Radioiodine Uptake Scan Interpretation
A patient with biochemically confirmed hyperthyroidism undergoes a radioiodine uptake scan. The scan reveals absent tracer uptake throughout the entire gland. Which of the following is the most likely explanation for this finding?
Correct Answer — C
Absent radioiodine uptake in a hyperthyroid patient means the thyroid is not actively synthesising or trapping iodine. This occurs in two scenarios: (1) thyroiditis — follicular cell damage causes passive hormone release, not active synthesis; the gland cannot trap iodine because synthesis machinery is disrupted; (2) excess iodine ingestion (Wolff-Chaikoff effect) — excess iodide acutely inhibits organification and uptake. All other causes of hyperthyroidism (Graves', toxic MNG, hot nodule, TSH adenoma) involve active hormone synthesis and show increased uptake.
Key concept
Absent ¹³¹I uptake in hyperthyroidism → thyroiditis or excess iodine ingestion. Active synthesis causes (Graves', MNG, hot nodule) → increased uptake.