Renal threshold for glucose is approximately 180 mg/dL. What occurs when plasma glucose reaches this level?

• A. All glucose is reabsorbed and none appears in urine
• B. Glucose is excreted in urine only after levels exceed 400 mg/dL
• C. Glucose begins to appear in urine when plasma glucose reaches about 180 mg/dL
• D. Glucose is completely reabsorbed at saturating rate

Answer: (C)

Renal handling of glucose depends on the proximal tubule’s reabsorptive capacity. Glucose is filtered freely at the glomerulus and is normally reabsorbed completely by SGLT transporters in the proximal tubule as long as the filtered load stays within that capacity. As plasma glucose rises, reabsorption keeps up until the transport system becomes saturated. At roughly 180 mg/dL, the amount filtered approaches what the tubule can reabsorb, so the transporters hit their maximum and cannot reclaim all the glucose. The excess glucose then appears in the urine, i.e., glycosuria begins. Below this threshold, no glucose is in the urine; beyond it, some glucose is excreted.