International convention: acronym - full term (i.e. separated by hyphen). Most countries have adopted this.
Sweden uses brackets instead.
Netherlands and Estonia add the acronym as a separate synonym.
All countries generally add acronyms only as acceptable synonyms, but allow some acronyms as preferred terms: e.g. MRI, HIV, DNA. It is hard to draw a clear line though when an acronym is allowed as a preferred term.
Belgium uses a set of strict rules concerning acronyms, which Nathalie De Sutter is willing to share with Ole.
Maria-Cornelia Wermuth : from a terminological point of view, the acronym is a synonym.
However, Norway for instance has a rule that each description should be unique, also acceptable synonyms across concepts; that would clash with adding acronyms as separate synonyms, as they are frequently used for multiple terms.