**Core Concept**
Radioisotope therapy is a treatment modality used for managing thyroid cancer metastasis and ablation. It involves the administration of radioactive isotopes that selectively target and destroy cancer cells in the thyroid gland or its metastases.

**Why the Correct Answer is Right**
The correct radioisotope used for thyroid treatment of metastasis/ablation is Iodine-131 (I-131). I-131 is a radioactive isotope of iodine that is selectively taken up by thyroid cells, including cancer cells. This allows for targeted therapy, where the radiation from I-131 destroys the cancer cells while minimizing damage to surrounding healthy tissues. I-131 also has a relatively short half-life, making it safer for patients and easier to manage.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Strontium-89 is primarily used for treating bone metastases in various cancers, not thyroid cancer.
**Option B:** This option is incorrect because Technetium-99m is a radioactive isotope used for diagnostic imaging, not therapeutic purposes.
**Option C:** This option is incorrect because Iodine-123 is a radioactive isotope used for diagnostic imaging of the thyroid gland, not for treatment.

**Clinical Pearl / High-Yield Fact**
When using I-131 for thyroid cancer treatment, it's essential to note that patients must be in a state of iodine deprivation (e.g., with Lugol's solution) to increase the uptake of I-131 by cancer cells. This is because iodine-rich foods and supplements can reduce the efficacy of I-131 therapy.

**Correct Answer: C. Iodine-131**