Hormones are classified into two main types:

1. Peptide hormones (e.g. human growth hormone): These protein hormones are made up of amino acids and are water-soluble. They cannot pass through a cell membrane. They bind to receptors on the cell's surface and cause changes within the cell.
2. Steroid hormones (e.g. sex hormones - androgens, estrogens, and progesterone): These hormones are lipid-soluble and can pass through the cell membrane to another cell. Steroid hormones cause specific genes to be expressed or suppressed by influencing gene transcription in a cell.

Hormones are regulated by other hormones, glands and organs, and by a negative feedback mechanism.

• Tropic hormones regulate the release of other hormones. The anterior pituitary in the brain, the hypothalamus, and the thyroid gland secrete tropic hormones.
• Organs and glands help hormonal regulation by monitoring blood content. E.g. the pancreas will secrete the hormone glucagon if glucose levels are too low and insulin if glucose levels are too high.
• In negative feedback regulation, the initial stimulus is reduced by the response it provokes. E.g. the regulation of red blood cell production.