Approach to Hypophosphatemia

1 Confirm hypophosphatemia

Serum phosphate varies with meals, carbohydrate load, and time of day. Confirm a low value before starting a workup.

Repeat the measurement

Draw a fasting, morning sample.
Avoid samples drawn after glucose infusion, insulin, or a carbohydrate-rich meal.
Measure calcium, magnesium, potassium, and creatinine at the same draw.
Use age-appropriate reference ranges. Normal values are higher in infants and children.

Severity in adults

Grade	mg/dL	mmol/L
Reference	2.5–4.5	0.81–1.45
Mild	2.0–2.4	0.65–0.80
Moderate	1.0–1.9	0.32–0.64
Severe	<1.0	<0.32

2 Acute or chronic?

Acute hypophosphatemia is usually a shift of phosphate into cells or bone. Chronic hypophosphatemia usually reflects renal loss or reduced intestinal absorption.

Acute (hours to days). Look for redistribution

Refeeding after starvation or prolonged illness
Insulin therapy, including treatment of diabetic ketoacidosis
Respiratory alkalosis (sepsis, salicylate toxicity, alcohol withdrawal, anxiety)
Catecholamines and β-agonists
Hungry bone syndrome (rapid skeletal uptake after abrupt reversal of high bone turnover)
- After parathyroidectomy
- Potent antiresorptive therapy in high-turnover states such as Paget's disease
Rapid cell proliferation (leukemic blast crisis, hematopoietic recovery)
Losses during continuous kidney replacement therapy

Chronic (persistent or recurrent). Look for loss or poor absorption

If hypophosphatemia persists without a redistribution explanation, or recurs, move to Step 3 and test whether the kidney is wasting phosphate.

Acute and chronic causes can coexist. A patient with alcohol use disorder, for example, may have poor intake, tubular dysfunction, and refeeding all at once.

Go to Step 3 →

3 Assessment of renal phosphate handling

Use paired fasting serum and urine samples (ideally a second-morning void) drawn while the patient is hypophosphatemic. Samples drawn on oral phosphate supplements overstate urinary losses.

Calculate FEPO₄, TRP, and TmP/GFR with the calculator. In hypophosphatemia, an FEPO₄ above about 5% or a TmP/GFR below the age-specific reference range indicates renal phosphate wasting. TmP/GFR is the most reliable index, because TRP and FEPO₄ depend on filtered load. A 24-hour urine phosphate above about 100 mg/day also suggests renal wasting.

Open the calculator →

Kidney wasting phosphate → Step 4 Kidney conserving phosphate → Step 5

4 The kidney is wasting phosphate: identify the pattern

Measure intact FGF23, PTH, calcium, 25(OH)D, 1,25(OH)₂D, and urine calcium. Screen urine for glucose, amino acids, and low-molecular-weight protein when proximal tubular injury is possible. Select the pattern that fits to see its differential diagnosis.

FGF23 ↑ / inapp. normal1,25D ↓ / inapp. normalFGF23 elevated or inappropriately normal, with 1,25(OH)₂D low or inappropriately normalFGF23-mediated phosphate wasting →PTH or PTHrP ↑sort by Ca and 25(OH)DPTH or PTHrP elevatedPTH1 receptor-mediated phosphate wasting →FGF23 ↓1,25D ↑UCa ↑FGF23 appropriately low, 1,25(OH)₂D high, urine calcium highPrimary renal phosphate transporter defect →FGF23 ↓1,25D normal / ↓PTH normalFGF23 appropriately low, 1,25(OH)₂D normal or low, PTH normal, with or without glucosuria, aminoaciduria, bicarbonate loss, hypouricemia, proteinuria, or other solute lossProximal tubulopathy, isolated or generalized →

Interpreting FGF23 in hypophosphatemia

In FGF23-independent hypophosphatemia, FGF23 should be suppressed. A value inside the laboratory reference range is therefore inappropriate when phosphate is low, and patients with XLH or tumor-induced osteomalacia often have FGF23 in the "normal" range.

Hartley and colleagues studied 149 patients with FGF23-mediated or FGF23-independent hypophosphatemia. An intact FGF23 cut point of 27 pg/mL separated the two groups with 100% sensitivity and specificity in that cohort.
C-terminal FGF23 performed less well. Values of 45–90 RU/mL overlapped between genetic FGF23 excess and FGF23-independent disease, and using the laboratory upper limit of normal for C-terminal FGF23 would have missed more than half of FGF23-mediated cases. Prefer intact FGF23.
Cut points are assay-specific. Measure before starting phosphate or calcitriol, which raise FGF23.
Iron deficiency raises C-terminal FGF23. Severe vitamin D deficiency with secondary hyperparathyroidism can leave FGF23 unsuppressed; replete vitamin D and remeasure.

Hartley IR, Gafni RI, Roszko KL, et al. Determination of FGF23 levels for the diagnosis of FGF23-mediated hypophosphatemia. J Bone Miner Res. 2022;37(11):2174–2185. Link

Pattern A. FGF23-mediated phosphate wasting

FGF23 reduces proximal tubular sodium-phosphate cotransporters and suppresses 1α-hydroxylase, so 1,25(OH)₂D is low or inappropriately normal.

Diagnosis	Notes
Tumor-induced osteomalacia	Acquired, adult onset, bone pain and fractures. Localize with somatostatin-receptor PET/CT (68Ga-DOTATATE) and selective venous sampling if needed.
IV iron-associated hypophosphatemia	Ferric carboxymaltose and some other formulations raise intact FGF23. Ask about infusions in the prior weeks to months; can persist and cause osteomalacia with repeated dosing.
X-linked hypophosphatemia	PHEX variants; most common heritable form. Childhood rickets, short stature, dental abscesses, enthesopathy in adults.
Autosomal dominant hypophosphatemic rickets	FGF23 variants resist cleavage. Variable onset; flares with iron deficiency.
Autosomal recessive hypophosphatemic rickets	DMP1, ENPP1, FAM20C variants.
Fibrous dysplasia / McCune-Albright syndrome	GNAS mosaicism; FGF23 from dysplastic bone, proportional to lesion burden.
Cutaneous skeletal hypophosphatemia syndrome	Mosaic RAS pathway variants with epidermal or melanocytic nevi.
Rare ectopic FGF23 production	Including some malignancies and other rare skeletal dysplasias.

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Pattern B. PTH1 receptor-mediated phosphate wasting

PTH and PTHrP both act on the PTH1 receptor, which internalizes proximal tubular sodium-phosphate cotransporters. Use calcium and 25(OH)D to sort the causes.

PTH	Calcium	Other clues	Consider
High	High or high-normal	Urine calcium may be high. 25(OH)D often low from accelerated conversion to 1,25(OH)₂D; does not exclude the diagnosis	Primary or tertiary hyperparathyroidism
High	Low or normal	25(OH)D low; urine calcium low	Secondary hyperparathyroidism from vitamin D deficiency
Suppressed	High	PTHrP high	Humoral hypercalcemia of malignancy

Diagnosis	Notes
Primary hyperparathyroidism	Including normocalcemic variants once secondary causes are excluded.
Secondary hyperparathyroidism	Vitamin D deficiency from low intake or sun exposure, malabsorption (celiac disease, inflammatory bowel disease, pancreatic insufficiency, bariatric surgery), liver or kidney disease, or enzyme-inducing drugs. Intestinal malabsorption and PTH-driven phosphaturia combine. Replete vitamin D and remeasure; if phosphate wasting persists, revisit the other patterns.
Tertiary hyperparathyroidism	After kidney transplantation, or after long-term phosphate therapy in XLH.
Humoral hypercalcemia of malignancy	PTH suppressed, PTHrP elevated.

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Pattern C. Primary renal phosphate transporter defect

Phosphate loss lowers FGF23, which lets 1,25(OH)₂D rise and increases calcium absorption and urinary calcium. Watch for kidney stones and nephrocalcinosis. Calcitriol worsens hypercalciuria; these disorders are treated with phosphate alone.

Diagnosis	Notes
Hereditary hypophosphatemic rickets with hypercalciuria	SLC34A3 (NaPi-IIc) variants. Heterozygotes may have isolated hypercalciuria or stones.
Other sodium-phosphate cotransporter disorders	SLC34A1 (NaPi-IIa) and SLC9A3R1 (NHERF1) variants.

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Pattern D. Proximal tubulopathy, isolated or generalized

Phosphate wasting may be the first or only sign of proximal tubular injury, before full Fanconi syndrome develops. Check normoglycemic glucosuria, urine amino acids, serum bicarbonate and uric acid, and urine β2-microglobulin or retinol-binding protein.

Unlike Pattern C, 1,25(OH)₂D does not rise. The injured proximal tubule cells that waste phosphate also house 1α-hydroxylase, and impaired megalin/cubilin uptake lets filtered 25(OH)D bound to vitamin D-binding protein escape into the urine, so 25(OH)D may also be low.

Diagnosis	Notes
Tenofovir disoproxil fumarate	The most common drug cause. Much less frequent with tenofovir alafenamide. Still used for HIV PrEP and hepatitis B.
Other drugs	Adefovir, ifosfamide, cisplatin, aminoglycosides, deferasirox, valproate, imatinib and other tyrosine kinase inhibitors.
Light-chain proximal tubulopathy	Consider in older adults without a drug explanation. Check serum free light chains and immunofixation.
Heavy metals	Lead, cadmium, mercury.
Inherited Fanconi syndrome	Cystinosis, Wilson disease, Dent disease, Lowe syndrome, tyrosinemia, galactosemia, hereditary fructose intolerance, mitochondrial disorders.
Kidney transplantation	Persistent phosphaturia after transplant can have a tubular component.

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5 The kidney is conserving phosphate: find the intestinal cause

A low FEPO₄ and normal TmP/GFR mean the kidney is responding appropriately. Evaluate for reduced intake, impaired absorption, or phosphate binding in the gut.

Low phosphate intake or total undernutritionSee differential →GI malabsorption or lossSee differential →Reduced vitamin D effectSee differential →Phosphate binding in the gutSee differential →

Pattern E. Low phosphate intake or total undernutrition

Phosphate is abundant in food, so low intake alone rarely causes hypophosphatemia. It usually reflects global undernutrition. These patients are at high risk of refeeding hypophosphatemia.

Diagnosis	Notes
Prolonged fasting or starvation	Including prolonged NPO status without nutrition support.
Alcohol use disorder	Often combined with tubular dysfunction, magnesium deficiency, and refeeding.
Eating disorders	Anorexia nervosa.
Frailty and poor oral intake	Older adults, chronic illness.

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Pattern F. GI malabsorption or loss

Often accompanied by vitamin D deficiency. If secondary hyperparathyroidism develops, the picture can shift toward renal wasting (Pattern B).

Diagnosis	Notes
Chronic diarrhea or steatorrhea	Including high-output ostomies.
Short bowel syndrome	After extensive small-bowel resection.
Mucosal disease	Celiac disease, inflammatory bowel disease.
Pancreatic exocrine insufficiency	Chronic pancreatitis, cystic fibrosis.
Bariatric surgery	Especially malabsorptive procedures.

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Pattern G. Reduced vitamin D effect

Low 1,25(OH)₂D activity reduces active intestinal phosphate absorption. When secondary hyperparathyroidism is prominent, renal wasting appears and Pattern B applies.

Diagnosis	Notes
Vitamin D deficiency	Low intake, limited sun exposure, malabsorption.
Impaired 25-hydroxylation	Advanced liver disease.
Impaired 1α-hydroxylation	Chronic kidney disease; vitamin D-dependent rickets type 1 (CYP27B1).
Vitamin D receptor resistance	Vitamin D-dependent rickets type 2 (VDR).
Increased catabolism	Phenytoin, phenobarbital, carbamazepine, rifampin.

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Pattern H. Phosphate binding in the gut

Take a careful medication history, including over-the-counter antacids. Urine phosphate is typically very low.

Diagnosis	Notes
Antacids	Aluminum- and magnesium-containing products, especially in large or prolonged doses.
Calcium salts	Calcium carbonate and calcium acetate.
Prescription phosphate binders	Sevelamer, lanthanum carbonate, ferric citrate, sucroferric oxyhydroxide.
Absorption inhibitors	Tenapanor (reduces paracellular absorption); nicotinamide.

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