**Question:** Autocrine hormone among the following:

A. Angiotensin II
B. Insulin
C. Thyrotropin
D. Norepinephrine

**Correct Answer:** B. Insulin

**Core Concept:** Autocrine hormones are signaling molecules produced by a cell or an organ and act on the same cell to regulate its function.

**Why the Correct Answer is Right:** Insulin is produced by beta cells of the pancreas and acts on target cells, primarily liver, muscle, and adipose tissue, to promote glucose uptake, storage, and utilization. It also inhibits gluconeogenesis and glycogenolysis, thereby lowering blood glucose levels. This makes insulin an autocrine hormone, as it acts on the same cells it is produced by.

**Why Each Wrong Option is Incorrect:**

A. Angiotensin II: This is an endocrine hormone produced by the kidneys and acts on target cells, such as smooth muscle cells in blood vessels, to regulate blood pressure and fluid balance. It is not an autocrine hormone.

B. Insulin (Correct Answer): As explained above, insulin is an autocrine hormone.

C. Thyrotropin: This is a hypothalamic hormone that stimulates the anterior pituitary gland to produce thyroid-stimulating hormone (TSH), which then acts on the thyroid gland to regulate thyroid hormone production. Thyrotropin is an endocrine hormone, not an autocrine one.

D. Norepinephrine: Produced by sympathetic neurons and released into the bloodstream, norepinephrine acts on target cells (e.g., cardiac muscle cells) through the alpha and beta receptors. It is an endocrine hormone, not an autocrine one.

**Clinical Pearl:** Understanding autocrine and endocrine hormones is essential for understanding how cells and organs communicate and maintain homeostasis in the body. Autocrine hormones are locally produced and act on neighboring cells, while endocrine hormones are produced in one part of the body and act on target cells in other parts.