**Core Concept**
Insulin is a hormone secreted by the pancreas that plays a crucial role in glucose metabolism. It acts on various cells in the body to facilitate glucose uptake and storage, thereby regulating blood glucose levels.

**Why the Correct Answer is Right**
Insulin exerts its effects by binding to its receptor on the surface of cells, particularly on adipose tissue, liver, and skeletal muscle cells. This binding activates the insulin receptor tyrosine kinase, which in turn triggers a cascade of intracellular signaling pathways. The primary effect of insulin is to increase the translocation of glucose transporter type 4 (GLUT4) to the cell surface, allowing glucose to enter the cell. Insulin also promotes glycogen synthesis, inhibits gluconeogenesis, and stimulates the uptake of amino acids and fatty acids.

**Why Each Wrong Option is Incorrect**
**Option A:** Insulin does not primarily act on the kidneys to regulate electrolyte balance.
**Option B:** Insulin does not primarily act on the adrenal glands to regulate cortisol secretion.
**Option C:** Insulin does not primarily act on the thyroid gland to regulate metabolism.

**Clinical Pearl / High-Yield Fact**
Insulin resistance, a condition where cells become less responsive to insulin, is a hallmark of type 2 diabetes. It is often associated with obesity, physical inactivity, and a diet high in sugar and saturated fats.

**Correct Answer:** D.