**Core Concept**
Radioiodine therapy is a treatment modality that utilizes radioactive isotopes of iodine to target and destroy thyroid tissue or other iodine-avid tissues. This principle is based on the selective uptake of iodine by the thyroid gland, allowing for targeted radiation therapy.

**Why the Correct Answer is Right**
Radioiodine therapy is primarily used to treat hyperthyroidism, particularly in cases of Graves' disease. The radioactive iodine (usually I-131) is taken up by the thyroid gland, leading to damage and eventual destruction of excess thyroid tissue. This results in a decrease in thyroid hormone production and normalizes thyroid function. The mechanism involves the selective uptake of iodine by the sodium-iodide symporter (NIS) in thyroid follicular cells, which is then incorporated into thyroid hormones.

**Why Each Wrong Option is Incorrect**
**Option A:** Radioiodine is not typically used to treat hypothyroidism, as it destroys thyroid tissue, potentially exacerbating the condition.
**Option B:** While radioiodine can be used to diagnose thyroid disorders (e.g., thyroid scintigraphy), it is not primarily used for this purpose.
**Option C:** Radioiodine is not commonly used to treat adrenal disorders, such as Cushing's syndrome or Addison's disease.

**Clinical Pearl / High-Yield Fact**
Radioiodine therapy is often preferred over surgery for treating hyperthyroidism, particularly in patients with large goiters or those who are poor surgical candidates. However, it's essential to note that radioiodine can cause temporary thyroiditis, leading to transient hypothyroidism.

**Correct Answer: D. Hyperthyroidism**