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I now have comprehensive material from Harrison's, Robbins (both editions), Goldman-Cecil, and Tietz, plus three excellent images. I'll compile the full detailed answer.

Thyroiditis — A Comprehensive Overview

Thyroiditis encompasses a diverse group of disorders characterized by thyroid inflammation. They are classified by onset and duration (acute/subacute/chronic), presence or absence of pain, and underlying etiology (autoimmune, infectious, drug-induced, post-radiation). — Harrison's Principles of Internal Medicine 22E

Classification

Category	Type	Key Feature
Acute	Suppurative / radiation / drug-induced	Bacterial/fungal infection, acute pain, fever
Subacute	De Quervain's (granulomatous)	Painful, post-viral, transient thyrotoxicosis
Subacute	Silent / postpartum (lymphocytic)	Painless, autoimmune, transient thyrotoxicosis
Chronic	Hashimoto's (autoimmune)	Most common; goiter → hypothyroidism
Chronic	Riedel's thyroiditis	Rare; invasive fibrosis
Chronic	Drug-induced	Amiodarone, IFN, checkpoint inhibitors, TKIs

1. Acute (Suppurative) Thyroiditis

Etiology & Pathogenesis

Rare. Caused by bacterial infection (Staphylococcus, Streptococcus, Enterobacter) or fungal organisms (Aspergillus, Candida, Coccidioides, Histoplasma, Pneumocystis — particularly in immunocompromised patients). In children and young adults, the most common predisposing cause is a piriform sinus fistula (remnant of the 4th branchial pouch), predominantly left-sided. In the elderly, risk factors include long-standing goiter and degeneration in thyroid malignancy. — Harrison's 22E

Clinical Features

Thyroid pain, often referred to throat or ears
Small, tender, possibly asymmetric goiter
Fever, dysphagia, erythema over thyroid
Cervical lymphadenopathy
Thyroid function usually normal

Investigations

↑ ESR, ↑ WBC
FNA: polymorphonuclear leukocyte infiltration; culture identifies organism
CT/ultrasound: localizes abscess

Complications

Tracheal obstruction, septicemia, retropharyngeal abscess, mediastinitis, jugular venous thrombosis — uncommon with prompt antibiotics.

Treatment

Antibiotics guided by Gram stain and culture
Surgical drainage if abscess forms

2. Subacute (De Quervain's / Granulomatous / Viral) Thyroiditis

Etiology

The most common cause of a painful thyroid in outpatient practice. Multiple viruses implicated: mumps, coxsackie, influenza, adenoviruses, echoviruses, and SARS-CoV-2 (including post-COVID-vaccine cases). Peak incidence: 30–50 years; women 3× more commonly than men. — Harrison's 22E

Pathophysiology

Characteristic patchy inflammatory infiltrate with disruption of thyroid follicles and multinucleated giant cells within follicles → granulomas with fibrosis → eventual return to normal.

The sequence of thyroid function changes:

Thyrotoxic phase: Follicular destruction → release of stored T4 and T3 → ↑ T4/T3, suppressed TSH, low/absent radioiodine uptake (<5%)
Hypothyroid phase: Stored hormone depleted → ↓ free T4, ↑ TSH
Recovery phase: T4/T3/TSH return to normal over ~6 months

The T4:T3 ratio during thyrotoxic phase is lower than in Graves' disease (where T3 is disproportionately elevated), reflecting passive hormone release rather than synthesis.

Clinical Course

Clinical phases of subacute thyroiditis. During the thyrotoxic phase, UT4 is elevated and TSH is suppressed. ESR peaks early and declines with inflammation. The hypothyroid phase follows as hormone stores are depleted, with TSH rising above normal, before eventual recovery. — Harrison's 22E

Painful, enlarged thyroid (exquisitely tender); pain referred to jaw or ear
Malaise, preceding URI symptoms
Fever and features of thyrotoxicosis/hypothyroidism depending on phase
Permanent hypothyroidism in ~15%, especially those with underlying thyroid autoimmunity

Investigations

↑ ESR (hallmark — often very high), ↑ T4/T3, suppressed TSH (thyrotoxic phase)
Radioiodine uptake: very low (<5%) during thyrotoxic phase
Thyroid antibodies: negative (distinguishes from Hashimoto's)
FNA: granulomatous inflammation with giant cells (if needed)

Histology

H&E of subacute (De Quervain's) thyroiditis showing granulomatous inflammation with multinucleated giant cells, disrupted follicles, lymphocytic infiltrate, and diminished colloid

High-power H&E: granulomatous thyroiditis with multinucleated foreign body giant cells, epithelioid histiocytes, and disrupted follicles. No caseation (distinguishes from TB). — WebPathology

Treatment

Mild-moderate: Aspirin 600 mg every 4–6 h or NSAIDs (+ PPI for gastroprotection)
Severe or inadequate response: Prednisone 15–40 mg/day, tapered over 6–8 weeks guided by symptom and ESR response
Thyrotoxic symptoms: β-blockers (propranolol) — antithyroid drugs are NOT effective (no excess synthesis)
Hypothyroid phase: Levothyroxine if symptomatic; usually temporary

3. Hashimoto's Thyroiditis (Chronic Autoimmune / Chronic Lymphocytic Thyroiditis)

Epidemiology

Most common cause of hypothyroidism in iodine-sufficient regions. Most prevalent between 45–65 years. Female:male ratio 10:1 to 20:1. First described by Hakaru Hashimoto in 1912 ("struma lymphomatosa"). — Robbins & Cotran Pathologic Basis of Disease; Tietz Lab Medicine

Pathogenesis

Breakdown of self-tolerance to thyroid autoantigens (thyroglobulin, thyroid peroxidase). Strong genetic component with polymorphisms in:

CTLA4 (cytotoxic T lymphocyte-associated antigen-4)
PTPN22 (protein tyrosine phosphatase non-receptor type 22)
IL2RA (IL-2 receptor α chain)

These are regulators of T-cell responses. Environmental risk factors include smoking, iodine excess, selenium deficiency, and infections. — Robbins & Cotran

Multiple immune mechanisms drive thyroid cell death:

CD8+ cytotoxic T cell–mediated destruction of follicular cells
Cytokine-mediated injury: CD4+ Th1 cells → IFN-γ → macrophage recruitment and follicular damage
Antibody-mediated mechanisms: Anti-TPO and anti-thyroglobulin antibodies → complement-dependent or ADCC-mediated injury (role still debated — may be consequence as well as cause)

Morphology

Thyroid diffusely enlarged, well-demarcated, pale yellow-tan and firm on cut surface
Extensive lymphoplasmacytic infiltrate with prominent germinal centers
Hürthle cell (oncocyte) change: follicular epithelium replaced by cells with abundant eosinophilic, granular cytoplasm — a metaplastic response to chronic injury
Atrophic follicles in advanced disease
With progression: fibrosis (fibrous variant ~10–12% of cases — dense hyalinized stroma with residual follicular islands; capsule intact, distinguishing it from Riedel's)

Low-power H&E of fibrous Hashimoto thyroiditis showing dense hyalinized fibrous bands replacing thyroid parenchyma, with residual lymphoid follicles and germinal centers

Low-power H&E: Fibrous variant of Hashimoto thyroiditis. Dense fibrotic stroma surrounds residual lymphoid follicles with germinal centers. The intact capsule distinguishes this from Riedel's thyroiditis. — WebPathology

Clinical Features

Goiter: usually firm, symmetric, non-tender; may be lobulated
Gradual onset of hypothyroidism: fatigue, weight gain, cold intolerance, constipation, dry skin, bradycardia, myxedema
Can pass through transient thyrotoxicosis ("Hashitoxicosis") early in disease
Progresses from euthyroid → subclinical hypothyroidism → overt hypothyroidism
Associated with other autoimmune diseases: type 1 diabetes, SLE, RA, Sjögren syndrome, pernicious anemia, Addison's disease

Investigations

Anti-TPO antibodies: positive in ~90% (most sensitive marker)
Anti-thyroglobulin antibodies: positive in 20–50%
TSH, free T4: depends on phase (may be normal, ↑ TSH with normal T4 in subclinical, or ↑ TSH + ↓ T4 in overt hypothyroidism)
Thyroid ultrasound: hypoechoic and heterogeneous parenchyma
Radioiodine uptake: variable (elevated if TSH is high, low in destructive phase)

Treatment

Overt hypothyroidism: Levothyroxine (goal: normalize TSH to 0.5–2.5 mU/L)
Subclinical hypothyroidism with high antibody titers: levothyroxine recommended
Goiter reduction: short-term TSH-suppressing doses may reduce goiter size
Monitoring: serial TSH; lifelong follow-up needed

Complications & Associated Malignancy

Primary B-cell lymphoma of the thyroid (MALT lymphoma): rare but well-established risk from prolonged intrathyroidal B-cell stimulation
Papillary thyroid carcinoma: increased frequency, particularly in women
FNA biopsy indicated for suspicious nodules

4. Silent (Painless) Thyroiditis & Postpartum Thyroiditis

Overview

Both are variants of destructive autoimmune thyroiditis, essentially the same disorder — distinguished only by timing: postpartum thyroiditis occurs in the first 12 months after delivery. — Goldman-Cecil Medicine; Tietz

Epidemiology

Postpartum thyroiditis: up to 10% of pregnancies; ~20× more common than Graves' in postpartum period; risk ↑ with positive anti-TPO in first trimester, personal/family history of autoimmunity
Sporadic painless thyroiditis: less common; ages 30–60; slightly more common in women (1.5:1)
50% of anti-TPO positive women in first trimester will develop postpartum thyroiditis

Clinical Phases

Classic triphasic pattern (experienced by ~20% of postpartum cases):

Thyrotoxic phase (1–3 months): palpitations, tachycardia, heat intolerance; non-tender goiter; symptoms milder than Graves'
Euthyroid interlude (1–2 months)
Hypothyroid phase (up to 12 months): fatigue, cold intolerance, constipation

~50% experience only hypothyroidism; ~30% only thyrotoxicosis.

Key Distinguishing Features from Graves' Disease

Feature	Silent/Postpartum Thyroiditis	Graves' Disease
Thyroid pain	Absent	Absent
Goiter	Non-tender	Non-tender, with bruit
Ophthalmopathy/dermopathy	Absent	Present
T3:T4 ratio	<20 (passive release)	T3 preferentially elevated
RAIU	Low/undetectable	High
TRAb	Negative	Positive
Anti-TPO	Usually positive	Often positive
Color Doppler	↓ flow	↑ flow

Treatment

Thyrotoxic phase: β-blockers if symptomatic; antithyroid drugs are contraindicated (no excess synthesis)
Hypothyroid phase: Levothyroxine if TSH >10 mIU/L or symptomatic
Women with history of postpartum thyroiditis: annual TSH measurement (↑ risk of permanent hypothyroidism — up to 30–50% at 1 year remain hypothyroid)

5. Drug-Induced Thyroiditis

Several drugs can cause thyroiditis, presenting as subacute or chronic disease:

Drug	Pattern
Amiodarone	Type 1 (iodine-induced hyperthyroidism) or Type 2 (destructive thyroiditis)
Interferon-α	Autoimmune or destructive thyroiditis
Tyrosine kinase inhibitors	Hypothyroidism or destructive thyroiditis
Immune checkpoint inhibitors (anti-PD-1, anti-CTLA-4)	Autoimmune thyroiditis, hypo- or hyperthyroidism
Lithium	Hypothyroidism

Amiodarone is particularly notable: it contains 37% iodine by weight and can cause thyrotoxicosis (both excess synthesis and destructive release) or hypothyroidism. Checkpoint inhibitor–induced thyroiditis is increasingly recognized as a major immune-related adverse event.

6. Riedel's Thyroiditis

Features

Rare, fibrosing disorder of unknown etiology; now considered part of the IgG4-related disease spectrum
Dense, keloid-like fibrosis that extends beyond the thyroid capsule into adjacent soft tissues (strap muscles, trachea, esophagus, carotid vessels) — this extracapsular invasion distinguishes it from fibrous Hashimoto's
Thyroid is rock-hard ("iron-hard goiter"), mimicking anaplastic carcinoma
Associated with other fibrosclerotic conditions: retroperitoneal fibrosis, orbital pseudotumor, sclerosing cholangitis
Symptoms: compressive symptoms (dysphagia, stridor, dysphonia), hypothyroidism
Treatment: glucocorticoids, tamoxifen; surgical release for tracheal compression; thyroid hormone replacement

Key Differential Diagnosis Summary

Parameter	De Quervain's	Hashimoto's	Postpartum	Acute Suppurative	Riedel's
Pain	Yes	No	No	Yes (severe)	No
Fever	Mild-mod	No	No	High	No
Thyrotoxicosis	Transient	Occasional	Transient	No	No
Hypothyroidism	Transient	Permanent	Often transient	No	Variable
Anti-TPO	Negative	Positive (90%)	Positive	Negative	Negative
ESR	Very high	Normal/mild↑	Normal	↑	Normal
RAIU	Very low	Variable	Very low	Normal	Normal/low
Histology	Granulomas, giant cells	Lymphoid follicles, Hürthle cells	Lymphocytic	PMN infiltrate	Dense fibrosis

References

Harrison's Principles of Internal Medicine 22E, pp. 3081–3082
Robbins & Cotran Pathologic Basis of Disease, pp. 998–1001
Robbins & Kumar Basic Pathology, p. 3184–3192
Goldman-Cecil Medicine (International Edition), pp. 3102–3135
Tietz Textbook of Laboratory Medicine 7E, pp. 3870–3936

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