In pharmacology, the international unit is a unit of measurement for the amount of a
substance; the mass or volume that constitutes one international unit varies based on which
substance is being measured, and the variance is based on the biological activity or effect,
for the purpose of easier comparison across substances. International units are used to
quantify vitamins, hormones, some medications, vaccines, blood products, and similar biologically
active substances. The name international unit has often been
capitalized, although major English-language dictionaries treat it as a common noun and
thus lowercase it. The name has several accepted abbreviations. It is often abbreviated as
IU in English; as UI in Romance languages; as IE in several Germanic languages; or as
other forms. In order to remove the possibility of having the letter "I" confused with the
digit "1", some hospitals have it as a stated policy to omit the "I", that is, to only use
U or E when talking and writing about dosages, while other hospitals require the word "Units"
to be written out entirely. Many biological agents exist in different
forms or preparations. The goal of the IU is to be able to compare these, so that different
forms or preparations with the same biological effect will contain the same number of IUs.
To do so, the WHO Expert Committee on Biological Standardization provides a reference preparation
of the agent, arbitrarily sets the number of IUs contained in that preparation, and
specifies a biological procedure to compare other preparations of the same agent to the
reference preparation. Since the number of IUs contained in a new substance is arbitrarily
set, there is no equivalence between IU measurements of different biological agents. For instance,
one IU of vitamin E cannot be equated with one IU of vitamin A in any way, including
mass or efficacy. Despite its name, IU is not part of the International
System of Units used in physics and chemistry. The IU should not be confused with the enzyme
unit, also known as the International unit of enzyme activity and abbreviated as U.
Equality and equivalency of IU for different substances
To define the IU for a substance, an international collaborative study is organized by the WHO
Expert Committee on Biological Standardization using various assay systems in several laboratories.
The main goal of the study is to reach a consensus regarding methods of analysis and the approach
to standardization so that results can be compared.
The study is performed using highly purified preparations of the substance, typically in
lyophilized form, called "international reference preparations" or IRPs. Each preparation is
divided into precisely weighed samples, with each sample stored in its own ampoule labeled
with a code corresponding to the source IRP. Assays are performed using these samples and
are calibrated against the previously available IU standard. These results can be quite variable;
the final IU value for samples of a given IRP are determined by consensus. The IRP that
provides the best results and shows the best long term stability is selected to define
the next IU. This IRP is then referred to as the "international standard."
Mass equivalents of 1 IU For some substances, the precise mass equivalent
of one IU is later established. If that happens, the former IU mass for that substance is officially
abandoned in favor of a newly established mass. The unit count often will still remain
in use. Vitamin A: 1 IU is the biological equivalent
of 0.3 μg retinol, or of 0.6 μg beta-carotene 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, or of
0.45 mg of dl-alpha-tocopherol acetate. Insulin: 1 IU is equivalent to 0.0347 mg of
human insulin. See also
United States Pharmacopeia References
External links WHO reference preparations