A 42-year-old woman presents with progressive fatigue, weight gain, cold intolerance, and delayed relaxation of her Achilles reflex. On examination, she has a firm, rubbery goiter and non-pitting edema of the face and hands. Thyroid biopsy shows dense lymphocytic infiltrate with germinal centers, atrophic follicles, and the structure marked **B** — large eosinophilic granular cells packed with mitochondria. Which of the following best describes the pathophysiologic basis of her primary hypothyroidism?

Explanation

## Why "Autoimmune lymphocytic destruction of thyroid follicles with anti-TPO and anti-thyroglobulin antibodies causing Hashimoto thyroiditis" is right The structure marked **B** — Hürthle (oncocytic/oxyphil) cells with large eosinophilic granular cytoplasm packed with mitochondria — is the pathognomonic histologic hallmark of Hashimoto thyroiditis. These cells represent thyroid follicular epithelium undergoing autoimmune destruction. The clinical presentation (firm rubbery goiter, delayed Achilles reflex, myxedema, female predominance, age 30–50 years) and histology (dense lymphocytic infiltrate with germinal centers, atrophic follicles) are diagnostic of Hashimoto thyroiditis, an autoimmune condition mediated by anti-TPO antibodies (90% sensitive) and anti-thyroglobulin antibodies (60–80%), which destroy thyroid tissue and cause primary hypothyroidism. (Robbins 10e Ch 24; Harrison 21e Ch 384) ## Why each distractor is wrong - **Iodine deficiency**: While iodine deficiency is a common cause of hypothyroidism globally, it does NOT produce Hürthle cells, lymphocytic infiltration, or germinal centers. The firm rubbery goiter and autoimmune serology exclude iodine deficiency, which is rare in iodine-sufficient countries like India. - **TSH-receptor stimulating antibodies**: This describes Graves disease (hyperthyroidism with diffuse hyperplasia and lymphocytic infiltration), NOT Hashimoto thyroiditis. Graves presents with thyrotoxicosis, warm tender goiter, and exophthalmos—opposite of this patient's hypothyroid state. Hashimoto features blocking antibodies (anti-TSH-receptor-blocking), not stimulating antibodies. - **Viral infection with transient thyroiditis**: Viral thyroiditis (e.g., subacute thyroiditis) presents with acute pain, elevated ESR, and transient hypothyroidism followed by recovery. It does NOT produce Hürthle cells, germinal centers, or chronic lymphocytic infiltration. The chronic histology and positive autoimmune serology exclude viral thyroiditis. **High-Yield:** Hürthle (oncocytic) cells = Hashimoto thyroiditis; anti-TPO > anti-thyroglobulin; most common cause of hypothyroidism in iodine-sufficient regions; female 7:1 predominance; firm rubbery goiter (vs soft vascular Graves). [cite: Robbins 10e Ch 24; Harrison 21e Ch 384]