thirst vs polydipsia

thirst and polydipsia both are nouns.

Word	Noun	Adjective	Verb	Adverb
thirst	Yes	No	Yes	No
polydipsia	Yes	No	No	No

As nouns, polydipsia is a hyponym of thirst; that is, polydipsia is a word with a more specific, narrower meaning than thirst:

thirst: a physiological need to drink
polydipsia: excessive thirst (as in cases of diabetes or kidney dysfunction)

Other hyponyms of thirst include dehydration.

thirst (noun)	polydipsia (noun)
strong desire for something (not food or drink)	excessive thirst (as in cases of diabetes or kidney dysfunction)
a physiological need to drink

thirst (verb)	polydipsia (verb)
have a craving, appetite, or great desire for
feel the need to drink