Osteomalacia classification
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief:
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Overview
Classification
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [2] Associate Editor(s)-in-Chief: Ibrahim Elkhayat, M.B.B.Ch.[3]
Osteomalacia is most usefully classified by the mechanism of the mineralization defect. The major categories are calcipenic osteomalacia, phosphopenic osteomalacia, and inhibitor/matrix osteomalacia. Phosphopenic osteomalacia is further divided into FGF23-mediated and FGF23-independent forms. Each category can also be classified as acquired or hereditary when applicable. This mechanism-based framework links classification to characteristic biochemical patterns and cause-directed management.[1][2]
Mechanism-based classification
| Category | Defining feature | Major subtypes/examples |
|---|---|---|
| Calcipenic osteomalacia | Inadequate calcium and/or vitamin D availability, usually with secondary hyperparathyroidism; renal phosphate wasting may occur as a consequence. | Acquired: nutritional vitamin D deficiency, dietary calcium deficiency, malabsorption, chronic liver disease, CKD/nephrotic syndrome, drug-induced disease. Hereditary: vitamin D–dependent rickets/osteomalacia (VDDR). |
| Phosphopenic osteomalacia | Chronic hypophosphatemia caused by renal phosphate wasting, typically with preserved serum calcium. | FGF23-mediated: hereditary XLH, ADHR, ARHR; acquired tumor-induced osteomalacia, fibrous dysplasia/McCune-Albright syndrome, and iron-infusion-associated phosphate wasting. FGF23-independent: Fanconi syndrome and hereditary hypophosphatemic rickets with hypercalciuria (HHRH). |
| Inhibitor/matrix osteomalacia | Calcium and phosphate availability are not the primary abnormality, but mineralization is impaired by an intrinsic enzymatic defect, abnormal matrix, or exogenous mineralization inhibitor. | Hypophosphatasia; aluminum or fluoride exposure; classically first-generation bisphosphonate etidronate at high/prolonged doses; other primary matrix/mineralization defects. |
Calcipenic osteomalacia
Calcipenic osteomalacia results from inadequate calcium and/or vitamin D availability. It may be acquired or hereditary.
Acquired calcipenic osteomalacia
Major acquired forms include:
- Nutritional vitamin D deficiency, including deficiency associated with limited sun exposure, darker skin pigmentation, or inadequate intake.
- Dietary calcium deficiency.
- Malabsorption, including celiac disease and after bariatric surgery, as well as gastrointestinal, pancreatic, or hepatobiliary disease.
- Chronic liver disease.
- Chronic kidney disease or nephrotic syndrome.
- Drug-induced disease, including osteomalacia associated with some anticonvulsants.[4]
Hereditary calcipenic osteomalacia
Hereditary vitamin D–dependent rickets/osteomalacia (VDDR) includes disorders affecting vitamin D activation, metabolism, or signaling:
- VDDR1A — CYP27B1-related 1α-hydroxylase deficiency.
- VDDR1B — CYP2R1-related 25-hydroxylase deficiency.
- VDDR2A — vitamin D receptor-related disease.
- VDDR2B — a rare form with impaired vitamin D receptor–DNA (VDRE) interaction; the precise genetic cause is not firmly established, with reported HNRNPC involvement and some sources describing the cause as unknown.[5][6]
- VDDR3 — accelerated inactivation of vitamin D metabolites due to gain-of-function CYP3A4 activity.[7]
Phosphopenic osteomalacia
Phosphopenic osteomalacia is characterized by chronic hypophosphatemia resulting from renal phosphate wasting. It is divided into FGF23-mediated and FGF23-independent forms.
FGF23-mediated phosphopenic osteomalacia
Hereditary forms include:
- X-linked hypophosphatemia (XLH), associated with PHEX abnormalities.
- Autosomal dominant hypophosphatemic rickets (ADHR).
- Autosomal recessive hypophosphatemic rickets (ARHR), including:
Acquired forms include:
- Tumor-induced osteomalacia caused by FGF23-secreting phosphaturic mesenchymal tumors.
- Fibrous dysplasia/McCune-Albright syndrome.
- Iron-infusion-associated phosphate wasting, particularly with ferric carboxymaltose.[10][11]
FGF23-independent phosphopenic osteomalacia
These disorders cause renal tubular phosphate loss without primary FGF23 excess. Examples include:
- Fanconi syndrome, including inherited forms associated with cystinosis or Wilson disease.
- Acquired Fanconi syndrome, including that associated with multiple myeloma or tenofovir disoproxil fumarate.
- Hereditary hypophosphatemic rickets with hypercalciuria (HHRH), associated with SLC34A3.[12]
Inhibitor/matrix osteomalacia
This category includes disorders in which calcium and phosphate availability is not the primary abnormality, but mineralization is impaired by an intrinsic enzymatic defect, abnormal matrix, or an exogenous inhibitor.
Examples include:
- Hypophosphatasia, associated with deficient tissue-nonspecific alkaline phosphatase activity and accumulation of inorganic pyrophosphate.
- Aluminum-associated osteomalacia.
- Fluoride-associated osteomalacia.
- Osteomalacia from mineralization inhibitors, classically the first-generation bisphosphonate etidronate at high or prolonged doses; this is not a typical feature of modern nitrogen-containing bisphosphonates at standard doses.[13][14]
Hypophosphatasia is distinctive in showing a low serum alkaline phosphatase, in contrast to the elevated ALP typical of most other osteomalacia. Recognizing this distinction can prevent misclassification as osteoporosis and inappropriate antiresorptive therapy.[15][16]
Acquired versus hereditary classification
A second classification axis separates osteomalacia into acquired and hereditary forms. This distinction is particularly important within calcipenic and FGF23-mediated phosphopenic osteomalacia.
| Classification axis | Hereditary examples | Acquired examples |
|---|---|---|
| Calcipenic | VDDR1A, VDDR1B, VDDR2A, VDDR2B, VDDR3 | Nutritional deficiency, dietary calcium deficiency, malabsorption, CKD/liver disease, drug-induced disease |
| FGF23-mediated phosphopenic | XLH, ADHR, ARHR1, ARHR2, ARHR3 | Tumor-induced osteomalacia, fibrous dysplasia/McCune-Albright syndrome, iron-infusion-associated phosphate wasting |
| FGF23-independent phosphopenic | HHRH; inherited Fanconi syndromes | Acquired Fanconi syndrome, including selected drug- or malignancy-associated cases |
Biochemical orientation to the classification
The mechanism-based categories have characteristic biochemical patterns, although these are not diagnostic cutoffs:
- Calcipenic osteomalacia generally shows low or low-normal calcium, elevated PTH, and elevated ALP; hypophosphatemia may result from PTH-mediated renal phosphate wasting.
- Phosphopenic osteomalacia generally shows normocalcemia with hypophosphatemia and elevated ALP.
- Inhibitor/matrix osteomalacia should be considered when calcium and phosphate abnormalities do not explain the mineralization defect; low ALP is an important clue to hypophosphatasia.[1][3]
Detailed biochemical thresholds and interpretation belong in the Laboratory findings microchapter.
Special consideration: CKD and renal osteodystrophy
In chronic kidney disease, osteomalacia represents a possible low-turnover mineralization-defect pattern within the broader CKD-MBD/renal osteodystrophy framework. Classification in this setting may therefore be expressed using the renal osteodystrophy turnover-mineralization-volume framework rather than forcing every case into the general calcipenic/phosphopenic scheme.[17][18]
Alternative classification schemes
Some published frameworks classify chronic hypophosphatemia into broader groups such as FGF23-related disease, primary tubular phosphate dysfunction, disorders of vitamin D metabolism, and PTH1R-mediated disorders. These schemes overlap substantially with the mechanism-based classification above and may be encountered in specialized literature.[7]
Key classification points
- The primary clinical distinction is calcipenic versus phosphopenic osteomalacia.
- Phosphopenic disease should then be separated into FGF23-mediated and FGF23-independent forms.
- Hypophosphatasia belongs to the inhibitor/matrix category and is suggested by low ALP rather than the elevated ALP typical of most osteomalacia.
- Acquired and hereditary forms should be distinguished whenever the clinical context permits.
- Classification is clinically important because osteomalacia may be mistaken for osteoporosis, particularly when bone mineral density is low, leading to inappropriate treatment.[19]
References
- ↑ 1.0 1.1 Paccou J; Tsourdi E; Anastasilakis AD; Lems WF; Compston J (2026). "Pathophysiology, diagnosis and management of secondary osteoporosis". Nature Reviews. Endocrinology. 22 (8): 452–462. doi:10.1038/s41574-026-01252-x.
- ↑ da Silva MMR; Bilezikian JP; de Paula FJA (2025). "Phosphate metabolism: its impact on disorders of mineral metabolism". Endocrine. 88 (1): 1–13. doi:10.1007/s12020-024-04092-9.
- ↑ 3.0 3.1 Uday S; Högler W (2025). "Differential Diagnosis of Heritable and Acquired Osteomalacia in Children: Biochemical and Biomaterial Signatures". Calcified Tissue International. 116 (1): 91. doi:10.1007/s00223-025-01398-2. PMID 40586907 Check
|pmid=value (help). - ↑ Collins MT; Marcucci G; Anders HJ; et al. (2022). "Skeletal and extraskeletal disorders of biomineralization". Nature Reviews. Endocrinology. 18 (8): 473–489. doi:10.1038/s41574-022-00682-7.
- ↑ Elder CJ; Bishop NJ (2014). "Rickets". The Lancet. 383 (9929): 1665–1676. doi:10.1016/S0140-6736(13)61650-5.
- ↑ "Vitamin D-dependent rickets". National Library of Medicine, MedlinePlus. Missing or empty
|url=(help) - ↑ 7.0 7.1 Ito N; Hidaka N; Kato H (2024). "The Pathophysiology of Hypophosphatemia". Best Practice & Research. Clinical Endocrinology & Metabolism. 38 (2): 101851. doi:10.1016/j.beem.2023.101851.
- ↑ Shore RM (2022). "Disorders of phosphate homeostasis in children, part 2: hypophosphatemic and hyperphosphatemic disorders". Pediatric Radiology. 52 (12): 2290–2305. doi:10.1007/s00247-022-05373-z.
- ↑ Trombetti A; Al-Daghri N; Brandi ML; et al. (2022). "Interdisciplinary management of FGF23-related phosphate wasting syndromes: a Consensus Statement on the evaluation, diagnosis and care of patients with X-linked hypophosphataemia". Nature Reviews. Endocrinology. 18 (6): 366–384. doi:10.1038/s41574-022-00662-x.
- ↑ Brandi ML; Clunie GPR; Houillier P; et al. (2021). "Challenges in the Management of Tumor-Induced Osteomalacia (TIO)". Bone. 152: 116064. doi:10.1016/j.bone.2021.116064. PMID 34147708 Check
|pmid=value (help). - ↑ Narasimhan S; Lavik A; Auron M (2025). "Rickets". Pediatrics in Review. 46 (9): 494–509. doi:10.1542/pir.2024-006494.
- ↑ Sam A; Meeran K; Hill N (2023). Osteomalacia. Endocrinology and Diabetes.
- ↑ Watts NB; Chesnut CH; Genant HK; et al. (2020). "History of Etidronate". Bone. 134: 115222. doi:10.1016/j.bone.2020.115222.
- ↑ Whyte MP (2016). "Hypophosphatasia — aetiology, nosology, pathogenesis, diagnosis and treatment". Nature Reviews. Endocrinology. 12 (4): 233–246. doi:10.1038/nrendo.2016.14.
- ↑ Ng E; Ashkar C; Seeman E; et al. (2023). "A Low Serum Alkaline Phosphatase May Signal Hypophosphatasia in Osteoporosis Clinic Patients". Osteoporosis International. 34 (2): 327–337. doi:10.1007/s00198-022-06597-3.
- ↑ Hummel LS; Blaschke M; Lamersdorf A; et al. (2026). "Low Alkaline Phosphatase in Adults: Implications for the Diagnosis of Hypophosphatasia During Evaluation for Osteoporosis". Osteoporosis International. doi:10.1007/s00198-026-08161-9.
- ↑ Palmer SC; Strippoli GFM; Craig JC; et al. (2022). Chronic Kidney Disease-Mineral and Bone Disorder. Evidence-Based Nephrology (2nd ed.).
- ↑ Ott SM (2017). "Renal Osteodystrophy—Time for Common Nomenclature". Current Osteoporosis Reports. 15 (3): 187–193. doi:10.1007/s11914-017-0367-y.
- ↑ Jha S; Chapman M; Roszko K (2019). "When Low Bone Mineral Density and Fractures Is Not Osteoporosis". Current Osteoporosis Reports. 17 (5): 324–332. doi:10.1007/s11914-019-00529-7.