altered has 2 distinct synonyms spread across 2 of its 3 senses. 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 unaltered.
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.
changed in form or character without becoming something else
“the altered policy promised success”
having testicles or ovaries removed
changed in order to improve or made more fit for a particular purpose
“seeds precisely adapted to the area”
Synonyms, antonyms and the broader and narrower relations for altered come from Princeton University’s WordNet. Full sources and licences.