Handling and measurement
IU (International Unit)
Last updated
A measure of how much biological activity something has, rather than how much it weighs. Each substance that uses IU has its own agreed reference preparation defining what one IU of that substance means. That makes IU substance-specific: one IU of one thing has no fixed relationship to one IU of another, and there is no general formula converting IU to milligrams.
Why it matters here
You cannot compare or convert an IU figure without knowing which substance it belongs to, and a conversion found for one substance is simply wrong for any other. When a label gives IU and another source gives mg for the same thing, the two only line up through a conversion published for that exact substance.
What it is not
IU is not the unit on an insulin syringe. That mark is a volume, one hundredth of a millilitre on a U-100 barrel, and it applies to whatever liquid happens to be in the barrel. IU measures activity and is defined per substance. They are the same word borrowed from two different systems, and reading one as the other is the most consequential mix-up in this whole vocabulary.
Also called
- iu
- i.u.
- international unit
- international units
- activity unit
- biological unit
IU is short for International Unit, agreed through the international biological standards system for substances whose activity could be measured before their exact mass or purity could. That history is why hormones and vitamins carry IU and ordinary chemicals do not.
Compounds carrying this term
None on file yet. This term is in the controlled vocabulary but no compound record currently uses it — which is a statement about our coverage, not about the term.