A 58-year-old man has thyrotoxicosis with weight loss, tremor, and atrial fibrillation. TSH is suppressed, free T4 is elevated. Radioactive iodine uptake is diffusely LOW. Serum thyroglobulin is also low. There is no thyroid tenderness. Which is the most likely diagnosis?
- A Graves disease
- B Thyrotoxicosis factitia (exogenous thyroid hormone intake) ✓
- C Subacute de Quervain thyroiditis
- D Toxic multinodular goiter
Correct answer: B. Thyrotoxicosis factitia (exogenous thyroid hormone intake)
Explanation
Thyrotoxicosis factitia is caused by exogenous thyroid hormone ingestion. It causes suppressed TSH, elevated free T4, LOW radioactive iodine uptake (no endogenous thyroid activity), and LOW serum thyroglobulin (thyroid is not synthesizing hormone). Graves disease shows HIGH uptake and elevated thyroglobulin. Subacute thyroiditis also shows low uptake but thyroglobulin is elevated due to follicular destruction releasing stored protein, and the thyroid is typically tender.
Reference: Williams Textbook of Endocrinology, 14th ed.
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