In a large autopsy series of patients with chronic thyroiditis, the most common histological finding was lymphocytic infiltration with germinal centers and fibrosis of the thyroid parenchyma. Which is the most common form of chronic thyroiditis?

Explanation

## Hashimoto Thyroiditis — The Most Common Chronic Thyroiditis ### Epidemiology and Prevalence **Key Point:** Hashimoto thyroiditis (chronic lymphocytic thyroiditis) is the most common cause of hypothyroidism in iodine-sufficient regions and the most frequent form of chronic thyroiditis worldwide. It accounts for the vast majority of autoimmune thyroid disease. ### Pathological Features The histological hallmark of Hashimoto thyroiditis includes: - **Lymphocytic infiltration** — predominantly CD8^+^ T cells and B cells - **Germinal centers** — evidence of active B-cell response and antibody production - **Follicular atrophy and destruction** — loss of thyroid follicles - **Fibrosis** — variable degree, can progress to extensive fibrosis - **Hürthle cells** (oncocytic change) — metaplasia of follicular epithelium ### Autoimmune Mechanism 1. Loss of immune tolerance to thyroid peroxidase (TPO) and thyroglobulin 2. Th1-mediated cellular immunity predominates 3. B-cell production of pathogenic antibodies (anti-TPO, anti-thyroglobulin) 4. Complement-mediated and antibody-dependent cellular cytotoxicity 5. Progressive follicular destruction → hypothyroidism ### Clinical Features - **Insidious onset** over months to years - Gradual development of hypothyroidism - Diffuse, firm, non-tender thyroid enlargement (goiter) - Female predominance (10:1 ratio) - Peak incidence: 30–50 years of age - Associated with other autoimmune conditions (type 1 diabetes, celiac disease, pernicious anemia) ### Laboratory Findings **High-Yield:** - **High titers of anti-TPO antibodies** (>95% of patients) - **Anti-thyroglobulin antibodies** (60–70% of patients) - Elevated TSH with low/normal free T4 (overt hypothyroidism) - Elevated TSH with normal free T4 (subclinical hypothyroidism) - Normal or low radioiodine uptake ### Stages of Hashimoto Thyroiditis | Stage | TSH | Free T4 | Antibodies | Symptoms | |-------|-----|---------|------------|----------| | **Euthyroid** | Normal | Normal | Present | None | | **Subclinical hypothyroidism** | Elevated | Normal | Present | Minimal | | **Overt hypothyroidism** | Elevated | Low | Present | Fatigue, weight gain, cold intolerance | ### Management - Levothyroxine replacement (first-line) - TSH monitoring to titrate dose - Lifelong treatment in most cases - Screen for associated autoimmune conditions ### Why Hashimoto is Most Common **Clinical Pearl:** Hashimoto thyroiditis is the leading cause of hypothyroidism in iodine-sufficient countries and accounts for ~90% of all thyroiditis cases. Its chronic, indolent course and high prevalence in the general population (1–2% of adults) make it the most frequently encountered form of thyroiditis in clinical practice and pathology specimens. **Mnemonic:** **HALT** — Hashimoto, Autoimmune, Lymphocytic infiltration, TPO antibodies.