polyuria

Polyuria simply means producing too much urine. In uncontrolled diabetes it happens because the blood is so full of sugar that the kidneys can no longer hold it all back; the excess glucose spills into the urine and, like a sponge, drags large amounts of water out with it. The result is frequent, voluminous urination — and the relentless thirst that follows as the body tries to replace the lost fluid.

Technically, polyuria is a urine output exceeding about 3 liters per day in adults. In diabetes it is caused by osmotic diuresis: once blood glucose rises above the kidney's reabsorption threshold (around 10 mmol/L, or 180 mg/dL), glucose appears in the urine and osmotically holds water in the tubules, so more water is excreted. This water loss in turn triggers thirst (polydipsia), one of the classic warning signs of diabetes.

Polyuria from diabetes mellitus must be distinguished from that of diabetes insipidus, where excessive urination stems not from glucose but from a lack of, or resistance to, antidiuretic hormone. In practice, checking the blood and urine glucose quickly tells the two apart. Heavy, persistent urination together with great thirst, especially with weight loss, is a strong reason to test for diabetes.