Radioactive iodine is ineffective in which thyroid condition due to low uptake?

• A. Grave's disease.
• B. Subacute thyroiditis.
• C. Hashimoto's thyroiditis.
• D. Papillary thyroid carcinoma.

Answer: (B)

Radioactive iodine uptake depends on functioning thyroid tissue actively trapping and organifying iodine. In subacute thyroiditis, the thyroid follicles are inflamed and damaged, releasing stored hormones but losing their ability to trap iodine. The gland’s overall activity for iodine uptake is reduced, so a radioactive iodine scan shows very low uptake and treatment with radioactive iodine is ineffective. This contrasts with conditions like Graves disease, where the gland is hyperfunctioning and avidly takes up iodine, making RAI therapy effective. Papillary thyroid carcinoma can also take up iodine in residual or metastatic tissue, which is why RAI is used in management, not inherently ineffective. Hashimoto’s thyroiditis can have variable uptake, but the classic pattern asked about—low uptake due to destructive inflammation—points to subacute thyroiditis.