subtle has 3 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. 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.
difficult to detect or grasp by the mind or analyze
“his whole attitude had undergone a subtle change”
working or spreading in a hidden and usually injurious way
“glaucoma is an insidious disease”
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 subtle.
Synonyms, antonyms and the broader and narrower relations for subtle come from Princeton University’s WordNet. Full sources and licences.