International unit

International Unit (IU) is a unit of measurement for the amount of a substance, based on measured biological activity or effect. The use of the International Unit is common in medicine and other fields, and it is used to quantify vitamins, hormones, some medications, vaccines, blood products, and similar biologically active substances.

Definition

The International Unit is not a standard of physical measure but is a unit of potency for biologically active substances. The actual mass or volume represented by an IU varies depending on the substance being measured. The definition of an IU for a particular substance is established by international agreement and is based on biological activity or effect.

Use in Medicine

In medicine, the International Unit is often used to measure the activity of many vitamins and drugs. For example, the antioxidant activity of Vitamin E can be measured in IU. Similarly, the biological effect of Insulin is often measured in IU. However, the standard is not universally used, and some substances are measured in milligrams (mg) or micrograms (μg).

Conversion to Other Units

Because the potency of a substance measured in International Units can vary depending on the substance, there is no universal conversion factor to other units of measure. However, for each specific substance, a conversion factor can be established. For example, one IU of Vitamin E is equivalent to 0.67 milligrams of the natural form of the vitamin.

Criticism and Controversy

The use of the International Unit has been criticized for its lack of clarity and potential for confusion. Because the IU measures biological effect rather than mass or volume, it can be difficult to compare doses of different substances or to convert between IU and other units of measure.

See Also

• Vitamin
• Insulin
• Measurement