A 28-year-old woman undergoes thyroidectomy for bilateral thyroid nodules. Histology shows sheets of polygonal cells with abundant granular eosinophilic cytoplasm (oncocytic cells), round nuclei, and central nucleoli. Calcitonin immunostain is negative; thyroglobulin is positive. Congo red shows amyloid stroma is ABSENT. Electron microscopy would show mitochondria-packed cytoplasm. What is the diagnosis?

• A Medullary thyroid carcinoma
• B Insular carcinoma (poorly differentiated thyroid carcinoma)
• C Hurthle cell (oncocytic) thyroid neoplasm ✓
• D Papillary thyroid carcinoma, oncocytic variant

Explanation

Hurthle cell (oncocytic) thyroid neoplasm is composed of follicular-derived oncocytes — cells with abundant granular eosinophilic cytoplasm packed with abnormal mitochondria (mitochondrial hyperplasia). They are thyroglobulin-positive and calcitonin-negative, distinguishing them from medullary carcinoma (which shows calcitonin positivity and amyloid stroma containing amyloid derived from procalcitonin). Hurthle cell carcinoma is defined by capsular or vascular invasion (like follicular carcinoma). It differs from conventional follicular carcinoma in that it is largely resistant to radioiodine (due to low sodium-iodide symporter expression). Papillary thyroid carcinoma oncocytic variant retains PTC nuclear features (grooves, pseudoinclusions).

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

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Written and medically reviewed by the StethoPrep medical team.