I thought that in Hebrew, thirsting and hungering were verbs only. Or at least more commonly. Can anyone shed light on this?
The present tense of verbs often doubles as an adjective. So if you say: הם צמאים למים it's a verb and be conjugated (although not common): הם צמאו, הם צמאים, הם יצמאו. But in the present tense there is no difference between the verb and the adjective.
To extend on AlmogL's answer, the past and future tenses of both exist, but are very archaic and literary. If you want to say "I was hungry" you wouldn't say רעבתי (it exists, but extremely literary), you'd say הייתי רעב. Same for צמא, same for future tense. The upshot of this is that they are used nowadays only as adjectives.