thinness has 7 distinct synonyms spread across 4 of its 5 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 thickness.
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.
relatively small dimension through an object as opposed to its length or width
“the tenuity of a hair”
the property of having little body fat
Synonyms, antonyms and the broader and narrower relations for thinness come from Princeton University’s WordNet. Full sources and licences.
the property of being very narrow or thin
“he marvelled at the fineness of her hair”
the property of being scanty or scattered; lacking denseness
a consistency of low viscosity
“he disliked the thinness of the soup”