A 56-year-old woman with Hashimoto's thyroiditis on levothyroxine develops palpitations and weight loss. TSH is 0.02 mU/L. Which antibody, if strongly positive, would suggest coexisting Graves' disease rather than over-replacement?
- A TSH receptor-stimulating antibody (TSAb/TRAb) ✓
- B Thyroid peroxidase antibody (TPO-Ab)
- C Thyroglobulin antibody (TgAb)
- D Antinuclear antibody (ANA)
Correct answer: A. TSH receptor-stimulating antibody (TSAb/TRAb)
Explanation
TSH receptor-stimulating antibodies (TRAb with stimulating activity, also called TSAb) are the pathogenic antibodies in Graves' disease that activate the TSH receptor autonomously. Their presence in a patient on levothyroxine with suppressed TSH strongly suggests coexisting Graves' disease superimposed on Hashimoto's thyroiditis — a recognised overlap syndrome. TPO-Ab and TgAb are markers of autoimmune thyroid disease in general and do not differentiate Graves from Hashimoto's. ANA is non-specific.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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