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.

While hormones in the blood come in contact with a number of cells, they influence only the cells which have receptors for each specific hormone.

• Endocrine signalling is when the hormones influence specific cells from a distance. For example, the pit...