Addison's Disease — Primary Adrenal Insufficiency

Addison's disease is primary adrenal insufficiency resulting from destruction or dysfunction of the adrenal cortex, leading to inadequate production of glucocorticoids, mineralocorticoids, and androgens. This disorder has significant clinical importance because it presents with nonspecific symptoms that are frequently missed on initial evaluation, yet untreated disease carries a mortality risk exceeding 5% from acute decompensation. The prevalence is approximately 40–60 cases per million in developed countries, with autoimmune adrenalitis responsible for ~80% of cases in iodine-replete regions; incidence is higher in women and typically manifests in the third to fifth decades of life. Recognition of Addison's disease is essential for board preparation as it appears frequently in USMLE Step 2 CK examinations, particularly in vignettes presenting with hypotension, hyponatremia, and hyperkalemia, or as an incidental finding on imaging performed for other indications. The classic constellation of fatigue, hyperpigmentation, and electrolyte abnormalities remains a high-yield diagnosis that examiners specifically test.

Addison's disease results from loss of >90% of adrenocortical parenchyma, triggering a cascade of hormonal deficiencies with profound metabolic consequences. Understanding the molecular basis of this disease illuminates why different etiologies produce identical clinical presentations:

The adrenal cortex synthesizes cortisol in the zona fasciculata through a series of enzymatic reactions initiated by the P450 side-chain cleavage enzyme (P450scc), which converts cholesterol to pregnenolone. This reaction is rate-limiting and tightly controlled by ACTH (adrenocorticotropic hormone) binding to melanocortin-2 receptors on adrenocortical cells. ACTH, in turn, is released from corticotroph cells in response to corticotropin-releasing hormone (CRH) from the hypothalamus. In primary adrenal insufficiency, when >90% of adrenocortical tissue is destroyed, cortisol production falls precipitously. This loss of negative feedback on the hypothalamus and anterior pituitary results in unopposed CRH and ACTH secretion, with ACTH levels rising to 10–100 times the upper limit of normal (>200 pg/mL is typical). Despite maximally elevated ACTH, the damaged adrenal cortex cannot mount an adequate cortisol response. Glucocorticoid deficiency manifests as loss of metabolic homeostasis: impaired hepatic gluconeogenesis, reduced vascular tone, decreased cardiac contractility, diminished cellular stress response, and loss of the cortisol-mediated suppression of inflammatory cytokines. This accounts for the profound fatigue, hypotension, fever, and cardiovascular instability characteristic of the disease. The circadian rhythm of cortisol secretion is abolished early in the disease course.

Aldosterone, synthesized in the zona glomerulosa by the enzyme aldosterone synthase (CYP11B2), is the principal mineralocorticoid hormone regulating sodium and potassium homeostasis. In Addison's disease, loss of adrenocortical tissue eliminates aldosterone production. Unlike ACTH, which can partially compensate for cortisol loss through upregulation of the HPA axis, the juxtaglomerular apparatus responds to loss of adrenal aldosterone by secreting renin, but the damaged adrenal cortex cannot respond to elevated renin stimulation. The loss of mineralocorticoid activity leads to: (1) renal sodium wasting with sodium concentration typically 120–130 mEq/L, (2) reciprocal potassium retention and hyperkalemia (often 5.5–7.0 mEq/L), (3) metabolic acidosis from renal tubular dysfunction, and (4) volume depletion leading to hypotension and prerenal azotemia. Hyperkalemia, unique to primary adrenal insufficiency (distinguishing it from secondary/central insufficiency), directly increases the resting membrane potential of cardiac myocytes and can trigger life-threatening arrhythmias. The sodium depletion causes osmotic shifts leading to hyposmolality, which combined with ADH-driven water retention (ADH is appropriately elevated in the setting of hypovolemia) produces hyponatremia.

The adrenal cortex normally secretes dehydroepiandrosterone (DHEA) and androstenedione, which contribute to secondary sexual hair and metabolic homeostasis in women. In Addison's disease, loss of these androgens contributes to loss of axillary and pubic hair, particularly in women. However, the most clinically striking consequence relates to ACTH itself: ACTH and related peptides are cleaved from the precursor molecule pro-opiomelanocortin (POMC). Elevated ACTH levels drive melanocyte-stimulating hormone (MSH) production from POMC, and MSH acts on melanocortin-1 receptors in melanocytes to increase production of eumelanin. This hyperpigmentation typically affects sun-exposed areas and areas of friction (elbows, knees, scars, oral mucosa, palmar creases), and the intensity of hyperpigmentation correlates roughly with ACTH levels, making it a biomarker of disease severity. This mechanism uniquely identifies primary adrenal insufficiency: secondary adrenal insufficiency (from pituitary disease) presents with fatigue and hypotension but characteristically features pallor, not hyperpigmentation, because ACTH is low.

In developed countries, >80% of Addison's disease results from autoimmune adrenalitis, wherein the immune system mounts a Th1-mediated attack against adrenocortical antigens. The primary target is 21-hydroxylase (CYP21A2), with >90% of autoimmune Addison's patients having circulating antibodies to 21-hydroxylase. These antibodies are complement-fixing IgG antibodies that cross the blood-brain barrier and activate complement on adrenocortical cell surfaces. Cytotoxic CD8+ T cells infiltrate the adrenal gland, and inflammatory cytokines including TNF-α, IL-6, and IFN-γ orchestrate gradual destruction of the adrenal cortex. Pathology shows lymphocytic infiltration with preservation of the medulla (which is non-adrenocortical in origin, derived from neural crest). The disease is often associated with other autoimmune conditions in the setting of autoimmune polyendocrine syndromes (APS), where polymorphisms in genes encoding immune regulation (AIRE, PTPN22, HLA) predispose to co-occurring thyroiditis, type 1 diabetes, and celiac disease. HLA-DR3 and HLA-DQ2 alleles show increased prevalence in autoimmune Addison's disease. Notably, the 21-hydroxylase antibody test has >99% specificity for autoimmune adrenalitis but only ~50% sensitivity, so absence of antibodies does not exclude autoimmune disease.

Recent reports suggest that viral infections (particularly adenovirus and cytomegalovirus) can trigger acute exacerbation in patients with underlying adrenal insufficiency, possibly through molecular mimicry or direct viral injury to remaining adrenocortical cells. This accounts for acute presentation in some patients who may have had subclinical disease.

Addison's disease results from diverse etiologies, with geographic, immunologic, and epidemiologic variations:

This is the predominant cause in iodine-replete regions. Women outnumber men by a 1.5–2:1 ratio. Patients typically present in the third to fifth decades. Autoimmune Addison's may occur in isolation or as part of autoimmune polyendocrine syndrome (APS): APS-1 (autoimmune regulator [AIRE] gene mutation) presents in childhood with adrenalitis, hypoparathyroidism, and candidiasis; APS-2 (more common, non-Mendelian inheritance) presents in adulthood with Addison's, autoimmune thyroid disease, and type 1 diabetes. Associations with other autoimmune conditions (celiac disease, pernicious anemia, Graves' disease) are frequent. The presence of 21-hydroxylase antibodies confirms autoimmune etiology with high specificity.

TB remains the second leading cause worldwide, particularly in endemic regions and immunocompromised populations. Mycobacterium tuberculosis establishes chronic infection in the adrenal gland, leading to caseous necrosis, fibrosis, and eventual loss of parenchyma. Historically, TB was responsible for the vast majority of Addison's disease cases; modern antituberculous therapy has reduced this proportion in developed countries. Patients with TB-related Addison's often have a history of pulmonary TB, though adrenal involvement can occur without active pulmonary disease. Adrenal calcification on imaging is suggestive but not pathognomonic for TB (also seen in histoplasmosis, aspergillosis, and amyloidosis). TB-related Addison's carries a higher mortality risk if not recognized, and these patients require concurrent treatment with antituberculous agents and adrenal replacement.

Endemic fungi cause chronic destructive infection, particularly in immunocompromised hosts (HIV/AIDS with CD4 <50 cells/μL, transplant recipients). Histoplasmosis is the most common infectious cause in some geographic regions with high prevalence. Progressive adrenal infiltration leads to insufficiency. Adrenal calcification and imaging findings suggesting granulomatous disease are clues to fungal etiology. These infections typically coexist with systemic fungal disease (respiratory, disseminated).

Advanced HIV (CD4 <50 cells/μL) is complicated by adrenal insufficiency in 25–50% of cases, attributable to opportunistic infection (CMV, TB, MAC, fungi), direct HIV invasion of adrenocortical cells, and immune reconstitution inflammatory syndrome (IRIS) following antiretroviral therapy initiation. CMV adrenalitis presents with rapid onset of symptoms and fulminant disease. Introduction of effective antiretroviral therapy has reduced the frequency of HIV-associated Addison's in developed countries but remains common in resource-limited settings.

Lung cancer, breast cancer, lymphoma, and melanoma can metastasize to the adrenal glands, causing bilateral adrenal dysfunction. Adrenalectomy for tumor resection is a separate iatrogenic cause. Adrenal insufficiency from metastatic disease usually occurs in the context of known advanced malignancy, but occasionally presents as the first manifestation of malignancy.

This is a peroxisomal disorder resulting from mutations in the ABCD1 gene, leading to accumulation of very long-chain fatty acids in adrenocortical and neurologic tissues. The cerebral form presents with progressive neurologic decline and adrenal insufficiency; the adrenomyeloneuropathy (AMN) form shows adrenal insufficiency with later-onset neurologic symptoms; and the cerebral childhood form (CCALD) is particularly severe. Males are predominantly affected (X-linked), though heterozygous females can develop disease. This is a crucial differential diagnosis in men presenting with Addison's disease, particularly if age <40 years, as presence of neurologic findings (spasticity, cognitive decline, neuropathy) should prompt testing with plasma very long-chain fatty acids and consideration of ABCD1 genetic testing. Treatment with adrenal hormone replacement does not halt neurologic progression.

Bilateral adrenalectomy for severe Cushing's syndrome (Nelson syndrome when performed for ectopic ACTH) or adrenocortical carcinoma permanently requires lifelong adrenal hormone replacement.

The clinical presentation of Addison's disease spans a spectrum from insidious chronic disease to life-threatening acute crisis, with symptomatology driven by the duration and severity of adrenocortical hormone deficiency:

Fatigue is the most common initial complaint, present in >90% of patients at diagnosis, and often represents the primary reason patients seek medical evaluation. This fatigue is characteristically severe, progressive, and unresponsive to rest; patients report inability to work full-time or perform usual activities. The mechanism involves multiple factors: (1) glucocorticoid deficiency impairs the stress response and energy metabolism, reducing ATP production and increasing oxygen consumption, (2) hypotension and reduced cardiac output limit oxygen delivery to tissues, and (3) anemia (present in 20–30% of patients from chronic disease and autoimmune hemolysis) decreases oxygen-carrying capacity. The fatigue is often accompanied by generalized weakness that is proximal (hip and shoulder girdle weakness is prominent) and non-neurological; patients may report inability to climb stairs or rise from chairs. Symptoms are often worse in the afternoon, corresponding to the normal nadir of cortisol secretion (normally the lowest levels occur at midnight; early morning levels peak around 6–8 AM).

Supine systolic blood pressure is typically 85–100 mmHg (normal >110 mmHg), with diastolic pressures often 50–60 mmHg. This results from combined glucocorticoid deficiency (reduced alpha-1 adrenergic receptor expression, reduced cardiac output) and mineralocorticoid deficiency (sodium wasting, hypovolemia). Patients frequently report dizziness or syncope on standing, particularly after prolonged recumbency or physical exertion. Orth

Supporting laboratory findings: hyponatremia, hyperkalemia, non-anion-gap metabolic acidosis, hypoglycemia, prerenal azotemia, mild normocytic anemia, and eosinophilia with relative lymphocytosis (cortisol normally suppresses both).

In suspected crisis, do not wait for results. Draw cortisol and ACTH, then treat immediately. If confirmatory testing will follow, dexamethasone is preferred for the initial dose because it does not cross-react in cortisol immunoassays, allowing a cosyntropin test to be performed afterward.