, leading to reduced T helper 1 and cytotoxic CD8+ T-cell activity.[87][88] Cortisol shifts the immune balance toward T helper 2-mediated humoral immunity by upregulating the expression of interleukins 4, 10, and 13.[89][90] The molecule also enhances regulatory T-cell activity, contributing to the prevention of excessive or autoreactive inflammation.[91]
The NF-κB pathway comprises a family of transcription factors that serve as central regulators of immune system activity, coordinating innate and adaptive immune responses and mediating inflammatory signaling.[92][93] This pathway governs the activation, differentiation, and survival of key immune cell populations, including macrophages, dendritic cells, and T and B lymphocytes, which are essential for effective host defense against pathogens.[94][95][96] Cortisol suppresses immune activity by inhibiting the NF-κB pathway, resulting in broad attenuation of both innate and adaptive immune responses.[97][98][99]
Central Nervous System
Cortisol exerts complex influences on neuromodulation, cognition, mood regulation, and stress adaptation through both rapid nongenomic and slower genomic mechanisms. Acute cortisol elevation during stress enhances adaptive coping, attention, vigilance, and emotional responsiveness.[100] Activity changes within key brain regions, including the hippocampus, amygdala, thalamus, and prefrontal cortex, mediate these effects.[101][102]
Chronic stress or sustained cortisol elevation produces detrimental neurocognitive and psychiatric effects. Long-term mood alterations include depression, anxiety, mania, and psychosis.[103][104][105][106] Cortisol suppresses rapid eye movement (REM) sleep, leading to sleep disturbances.[107][108]
Prolonged cortisol excess contributes to cognitive decline, characterized by impaired declarative memory, attention, and executive function.[109] Structural brain alterations have been observed, including hippocampal atrophy, amygdalar and prefrontal dysfunction, and ventricular enlargement, all of which underlie persistent neurocognitive impairment.[110][111][112] These neuroanatomical changes are associated with an elevated risk of neurodegenerative disorders, such as Alzheimer disease and Parkinson disease.[113]
Ophthalmologic Effects
Chronic cortisol elevation increases intraocular pressure, predisposing to the development of glaucoma.[114][115] Prolonged exposure to elevated cortisol is also associated with posterior subcapsular cataract formation.[116] Increased choroidal thickness and macular alterations correlate with heightened cortisol levels, contributing to the risk of central serous chorioretinopathy.[117]
Gastrointestinal Effects
During acute stress, cortisol exerts gastroprotective effects by maintaining gastric mucosal integrity, mucosal blood flow, and mucus secretion.[118] These mechanisms reduce susceptibility to ulcerogenic injury. In contrast, sustained cortisol elevation, whether endogenous or exogenous, impairs gastrointestinal mucosal defense and barrier function.[119] Cortisol also alters gastrointestinal motility and nutrient absorption.[120] The combined effects promote gut dysbiosis and increased intestinal permeability, leading to heightened mucosal inflammation.[121][122]
Mechanism
The HPA axis tightly regulates cortisol secretion. Corticotropin-releasing hormone (CRH), synthesized and secreted by the paraventricular nucleus of the hypothalamus, stimulates the anterior pituitary to release ACTH.[123] ACTH then acts on the adrenal cortex to promote cortisol synthesis and secretion. Magnocellular neurons in the paraventricular and supraoptic nuclei produce arginine vasopressin.[124][125] This hormone modulates HPA axis activity by enhancing the stimulatory effects of CRH on pituitary corticotropes.[126] ACTH exerts trophic effects on the zonae fasciculata and reticularis of the adrenal cortex through activation of the melanocortin 2 receptor.[127][128][127]
Cortisol exerts negative feedback on both the hypothalamus and anterior pituitary, inhibiting the secretion of CRH and ACTH, respectively. The HPA axis operates in a circadian pattern, with cortisol concentrations peaking in the early morning and reaching their lowest levels at night.[129]
Cortisol is primarily cleared by the liver and kidneys. In the liver, the molecule undergoes irreversible inactivation via A-ring reductases, specifically 5β-reductase and 5α-reductase.[130] In the kidneys, 11β-HSD2 converts cortisol to its inactive form, cortisone, through oxidation. This process is reversible, as 11β-HSD1 in other tissues can regenerate cortisol from cortisone through reduction.[131][132]
Clinical Significance
Cortisol levels are continuously monitored in the body to maintain homeostasis. Unregulated levels can be detrimental, as evidenced by the clinical syndromes explained below.
Hypercortisolism
Cushing syndrome occurs when the body is exposed to high cortisol levels for a prolonged period. The etiologies of Cushing syndrome are classified as either ACTH-dependent or ACTH-independent. In ACTH-dependent subtypes, excess ACTH results from a pituitary tumor or an ectopic source, such as a neuroendocrine tumor. In both cases, the overproduction of ACTH stimulates the adrenal glands to produce excess cortisol.
In ACTH-independent subtypes, the condition may be due to either an endogenous or exogenous cause. The endogenous form typically results from an adrenal tumor that autonomously secretes cortisol. The exogenous form is caused by excessive oral or injectable corticosteroid use.[133] Oral corticosteroids, such as prednisone, increase circulating cortisol levels. These agents are prescribed to alleviate symptoms associated with chronic inflammatory diseases, such as systemic lupus erythematosus and rheumatoid arthritis.
The symptoms of Cushing syndrome depend on the degree and duration of cortisol elevation. Common manifestations include weight gain, particularly in the face and abdomen; fat deposition between the shoulder blades; diabetes; hypertension; hirsutism in women; proximal muscle weakness; and osteoporosis.[134] The underlying cause determines treatment. Surgical intervention is the preferred approach, but glucocorticoid receptor antagonists may be used when surgery is contraindicated.
Hypocortisolism
Primary adrenal insufficiency, also known as Addison disease, most commonly results from autoimmune adrenalitis.[135] Other etiologies include malignancy, infection, and adrenal hemorrhage. Autoimmune adrenalitis occurs when immune-mediated destruction targets the adrenal cortex.[136]
Secondary adrenal insufficiency arises from inadequate production of ACTH by the anterior pituitary gland. This condition may result from pituitary pathology, although suppression of the HPA axis due to chronic exogenous glucocorticoid therapy represents the most frequent cause.[137] Tertiary adrenal insufficiency occurs when the hypothalamus fails to release sufficient CRH.
Clinical manifestations of adrenal insufficiency include fatigue, weight loss, hypotension, and hyperpigmentation of the skin.[138] Aldosterone deficiency contributes to electrolyte disturbances, with laboratory findings commonly showing hyponatremia and hyperkalemia. Glucocorticoid replacement therapy, such as hydrocortisone, is required to manage hypocortisolism. Dosage adjustments are necessary during acute stressors, including illness or surgery, to prevent adrenal crisis.[139]
