**Core Concept**
Decreased radioiodine uptake refers to the reduced ability of the thyroid gland to absorb and concentrate iodine from the bloodstream, which is essential for thyroid hormone production. This concept is crucial in diagnosing and managing thyroid disorders.

**Why the Correct Answer is Right**
Decreased radioiodine uptake is typically seen in conditions where the thyroid gland is inflamed or damaged, leading to a disruption in the normal functioning of the thyroid follicles. In subacute thyroiditis, also known as de Quervain's thyroiditis, the thyroid gland becomes inflamed due to a viral infection, resulting in the release of pre-formed thyroid hormones into the bloodstream and a subsequent decrease in radioiodine uptake. This condition is characterized by the presence of inflammatory cells in the thyroid gland, which can be visualized on histopathological examination.

**Why Each Wrong Option is Incorrect**
**Option A:** Toxic multinodular goiter is characterized by an increase in thyroid hormone production due to the growth of multiple nodules in the thyroid gland. In this condition, the thyroid gland is more active and has an increased radioiodine uptake.
**Option B:** Grave's disease is an autoimmune disorder that leads to an overproduction of thyroid hormones. The thyroid gland is hyperactive in Grave's disease, resulting in an increased radioiodine uptake.
**Option D:** Factitious thyroiditis, also known as factitious hypothyroidism, is a condition where a person intentionally causes thyroid damage, often by administering exogenous thyroid hormones. However, factitious thyroiditis is not a recognized medical condition and is not associated with decreased radioiodine uptake.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that decreased radioiodine uptake is often seen in conditions where the thyroid gland is inflamed or damaged, such as subacute thyroiditis. This is in contrast to conditions like Grave's disease, where the thyroid gland is hyperactive and has an increased radioiodine uptake.

**Correct Answer: C. Subacute thyroiditis**