Hereditary Hemochromatosis

Hereditary hemochromatosis (HH) is a genetic disorder of iron metabolism characterized by inappropriate intestinal iron absorption leading to progressive iron overload in multiple organ systems. It is the most common genetic disorder in populations of Northern European descent, with a prevalence of approximately 1 in 200-400 individuals in the United States and Europe, though clinical penetrance varies significantly. The disease has tremendous clinical importance because it is both highly treatable and easily missed if not actively screened for, particularly in asymptomatic individuals. Type 1 HH, caused by HFE gene mutations, accounts for >90% of cases in populations of European ancestry and presents classically in middle-aged men with cirrhosis, diabetes mellitus, and arthropathy. Early diagnosis and treatment with phlebotomy can prevent virtually all clinical complications and normalize life expectancy, making it an ideal screening target for medical boards and clinical practice.

The fundamental pathophysiology of hemochromatosis stems from inadequate regulation of hepcidin, a 25-amino acid peptide hormone that is the master regulator of systemic iron homeostasis. Hepcidin is synthesized by hepatocytes and acts on enterocytes and macrophages by binding to ferroportin (the only known iron exporter), causing its internalization and degradation, thereby reducing iron export into plasma. In normal physiology, hepcidin expression is tightly regulated by iron stores (via the iron-sensing BMP6/SMAD pathway), erythropoietic demand (via erythroferrone from erythroblasts), and inflammation (via IL-6/STAT3). In HFE-related hemochromatosis, mutations in the HFE protein impair the body's ability to sense iron stores, resulting in inappropriately low hepcidin levels despite elevated iron stores—this is the critical defect. Consequently, ferroportin remains functional on the basolateral surface of duodenal enterocytes, permitting continuous iron absorption regardless of body iron status. The human body lacks an active mechanism for iron excretion (unlike other minerals), so iron continuously accumulates at approximately 1-2 mg/day in the setting of unregulated absorption.

Once absorbed iron enters the circulation, it is transported via transferrin to target tissues. In hemochromatosis, transferrin saturation rises above the normal range of 20-45% (typically exceeding 60%), and excess non-transferrin-bound iron (NTBI) circulates and deposits in organs. NTBI is highly toxic because it catalyzes Fenton chemistry reactions generating reactive oxygen species (ROS), particularly hydroxyl radicals. Iron accumulates in parenchymal cells of the liver, pancreas, heart, and pituitary, as well as in iron-storage cells of joints and synovium. The pattern of organ involvement reflects tissue-specific iron uptake kinetics: the liver preferentially accumulates iron first and most extensively (cirrhosis develops after ~20 grams of iron accumulation), followed by the pancreas (insulin-producing beta cells are particularly vulnerable), myocardium, and anterior pituitary. Iron deposition in hepatocytes triggers oxidative stress, lipid peroxidation, and activation of stellate cells, ultimately leading to hepatic fibrosis and cirrhosis through a process independent of inflammation or viral hepatitis.

The HFE gene encodes a major histocompatibility complex class I-like molecule that serves as a co-receptor for the transferrin receptor 1 (TfR1) and interacts with transferrin receptor 2 (TfR2), both critical for iron sensing by hepatocytes. The most common mutation, C282Y (cysteine-to-tyrosine substitution at codon 282), disrupts a disulfide bond necessary for proper HFE-TfR1 interaction and protein folding, leading to misfolding, retention in the endoplasmic reticulum, and impaired iron sensing. The second most common variant, H63D (histidine-to-aspartate at codon 63), is less functionally significant and causes disease primarily when homozygous or compound heterozygous with C282Y (though homozygous H63D rarely causes clinically significant disease). Compound heterozygotes (C282Y/H63D) have intermediate iron accumulation and variable penetrance. Non-HFE mutations in HAMP (hepcidin), HJV (hemojuvelin), TFR2, and SLC40A1 (ferroportin) cause rare forms of iron overload with distinct inheritance patterns and variable clinical presentations; these are collectively termed "non-HFE hemochromatosis" or "secondary iron overload."

In the liver, iron-catalyzed oxidative stress damages hepatocytes and activates hepatic stellate cells through transforming growth factor-beta (TGF-β) signaling, leading to collagen deposition and progressive cirrhosis. Cirrhotic livers are at extraordinarily high risk for hepatocellular carcinoma (HCC), with annual incidence of 3-6% even after phlebotomy removes excess iron (because fibrosis/cirrhosis is irreversible). In the pancreas, iron deposition in beta cells impairs insulin synthesis and secretion, causing "bronze diabetes" (combination of hemochromatosis and diabetes mellitus); iron also accumulates in exocrine tissue, though exocrine insufficiency is less common than endocrine dysfunction. In the myocardium, iron deposits cause restrictive or dilated cardiomyopathy with conduction abnormalities and arrhythmias; myocardial involvement is a leading cause of death in untreated hemochromatosis. The anterior pituitary is vulnerable to iron deposition, resulting in hypogonadism (low luteinizing hormone and follicle-stimulating hormone secretion), impotence, and amenorrhea—these endocrine manifestations often precede hepatic cirrhosis. In joints, particularly the small joints of hands and wrists, iron deposits in synovium and cartilage trigger inflammatory arthropathy through oxidative mechanisms and altered cartilage turnover, creating a pattern that mimics rheumatoid arthritis.

The most frequent presenting symptom, occurring in ~80% of symptomatic patients. Fatigue in hemochromatosis results from iron-induced mitochondrial dysfunction and oxidative stress in multiple tissues, exacerbated by myocardial iron deposition reducing cardiac output, hepatic cirrhosis causing metabolic derangement, and hypogonadism (low testosterone) impairing vitality. Morning fatigue that worsens through the day is characteristic.

Affects 25-50% of symptomatic patients, particularly men. Iron deposition in synovial tissue and cartilage triggers an inflammatory arthritis that typically involves the second and third metacarpophalangeal (MCP) joints, wrists, knees, and shoulders (the characteristic "arthritis of hemochromatosis" MCP distribution differs from rheumatoid arthritis, which spares the MCP joints initially). Patients describe morning stiffness, joint swelling, and progressive limitation of motion; the arthropathy may be the earliest clinical manifestation and can progress to severe joint damage and disability even after iron removal. X-rays often show chondrocalcinosis (calcium pyrophosphate deposition), acute arthritis attacks may resemble gout, and the condition is notoriously refractory to anti-inflammatory therapy.

Erectile dysfunction and loss of libido affect 25-50% of symptomatic men, caused by iron deposition in the anterior pituitary impairing gonadotropin secretion. This may be the earliest endocrine manifestation and can precede cirrhosis. Testicular atrophy occurs, resulting in firm, small, painless testes on examination. Women may present with amenorrhea or premature menopause due to pituitary dysfunction.

Right upper quadrant pain or fullness may indicate hepatomegaly (present in ~75% of symptomatic patients), progressing to signs of cirrhosis including ascites, jaundice, and splenomegaly. Iron-induced cirrhosis is indistinguishable from cirrhosis of other etiologies once established; hepatic decompensation with variceal bleeding, encephalopathy, and renal failure may be the initial presentation if the disease is diagnosed late.

Occurs in approximately 50% of symptomatic patients with cirrhosis, though only 20% with non-cirrhotic iron overload. Hyperglycemia results from iron-induced beta cell dysfunction (both reduced insulin secretion and increased insulin resistance from hepatic cirrhosis). The term "bronze diabetes" reflects the hyperpigmentation that may occur from melanin deposition in skin (from hemosiderin in dermal fibroblasts and from melanocyte stimulation). Diabetes control is often difficult and may require insulin despite modest hyperglycemia, reflecting beta cell failure rather than simple insulin resistance.

Myocardial iron deposition presents with dyspnea, palpitations, orthopnea, and lower extremity edema from restrictive or dilated cardiomyopathy. Life-threatening arrhythmias (atrial fibrillation, ventricular tachycardia) and conduction abnormalities are common. Cardiac complications are a major cause of death in untreated hemochromatosis, particularly in younger patients with Type 2 (juvenile) hemochromatosis, where cardiomyopathy often dominates the clinical picture.

Occurs in approximately 25% of symptomatic patients and results from a combination of hemosiderin deposition in dermal macrophages and increased melanin production stimulated by iron overload. The hyperpigmentation is typically "slate gray" or bronze in appearance, affecting sun-exposed areas and the face; this is distinct from the jaundice of cirrhosis and gives rise to the historical term "bronze diabetes."

Approximately 80% of people homozygous for C282Y never develop clinical disease; those who do typically remain asymptomatic for years despite gradual iron accumulation. Screening programs have identified many asymptomatic individuals with iron overload, in whom early phlebotomy therapy prevents disease development—this is the highest-yield scenario for USMLE Step 2 CK. These patients often present incidentally on routine laboratory screening (elevated transferrin saturation or ferritin on a chemistry panel) or during family screening after a proband is diagnosed.

Because menstruation provides approximately 0.5 mg/day of iron loss, premenopausal women develop clinical disease infrequently. However, postmenopausal women accumulate iron at rates approaching men and may present in the sixth to eighth decade with cirrhosis and its complications, sometimes with minimal preceding symptoms. Pregnancy may have a protective effect through hemodilution and increased iron demands, but iron accumulation resumes promptly postpartum.

Presents dramatically in adolescents and young adults (often in the second to fourth decade) with severe, rapidly progressive iron overload. Hypogonadism and cardiomyopathy dominate the clinical picture, with cirrhosis developing quickly and cardiac complications frequently appearing before hepatic decompensation. These patients present with severe fatigue, sexual dysfunction, and syncope from arrhythmias.

May present atypically with macrophage iron loading (ferroportin disease) sparing hepatocytes, or with severe juvenile-onset disease and hepcidin mutations. The pattern of organ involvement may differ from HFE-HH.

The diagnosis of hemochromatosis begins with clinical suspicion, which should be raised in: (1) any patient with