**Core Concept**
Radioactive iodine (RAI) therapy is used primarily to ablate thyroid tissue in conditions like hyperthyroidism or thyroid cancer. It works by targeting thyroid cells due to their high iodine uptake, leading to cell death. The most common and well-documented complication is permanent hypothyroidism due to destruction of functional thyroid tissue.

**Why the Correct Answer is Right**
Radioactive iodine is selectively taken up by thyroid cells, causing radiation damage to these cells. This leads to a significant reduction or complete loss of thyroid function, resulting in hypothyroidism. This is a predictable, dose-dependent side effect and is the most frequent complication. While other complications like secondary leukemia or thyroid malignancy are rare, hypothyroidism occurs in up to 50–90% of patients depending on the dose and indication.

**Why Each Wrong Option is Incorrect**
Option A: Thyroid malignancy is not a complication of RAI therapy; in fact, RAI is used to treat thyroid cancer and may reduce the risk of recurrence.
Option C: Leukemia is a very rare complication of RAI, and not a recognized or significant side effect. The risk is extremely low and not clinically relevant compared to hypothyroidism.
Option D: "All" is incorrect because only hypothyroidism is a well-established, common, and direct complication.

**Clinical Pearl / High-Yield Fact**
After RAI therapy, patients require lifelong thyroid hormone replacement therapy due to hypothyroidism — this is a key point in post-treatment management and must be communicated clearly to patients.

✓ Correct Answer: B. Hypothyroidism