A 42-year-old woman presents with painless enlargement of the thyroid and hypothyroidism. Thyroidectomy specimen shows a diffusely enlarged gland with a firm, tan-white cut surface. Microscopy reveals dense lymphocytic infiltrate with well-formed germinal centers, small atrophic thyroid follicles, and follicular epithelial cells with abundant eosinophilic granular cytoplasm. The granular appearance of these cells is due to:

• A An abundance of mitochondria ✓
• B Accumulation of thyroglobulin within dilated rough endoplasmic reticulum
• C Accumulation of lysosomal autophagosomes from immune-mediated injury
• D Glycogen deposition secondary to cellular hypoxia

Explanation

The findings are classic for Hashimoto thyroiditis, and the eosinophilic granular cells are Hürthle cells (oxyphil/oncocytic metaplasia of follicular epithelium). Their granular cytoplasm results from a massive increase in mitochondria that crowd the cytoplasm. Dilated endoplasmic reticulum with thyroglobulin gives pale cytoplasm in active follicular cells, not the dense granular look, so option B is the tempting but incorrect distractor. Germinal centers plus Hürthle cells distinguish Hashimoto from Riedel thyroiditis, which shows dense fibrosis.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.