International unit

In pharmacology, the International Unit is a unit of measurement for the amount of a substance, based on measured biological activity or effect. "International Unit" is abbreviated as IU, or as UI from the French unité internationale, or as UI from the Italian unità internazionale, or as IE from the German Internationale Einheit.

The unit is used for vitamins, hormones, some medications, vaccines, blood products, and similar biologically active substances. Despite its name, the IU is not part of the International System of Units used in physics and chemistry.

Variance

The precise definition of one IU differs from substance to substance and is established by international agreement for each substance. There is no equivalence among different substances; for instance, one IU of vitamin E does not contain the same number of milligrams as one IU of vitamin A.

Equality

To define an IU of a substance, the WHO Expert Committee on Biological Standardization provides a reference preparation of the substance, arbitrarily sets the number of IUs contained in that preparation, and specifies a biological procedure to compare other preparations of that substance to the reference preparation. The goal in setting the standard is that different preparations with the same biological effect will contain the same number of IUs.

Equivalency

For some substances, the equivalent mass of one IU is later established. If that happens, the former IU definition for that substance is officially abandoned, in favor of a newly established weight. However, the unit count often remains in use nevertheless, because it is convenient. For example, vitamin E exists in a number of different forms, all having different biological activities. Rather than specifying the precise type and mass of vitamin E in a preparation, for the purposes of pharmacology it is sufficient, simply, to specify the number of IUs of vitamin E.

Mass equivalents of 1 IU

• Insulin: 1 IU is the biological equivalent of about 45.5 μg pure crystalline insulin (1/22 mg exactly).
  This corresponds to the old USP insulin unit, where one unit (U) of insulin is equal to the amount required to reduce the concentration of blood glucose in a fasting rabbit to 45 mg/dl (2.5 mmol/L).
• Vitamin A: 1 IU is the biological equivalent of 0.3 μg retinol, or of 0.6 μg beta-carotene in the USA , and of 0.3 μg beta-carotene in Canada

• Vitamin C: 1 IU is 50 μg L-ascorbic acid
• Vitamin D: 1 IU is the biological equivalent of 0.025 μg cholecalciferol/ergocalciferol
• Vitamin E: 1 IU is the biological equivalent of about 0.667 mg d-alpha-tocopherol (2/3 mg exactly), or of 1 mg of dl-alpha-tocopherol acetate

Difference from unit of enzyme activity

The IU should not be confused with the enzyme unit, also known as the International unit of enzyme activity and abbreviated as U.

External links

• WHO reference preparations
• International Unit Converter