**Core Concept**
Radioiodine treatment of Grave's disease involves the administration of radioactive iodine to destroy thyroid tissue, thereby reducing the production of thyroid hormones. The treatment can lead to various complications, including thyroid dysfunction.

**Why the Correct Answer is Right**
The most common complication of radioiodine treatment is hypothyroidism, which occurs due to the destruction of thyroid tissue. This results in a decrease in the production of thyroid hormones, leading to symptoms such as fatigue, weight gain, and cold intolerance. The destruction of thyroid tissue also leads to a decrease in the production of thyroid-stimulating hormone (TSH), which is usually elevated in hypothyroidism.

**Why Each Wrong Option is Incorrect**
**Option A:** Thyroiditis is a possible complication of radioiodine treatment, but it is less common than hypothyroidism. Thyroiditis occurs due to the inflammatory response to the destruction of thyroid tissue.
**Option B:** Thyroid storm is a rare but life-threatening complication of radioiodine treatment, typically occurring in patients with pre-existing thyroid dysfunction. It is not the most common complication.
**Option C:** Thyroid cancer is not a direct complication of radioiodine treatment, although it may be a concern in patients with a history of radiation exposure.

**Clinical Pearl / High-Yield Fact**
It is essential to monitor thyroid function tests (TFTs) regularly after radioiodine treatment to diagnose and manage hypothyroidism promptly.

**Correct Answer:** A. Hypothyroidism.