Why does high blood sugar cause extreme thirst?

Unquenchable thirst (polydipsia) and frequent urination (polyuria) are two of the most classic, hallmark symptoms of undiagnosed or poorly managed diabetes. It all comes down to how your kidneys handle excess sugar.

Normally, your kidneys filter glucose out of your blood and then reabsorb it so none is lost in your urine. However, when your blood sugar levels get dangerously high, your kidneys become completely overwhelmed. They can no longer reabsorb all that glucose, so the excess sugar spills over into your urine.

Because sugar draws water with it, this excess glucose pulls a massive amount of fluid from your bodily tissues to help flush it out. This causes you to urinate constantly and in large volumes. Because you are losing so much fluid so rapidly, your body becomes severely dehydrated. That intense, unquenchable thirst is simply your brain's desperate attempt to replace the fluids you are losing in the bathroom! Once your blood sugars are brought back down into a normal range, this cycle stops.