Glucosuria is caused by either a prerenal condition, i.e., hyperglycemia, or a renal condition, i.e., defective tubular absorption.

Diabetes Mellitus is the most common prerenal condition leaded to disease that results in  yperglycemia and glucosuria.  Patients with undiagnosed or inadequately controlled diabetes have blood glucose concentrations that exceed the renal threshold level (usually greater than 180 –200 mg/dL), the excess glucose will not be absorbed into the blood and will be eliminated from the body by way of urine.

In renal glucosuria, even having a normal blood glucose level, when having problems in  reabsorption in tubule can cause all the glucose result in urine. 

Other factors that might result in renal conditions are reduced glomelular blood flow. Glucose freely passes through the glomerular filtration barrier; however, if this barrier is compromised because of disease, the glomelular fitration rate (GFR) may be decreased. In these instances, hyperglycemia can be present, but because of the decreased GFR, only limited amounts of glucose are able to pass into the ultrafiltrate. The tubules are able to reabsorb all the glucose    presented to them, and glucosuria is not present.