International unit

Depending on the form (powder vs solution), production method, or just batch-to-batch differences, there is often some variation in their relative potencies, so that a simple measure of mass would not suffice.

Since the number of IUs contained in a new substance is arbitrarily set, there is no equivalence between IU measurements of different biological agents.

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.

There is a system in place also called the international unit to describe the relative strength of these different molecular forms.

Like the biological international unit, the IU for vitamins is also an arbitrary number that cannot be interconverted among different types of activities.

[9] This same commission also established early standards for biologics (antitoxins, insulins, pituitary extract and sex hormones) in the interwar period.

[15] The name international unit is often capitalized in English and other languages, although major English-language dictionaries treat it as a common noun and thus use lower case.