thirstiness vs thirsty

thirstiness is a noun but thirsty is not a noun.

Word	Noun	Adjective	Verb	Adverb
thirstiness	Yes	No	No	No
thirsty	No	Yes	No	No

thirstiness (noun)	thirsty (noun)
strong desire for something (not food or drink)
a physiological need to drink
a deficiency of moisture (especially when resulting from a permanent absence of rainfall)

thirstiness (adjective)	thirsty (adjective)
	feeling a need or desire to drink
	able to take in large quantities of moisture
	(usually followed by `for') extremely desirous
	needing moisture