While the blood is filtered in the kidneys (glomerular filtration), glucose enters the urine. This then flows through the small kidney tubes and the kidney transports the glucose back into the blood.
If there is excessive glucose in the blood, the kidneys no longer manage to transport everything back into the blood.
Then glucose remains in the urine and is excreted. Up to a blood sugar level of 160-180 mg/dl (= 8.9 – 10.0 mmol/l), the kidneys can usually return the entire glucose back to the blood. If the blood sugar level continues to rise, they will not make it any more at some point. This limit is also called the kidney threshold.
However, glucose is not exclusively found in urine with diabetes. Kidney disease can also reduce the renal threshold. For example, if the cells that should bring the glucose back into the blood are damaged.
Then glucose appears in the urine at normal blood sugar levels.
This can be caused by hereditary diseases, poisoning but also other kidney damage. Glucose is also a harmless, hereditary feature in the urine.
In these glucosuria, the glucose level in the urine is high. But unlike diabetes, blood sugar levels are normal and a sugar exposure test also provides normal results.
There are now special drugs for diabetics, so-called flozins (SGLT2 inhibitors), which increase the excretion of sugar in the urine. So here a high uric sugar value is a sign that the drug has the desired effect. These drugs also lower blood sugar levels after meals. Such drugs include dapagliflozin, empagliflozin and canagliflozin.
Occasional side effects due to the high sugar content in the urine can be infections in the genital area.