theorize has 7 distinct synonyms spread across 1 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. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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.
to believe especially on uncertain or tentative grounds
“Scientists supposed that large dinosaurs lived in swamps”
construct a theory about
“Galileo theorized the motion of the stars”
form or construct theories
“he thinks and theorizes all day”
Synonyms, antonyms and the broader and narrower relations for theorize come from Princeton University’s WordNet. Full sources and licences.