thoughtfulness has 7 distinct synonyms spread across 3 of its 4 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 2 recorded opposites are thoughtlessness, unthoughtfulness.
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.
a calm, lengthy, intent consideration
kind and considerate regard for others
“he showed no consideration for her feelings”
the trait of thinking carefully before acting
Synonyms, antonyms and the broader and narrower relations for thoughtfulness come from Princeton University’s WordNet. Full sources and licences.