Toddler drinks a ton

My 4 year old is ALWAYS thirsty, he can go through bottle after bottle of water or whatever i give him to drink. If i tell him he has enough for now all he does is cry and screams he is thirsty... he even gets so bad he will steal his brothers bottle and hide and drink. I brought up to doc and they did a finger poke for blood sugar in office and it was 88 so it was normal but i am worried we are missing something.

..is anyone elses child like this and nothing wrong just drinks alot?