A 35-year-old woman from Delhi presents with a 6-month history of progressive fatigue, weight gain, and cold intolerance. She reports a painless, diffuse enlargement of the thyroid gland. Laboratory studies show TSH 18 mIU/L (normal 0.5–5), free T4 0.5 ng/dL (normal 0.8–1.8), and elevated TPO antibodies (titer 1:1600). Thyroid ultrasound demonstrates diffuse hypoechoic echotexture with heterogeneous parenchyma. Fine-needle aspiration cytology (FNAC) shows lymphocytic infiltration with Hürthle cells. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Chronic Autoimmune Thyroiditis (Hashimoto's Thyroiditis) ### Pathognomonic Features **Key Point:** Hashimoto's thyroiditis is the most common cause of hypothyroidism in iodine-sufficient regions and is characterized by autoimmune destruction of the thyroid gland with positive TPO antibodies. ### Diagnostic Criteria | Feature | Hashimoto's | Subacute | Postpartum | Iodine Deficiency | |---------|-------------|---------|-----------|-------------------| | **Onset** | Insidious (months–years) | Acute (weeks) | Postpartum (6–12 mo) | Gradual | | **Neck pain** | Absent | Severe | Absent | Absent | | **Fever** | Absent | Common | Absent | Absent | | **TPO antibodies** | Positive (high titer) | Negative | May be positive | Negative | | **Thyroglobulin Ab** | Often positive | Negative | May be positive | Negative | | **TSH level** | Elevated | Suppressed (early) | Variable | Elevated | | **Free T4** | Low | Initially high | Variable | Low | | **ESR/CRP** | Normal | Markedly elevated | Normal | Normal | | **Ultrasound** | Diffuse hypoechoic | Heterogeneous hypoechoic | Variable | Diffuse hypoechoic | | **FNAC** | Lymphocytes + Hürthle cells | Granulomas | Lymphocytes | Colloid | ### Histopathology: The Gold Standard **High-Yield:** FNAC showing **lymphocytic infiltration with Hürthle cells** (oncocytic metaplasia of follicular epithelium) is virtually diagnostic of Hashimoto's thyroiditis. ### Pathophysiology 1. **Genetic predisposition** — HLA-DR3, HLA-DR5 association 2. **Loss of immune tolerance** — breakdown of regulatory T cell function 3. **Autoimmune attack:** - TPO antibodies bind thyroid peroxidase → complement activation - Thyroglobulin antibodies form immune complexes - CD8+ T cells infiltrate and destroy thyroid follicles 4. **Result:** Progressive thyroid fibrosis and hypothyroidism ### Clinical Presentation **Mnemonic: "COLD FACTS" (Hypothyroid Symptoms):** - **C**old intolerance - **O**besity/weight gain - **L**ethargy/fatigue - **D**ry skin - **F**atigue - **A**nemia (pernicious anemia association) - **C**onstipation - **T**emperature (low) - **S**low metabolism ### Why This Patient Has Hashimoto's Thyroiditis 1. **Insidious onset over 6 months** — chronic autoimmune process, not acute viral 2. **Positive TPO antibodies (titer 1:1600)** — hallmark of autoimmune thyroiditis 3. **Overt hypothyroidism (TSH 18, free T4 0.5)** — end-stage of autoimmune destruction 4. **Painless diffuse thyroid enlargement** — lymphocytic infiltration without inflammation 5. **FNAC with Hürthle cells** — pathognomonic for Hashimoto's 6. **Diffuse hypoechoic ultrasound** — reflects chronic lymphocytic infiltration and fibrosis ### Management Algorithm ```mermaid flowchart TD A[Hashimoto's Thyroiditis Diagnosed]:::outcome --> B{TSH Level?}:::decision B -->|TSH elevated + symptoms| C[Start Levothyroxine]:::action B -->|TSH elevated + no symptoms| D[Monitor TSH 6-monthly]:::decision D -->|TSH > 10| E[Start Levothyroxine]:::action C --> F[Initial dose: 25-50 mcg daily]:::action F --> G[Titrate by 25-50 mcg every 6-8 weeks]:::action G --> H[Target TSH 0.5-2.5 mIU/L]:::action H --> I[Monitor TSH annually once stable]:::action I --> J[Long-term replacement therapy]:::outcome ``` **Clinical Pearl:** Levothyroxine should be taken on an empty stomach, 30–60 minutes before breakfast, to maximize absorption. Calcium, iron, and proton pump inhibitors interfere with absorption and should be separated by ≥4 hours. **Warning:** Do NOT confuse Hashimoto's (autoimmune, TPO+) with subacute thyroiditis (viral, TPO−, painful). The presence of high-titer TPO antibodies is the key discriminator. [cite:Harrison 21e Ch 405]