**Core Concept**
Hyperthyroidism is characterized by an excessive production of thyroid hormones, leading to increased metabolic rate, cardiac contractility, and cardiac output. The increased cardiac output in hyperthyroidism is primarily due to increased contractility and decreased peripheral vascular resistance.

**Why the Correct Answer is Right**
The increased contractility in hyperthyroidism is mediated by the increased levels of thyroid hormones, which stimulate the beta-1 adrenergic receptors in the heart. This leads to an increase in the activity of adenylate cyclase, resulting in increased intracellular cAMP, which ultimately leads to increased cardiac contractility. Additionally, the increased levels of thyroid hormones also lead to a decrease in peripheral vascular resistance, which contributes to the increased cardiac output.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because increased cardiac contractility is a primary mechanism of increased cardiac output in hyperthyroidism. The increased contractility is not due to a decrease in cardiac output.
* **Option B:** This option is incorrect because decreased peripheral vascular resistance is a contributing factor to the increased cardiac output in hyperthyroidism. The decreased peripheral vascular resistance is not a mechanism that decreases cardiac output.
* **Option C:** This option is incorrect because increased heart rate is a secondary mechanism of increased cardiac output in hyperthyroidism. The increased heart rate is a result of the increased contractility and decreased peripheral vascular resistance.

**Clinical Pearl / High-Yield Fact**
In hyperthyroidism, the increased cardiac output is often accompanied by other clinical manifestations such as palpitations, tremors, and heat intolerance.

**Correct Answer: C.**