**Core Concept**
Ghrelin is a peptide hormone that plays a crucial role in regulating appetite, energy balance, and glucose metabolism. It is secreted by the stomach and acts on the hypothalamus to stimulate food intake. Ghrelin's action is opposite to that of leptin, which suppresses appetite.

**Why the Correct Answer is Right**
Ghrelin acts on the growth hormone secretagogue receptor 1a (GHSR1a) in the hypothalamus, leading to increased appetite and food intake. It also stimulates the release of growth hormone from the pituitary gland, which has anabolic effects on the body. Ghrelin's secretion is increased in fasting states and decreased after eating, suggesting its role in regulating energy balance.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because ghrelin does not primarily regulate blood pressure. The renin-angiotensin-aldosterone system (RAAS) is responsible for blood pressure regulation.
* **Option B:** This option is incorrect because ghrelin does not primarily regulate glucose metabolism through insulin secretion. Insulin is secreted by the pancreas in response to high blood glucose levels.
* **Option C:** This option is incorrect because ghrelin does not primarily regulate body temperature. The hypothalamus is responsible for regulating body temperature through the sympathetic and parasympathetic nervous systems.

**Clinical Pearl / High-Yield Fact**
Ghrelin levels are increased in fasting states and decreased after eating, suggesting its role in regulating energy balance. Ghrelin agonists, such as ghrelin mimetics, are being investigated for their potential to treat cachexia and other metabolic disorders.

**Correct Answer:** D.