Tell me all reasons for icthing

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I now have very rich, authoritative content from multiple dermatology and clinical textbooks. Let me compile the comprehensive answer.

All Causes of Itching (Pruritus)

Pruritus is classified into dermatological, systemic/internal, neurological/neuropathic, pharmacological, pregnancy-related, and psychogenic categories. Here is the full breakdown:

1. Primary Skin (Dermatological) Causes

These are conditions where itching originates directly from the skin:

Condition	Notes
Atopic dermatitis (eczema)	Most common chronic pruritic skin disease
Contact dermatitis	Allergic or irritant reaction to substances
Psoriasis	Silvery plaques, often pruritic
Urticaria (hives)	Histamine-mediated wheals
Scabies	Sarcoptes scabiei mite infestation; intense nocturnal itch
Dermatitis herpetiformis	Associated with gluten/celiac disease
Lichen planus	Violaceous, itchy papules
Xerosis (dry skin)	Especially common in elderly (winter itch / asteatotic eczema)
Bullous pemphigoid	Autoimmune blistering disease with severe itch
Insect bites / pediculosis	Lice, fleas, bed bugs
Tinea infections	Fungal infections (ringworm, athlete's foot)
Seborrheic dermatitis	Scalp and face; flaking and itch
Miliaria (heat rash)	Blocked sweat ducts
Prurigo nodularis	Nodular lesions from chronic scratching
Lichen simplex chronicus	Thickened skin from repeated scratching
Folliculitis	Infected hair follicles
Sunburn / photodermatitis	UV-induced skin damage

Andrews' Diseases of the Skin, p. 2504 (Pruritic Dermatoses)

2. Systemic / Internal Causes

These are conditions where itching occurs due to an internal disease, often without primary skin lesions:

Renal

Chronic Kidney Disease (CKD) / Uremic pruritus - the most common systemic cause; affects 20-80% of patients with chronic renal failure. Mechanisms include xerosis, secondary hyperparathyroidism, elevated serum histamine, iron deficiency anemia, and hypervitaminosis A.

Hepatic / Cholestatic

Obstructive liver disease / cholestasis (e.g., primary biliary cirrhosis, primary sclerosing cholangitis)
Hepatitis C (with or without jaundice)
Intrahepatic cholestasis of pregnancy

Endocrine / Metabolic

Diabetes mellitus
Hypothyroidism and hyperthyroidism
Hyperparathyroidism

Hematological

Iron deficiency anemia
Polycythemia vera - affects >1/3 of patients; classically triggered by warm water bathing (aquagenic pruritus)
Hodgkin lymphoma - pruritus is a first symptom in 7% of patients; incidence 10-30%; often severe with burning quality
Chronic lymphocytic leukemia (CLL)
Cutaneous T-cell lymphoma (mycosis fungoides)
Myeloma

Malignancy

Internal solid organ cancers - generalized pruritus without skin lesions can be a paraneoplastic phenomenon; workup is indicated

Infectious

HIV/AIDS - severe, treatment-resistant pruritus; associated with elevated IgE, eosinophilia, and Th2-cytokine profile
Intestinal parasites (eosinophilia on CBC is a screening clue)
Hepatitis C

Autoimmune / Connective Tissue

Dermatomyositis (connective tissue disease)
Systemic lupus erythematosus

Other Systemic

Carcinoid syndrome (serotonin release)
Multiple sclerosis (also has neurological mechanisms)
Andrews' Diseases of the Skin, p. 2408-2418; Dermatology 2-Volume Set 5e, p. 1863

3. Neurological / Neuropathic Causes

Neuropathic pruritus arises from disease, compression, or irritation of afferent nerves - not from skin disease:

Brachioradial pruritus - cervical/thoracic spine pathology
Notalgia paresthetica - mid-back itch from thoracic nerve entrapment
Postherpetic pruritus - after shingles (herpes zoster) healing
Multiple sclerosis - MS plaques can generate pruritus
Brain tumors / stroke - central nervous system lesions
Small fiber neuropathy
Trigeminal trophic syndrome
Post-stroke pruritus
Dermatology 2-Volume Set 5e, p. 1898 (Neuropathic Etiologies of Pruritus)

4. Drug-Induced (Pharmacological) Causes

Virtually any drug can cause a pruritic reaction. Specific mechanisms:

Mechanism	Drugs
Cholestasis	Erythromycin estolate, estrogens (oral contraceptives), captopril, sulfonamides, chlorpromazine
Hepatotoxicity	Acetaminophen, anabolic steroids, isoniazid, minocycline, amoxicillin-clavulanic acid, halothane, phenytoin
Xerosis	Retinoids, beta-blockers, calcium channel blockers, hydrochlorothiazide, tamoxifen, busulfan
Phototoxicity	8-methoxypsoralen
Neurologic (opioid-mediated)	Morphine, codeine, fentanyl, tramadol, butorphanol
Stimulant-induced	Methamphetamine, cocaine
Increased serotonin	SSRIs (sertraline, fluoxetine)
Increased bradykinin	ACE inhibitors
Biologic agents	Anti-EGFR monoclonal antibodies (used in cancer), IL-2

Dermatology 2-Volume Set 5e, p. 1877-1882

5. Pregnancy-Related Causes

Approximately 20% of pregnant women experience pruritus:

Intrahepatic cholestasis of pregnancy (most clinically significant)
Pruritic urticarial papules and plaques of pregnancy (PUPPP)
Pemphigoid gestationis (autoimmune)
Atopic eruption of pregnancy
Prurigo of pregnancy
Dermatology 2-Volume Set 5e, p. 1872

6. Psychogenic / Psychiatric Causes

Chronic idiopathic pruritus - can be linked to cutaneous, systemic, neurologic, or psychiatric etiologies
Anorexia nervosa
Depression and anxiety disorders
Obsessive-compulsive disorder (delusional parasitosis / Ekbom syndrome)
Psychogenic excoriation

Psychogenic pruritus is characterized by: temporal association with psychiatric symptoms, absence of nocturnal awakening, and response to psychotropic drugs. - Kaplan & Sadock's Comprehensive Textbook of Psychiatry

7. Local / Regional Causes

Pruritus ani - perianal itching; causes include hemorrhoids, ectropion, anal fissure/fistula, neoplasm, fungal/bacterial infection, pinworms, contact dermatitis, dietary irritants (coffee, spices, citrus)
Pruritus vulvae - causes include unspecified dermatitis (54%), lichen sclerosus (13%), candidiasis (10%), dysesthetic vulvodynia (9%), psoriasis (5%)
Pruritus scroti
Scalp pruritus - seborrheic dermatitis, psoriasis, pediculosis capitis
Schwartz's Principles of Surgery, p. 2638; Andrews' Diseases of the Skin, p. 2589

Quick Summary by Category

Category	Key Examples
Skin disease	Eczema, psoriasis, scabies, urticaria, dry skin
Kidney	CKD/uremia (most common systemic cause)
Liver	Cholestasis, hepatitis C, PBC
Blood	Polycythemia vera, Hodgkin lymphoma, CLL
Endocrine	Diabetes, thyroid disease
Nerve	Postherpetic, brachioradial, MS, stroke
Drugs	Opioids, SSRIs, ACE inhibitors, retinoids
Infection	HIV, parasites, fungal
Cancer	Solid tumors (paraneoplastic), lymphoma
Pregnancy	Cholestasis, PUPPP, pemphigoid gestationis
Psychiatric	Anxiety, depression, delusional parasitosis

Clinical pearl: Generalized pruritus without a primary rash (sine materia) always warrants a systemic workup including CBC with differential, liver/renal/thyroid panels, fasting glucose, hepatitis C serology, HIV (if risk factors), urinalysis, serum protein electrophoresis, and chest X-ray as a minimum. Eosinophilia is a useful screen for parasitic disease. - Andrews' Diseases of the Skin

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