**Core Concept**
Follicular carcinoma of the thyroid is a type of differentiated thyroid cancer that often retains its ability to take up iodine, even after metastasis. This characteristic allows for the use of radioactive iodine therapy as a treatment option.

**Why the Correct Answer is Right**
Radioiodine therapy involves the administration of radioactive iodine (I-131) to destroy cancer cells that have metastasized. Since follicular carcinoma cells retain iodine uptake, they can be selectively targeted and destroyed by radioactive iodine. This treatment is particularly effective for metastatic disease that is not amenable to surgical resection. The radioactive iodine is taken up by the cancer cells, leading to cell death and reduction in tumor burden.

**Why Each Wrong Option is Incorrect**
**Option B:** Surgery is typically reserved for localized disease or when there is a need for tissue diagnosis. In the case of metastatic follicular carcinoma, surgery would not be effective in treating the disseminated disease.
**Option C:** Thyroxine is used to suppress thyroid-stimulating hormone (TSH) production, which can slow the growth of thyroid cancer cells. However, it does not directly target or destroy metastatic cancer cells.
**Option D:** Observation is not a viable treatment option for metastatic follicular carcinoma, as it would allow the disease to progress and potentially worsen the patient's prognosis.

**Clinical Pearl / High-Yield Fact**
It is essential to note that radioactive iodine therapy is most effective when the thyroid-stimulating hormone (TSH) levels are suppressed, as high TSH levels can stimulate the growth of thyroid cancer cells.

✓ Correct Answer: A. Radioiodine.