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 has been changed. When that happens, the former IU mass for that substance is officially abandoned in favor of a newly established mass. Such a change has happened with the immunoassay standards for prolactin: as one batch of standard (84/500; 53 mIU ≈ 2.5 μg) was running out of stock, a new standard (83/573; 67 mIU ≈ 3.2 μg) was calibrated against the old one (as 67.2 mIU) and replaced the former. Micronutrients: 1 IU Vitamin A = 0.3 μg retinol (~0.1 nmol) = 0.6 μg beta-carotene 1 IU Vitamin D = 0.025 µg D2/D3 ≈ 0.65 pmol 1 IU Vitamin E = mg d-alpha-tocopherol = 0.90 mg of dl-alpha-tocopherol Peptides: 1 IU insulin ≙ 0.0347 mg human insulin (28.8 IU/mg) 1 IU Oxytocin ≙ 1.68 μg of pure peptide Languages 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. The name has several accepted abbreviations. It is usually abbreviated as IU in English, and UI in Romance languages (for example Spanish unidad internacional, French unité internationale, Italian unità internazionale, Romanian unitate internațională), IE in several Germanic languages (for example German internationale Einheit, Dutch internationale eenheid) or as other forms (for example Russian МЕ, международная единица [mezhdunarodnaya yedinitsa], Hungarian NE, nemzetközi egység). In order to remove the possibility of having the letter "I" confused with the digit "1", some hospitals have it as a stated policy omit the "I", that is, to only use U or E when talking and writing about dosages, while other hospitals require the word "units" (or words "international units") to be written out entirely. (For example, "three international units per litre" may be abbreviated "3 U/L". The "liter" sign (L) is less affected, as less confusing written forms are used.) See also United States Pharmacopeia (USP) References Notes Citations External links WHO reference preparations Online converter with authoritative references for all substances Units of measurement Dosage forms