registered has no recorded synonym. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is unregistered.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
(of animals) officially recorded with or certified by a recognized breed association; especially in a stud book
“a registered Percheron”
listed or recorded officially
“record is made of `registered mail' at each point on its route to assure safe delivery”
1 of the 3 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for registered.
Synonyms, antonyms and the broader and narrower relations for registered come from Princeton University’s WordNet. Full sources and licences.