**Core Concept**
Riedel's thyroiditis is a rare form of thyroiditis characterized by the replacement of thyroid gland tissue with dense fibrous tissue, leading to a hard, non-tender thyroid gland. This condition is often associated with autoimmune thyroiditis, particularly Hashimoto's thyroiditis.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of Riedel's thyroiditis. The condition is characterized by the deposition of collagen and other fibrous proteins, which leads to the replacement of thyroid gland tissue. This process is often associated with autoimmune thyroiditis, and the fibrous tissue can cause compression of surrounding structures, including the trachea and esophagus. The fibrosis in Riedel's thyroiditis is a result of chronic inflammation and scarring, which can be caused by the autoimmune response against the thyroid gland.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because Riedel's thyroiditis is not typically associated with a hot nodule on thyroid scan. While some cases may present with a nodular thyroid gland, the characteristic feature of Riedel's thyroiditis is the replacement of glandular tissue with fibrous tissue.

**Option B:** This is incorrect because Riedel's thyroiditis is not a form of subacute thyroiditis. While both conditions are forms of thyroiditis, they have distinct pathophysiological mechanisms and clinical presentations.

**Option C:** This is incorrect because Riedel's thyroiditis is not typically associated with hyperthyroidism. In fact, many patients with Riedel's thyroiditis may present with hypothyroidism due to the destruction of thyroid gland tissue.

**Clinical Pearl / High-Yield Fact**
Riedel's thyroiditis can be challenging to diagnose, and the condition often requires a combination of clinical, laboratory, and imaging studies. A key feature of the condition is the presence of a hard, non-tender thyroid gland, which can be confirmed by thyroid palpation and imaging studies.

**Correct Answer:** C.