thing-in-itself vs noumenon

thing-in-itself and noumenon both are nouns.

Word	Noun	Adjective	Verb	Adverb
thing-in-itself	Yes	No	No	No
noumenon	Yes	No	No	No

As nouns, thing-in-itself and noumenon are synonyms defined as:

thing-in-itself and noumenon: the intellectual conception of a thing as it is in itself, not as it is known through perception

thing-in-itself (noun)	noumenon (noun)
the intellectual conception of a thing as it is in itself, not as it is known through perception	the intellectual conception of a thing as it is in itself, not as it is known through perception