Physiology · Renal, fluids and acid–base

Glucose transport maximum and renal threshold !

Measurement
Exam priority: medium

The kidney reclaims all filtered glucose up to a limit; when blood glucose goes past it, glucose spills into the urine.

Updated 2026-11-01

Also called: Glucose Tm, Glucosuria, Renal threshold for glucose

See it in 3D: Kidneys

What it is

Glucose is freely filtered and the proximal tubule reclaims it with sodium-coupled transporters that can saturate. As long as the filtered load is below their capacity, there is no glucose in the urine. The renal threshold is the plasma glucose at which glucosuria begins; the transport maximum (Tm) is the whole kidney’s maximal reabsorption. Between the two lies a curved zone (“splay”) because not all nephrons saturate at once.

The nephron: segments, transporters and diuretics

Na⁺ reabsorbed ≈ 65–70%Water reabsorbed ≈ 65–70%

Proximal tubule

What it does: Reabsorbs ≈ 65–70% of filtered Na⁺ and water (isosmotically), nearly all glucose and amino acids, ≈ 80% of HCO₃⁻ and most K⁺ and phosphate. Secretes organic acids and bases (many drugs) and makes NH₄⁺ from glutamine.
Transporters: NHE3 (Na⁺/H⁺) with carbonic anhydrase; SGLT2 (early segment, ≈ 90% of glucose) and SGLT1; NaPi-2a (phosphate); basolateral Na⁺/K⁺-ATPase.
Hormones: Ang II stimulates NHE3 (more Na⁺ and HCO₃⁻: contraction alkalosis); PTH inhibits NaPi-2a (phosphaturia) and activates 1α-hydroxylase (active vitamin D).
Drugs: Carbonic anhydrase inhibitors (1), SGLT2 inhibitors (2) and osmotic diuretics (3, with the thin descending limb).
Disorders: Fanconi syndrome (loses glucose, amino acids, phosphate and HCO₃⁻); type 2 (proximal) renal tubular acidosis; familial renal glucosuria (SGLT2).

Glomerulus and arterioles
Proximal tubule
Thin descending limb
Thin ascending limb
Thick ascending limb
Distal convoluted tubule
Collecting tubule and duct
Juxtaglomerular apparatus (macula densa)
Cortex
Medulla
Interstitial osmolality (≈ 300 → 1200 mOsm/kg)

Illustrative schematic drawn by Durgical (a long-loop juxtamedullary nephron); proportions and percentages are approximate and not for clinical decisions.

Equations

Filtered loadFL = GFR × P_glu

P_glu: plasma glucose (mg/mL)

ExcretionE = FL − R

R: reabsorption (at most, the Tm)

If FL ≤ Tm, E ≈ 0 (apart from the splay zone).

Approximate figures (adult)

Parameter	Value	Note
Fasting plasma glucose	≈70–100 mg/dL
Renal threshold for glucose	≈180–200 mg/dL	lower in pregnancy and with SGLT2 inhibitors
Transport maximum (Tm)	≈300–375 mg/min	varies between sources and people
Reabsorption by SGLT2 / SGLT1	≈90 % / ≈10 %	early and late proximal tubule

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

Clinical notes

In poorly controlled diabetes, glucosuria causes osmotic diuresis: polyuria, polydipsia, sodium and potassium loss, and a risk of dehydration.
In pregnancy GFR rises: filtered load increases and glucosuria can occur with normal blood glucose.
A urine dipstick detects glucose only once plasma glucose has passed the threshold: it is not a test for diagnosing diabetes.

Test yourself

Think of the answer, then tap the question to see it.

With a GFR of 120 mL/min, plasma glucose of 400 mg/dL and a Tm of 375 mg/min, how much glucose is excreted?

Filtered load = 1.2 dL/min × 400 mg/dL = 480 mg/min. 375 is reabsorbed; ≈105 mg/min is excreted.

Why does glucose appear in urine before the whole kidney reaches its Tm?

Nephrons differ in reabsorptive capacity and in load: some saturate before others. That heterogeneity produces the splay.

Why can heavy glucosuria cause hypernatraemia and dehydration?

Glucose left in the lumen prevents water reabsorption (osmotic diuresis): water is lost in greater proportion than sodium, and extracellular volume falls.

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See it in 3D

The structures of Durgical's 3D anatomy where it happens.

Kidneys

In Biochemistry

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Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 25.6 Tubular Reabsorption (OpenStax (Rice University), 2022); Type 2 Diabetes (StatPearls (NCBI Bookshelf), 2024); Type 1 Diabetes (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 28). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.