Insulin is a Peptide Hormone (a small protein) secreted by the $\beta$-cells of the pancreatic islets.

• It consists of 51 amino acids arranged in two polypeptide chains ($A$ and $B$) linked together by disulfide bridges.
• It plays a crucial role in regulating carbohydrate metabolism by facilitating the uptake of glucose into cells.
• In contrast, hormones like testosterone or cortisol are Steroid Hormones (lipid-derived).