"I am thirsty!"

Translation: Mám žízeň!

[foo__bar]

what about using "žízniv"?

[kacenka9]

well, technically it is correct. In reality somebody used this last time in about 1874. It sounds like something you would only read in a historical novel. The same goes for "jsem hladov". If you used it in a restaurant, the waiter would likely laugh till he cries and than bring you something to drink.

Though correct I am hesitant to add as an acceptable answer simply because in real life is is non existent.

And, of course, It would be "Já jsem žízniv/hladov". Not "Já mám..."

[VladaFu]

Even "Jsem žíznivý." is a bit strange, but the short forms of adjectives are really obsolete with a few exceptional phrases.