[PhasmaFelis]

Someone always claims this.

If I only drink when I'm thirsty, then half the time I'll be depressed and headachey and unfocused and not thirsty until I idly grab a drink and suddenly find myself standing in front of the sink chugging three glasses of water.

If your thirst system is finely tuned, that's nice for you. I and millions of others could use a better system.

[dp-hackernews]

If your spit isn't wet like water, then you could do with a drink, although being in an anxious state may indicate a false positive.

Thirst is an early'ish warning, but not early enough if you've eaten food that needs more water in order for your body to process it than you have consumed prior to eating, or you have diarrhea, or you are low on electrolytes, or your kidneys have problems monitoring and maintaining the balance of said electrolytes.

See also:

https://pubmed.ncbi.nlm.nih.gov/31082167/