A 14-year-old girl presents with heat intolerance, tremor, weight loss despite increased appetite, and palpitations. Examination reveals a diffuse, non-tender goiter with a bruit, lid lag, and exophthalmos. TSH is suppressed and free T4 is elevated. Which investigation best distinguishes Graves disease from painless thyroiditis in this patient?
- A Anti-thyroid peroxidase (anti-TPO) antibody titer
- B Erythrocyte sedimentation rate (ESR)
- C Thyroid uptake scan showing diffusely increased uptake ✓
- D Serum thyroglobulin level
Correct answer: C. Thyroid uptake scan showing diffusely increased uptake
Explanation
Graves disease shows diffusely increased radioactive iodine uptake due to TSH-receptor antibodies stimulating the gland. Painless (silent) thyroiditis shows markedly reduced uptake because thyroid hormone leaks from a damaged gland rather than being newly synthesized. Anti-TPO antibodies can be positive in both Graves and autoimmune thyroiditis, so they do not reliably distinguish them. ESR helps distinguish subacute (de Quervain) thyroiditis, which is painful and elevated, but not Graves from painless thyroiditis.
Reference: Williams Textbook of Endocrinology, 14th ed.
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