A steroid is a organic compound, either natural or man-made, which has four cycloalkane rings in its structure. Plants, animals and fungi make hundreds of different natural steroids.

Steroid hormones are steroids which act as hormones in the body. Natural steroid hormones are usually made from cholesterol in the adrenal glands and gonads. Cholesterol is a plentiful steroid in animal cells: it is important for cell membranes.

Examples of steroid hormones include

• estrogen,
• testosterone,
• progesterone,
• Vitamin D₃ or cholecalciferol.