What does the medical abbreviation IU mean?

The medical abbreviation IU stands for International Unit. It is a standardized unit used to measure the activity of certain substances, such as vitamins, hormones, and enzymes. The IU is based on the biological activity of a substance, which is determined by comparing it to a standard preparation. For example, one IU of vitamin C is equal to the biological activity of 0.05 milligrams of ascorbic acid.