A 58-year-old woman with a history of neck irradiation presents with fatigue, cold intolerance, and constipation. TSH is 48 mIU/L, free T4 is 0.4 ng/dL. Anti-TPO antibodies are negative. What is the most likely underlying mechanism?
- A Autoimmune destruction of thyroid follicular cells
- B Radiation-induced direct follicular cell damage and fibrosis ✓
- C TSH receptor blocking antibodies
- D Iodine organification defect
Correct answer: B. Radiation-induced direct follicular cell damage and fibrosis
Explanation
A history of external beam radiation to the neck is a classic non-autoimmune cause of hypothyroidism, resulting from direct radiation-induced follicular cell death and progressive fibrosis over months to decades. The absence of anti-TPO antibodies argues against Hashimoto's thyroiditis. TSH receptor blocking antibodies characterize atrophic thyroiditis; organification defects are congenital.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.