1000th vs thousandth

1000th is not a noun while thousandth is a noun.

Word	Noun	Adjective	Verb	Adverb
1000th	No	Yes	No	No
thousandth	Yes	Yes	No	No

As adjectives, 1000th and thousandth are synonyms defined as:

1000th and thousandth: the ordinal number of one thousand in counting order

1000th (noun)	thousandth (noun)
	one part in a thousand equal parts
	position 1,000 in a countable series of things

1000th (adjective)	thousandth (adjective)
the ordinal number of one thousand in counting order	the ordinal number of one thousand in counting order