Chronic kidney disease (CKD) is characterized by a progressive decline in renal function occurring over months or years. Affecting approximately 10% of the global population, CKD imposes substantial morbidity, mortality, and healthcare costs worldwide1. Clinically, CKD is defined by the presence of structural or functional kidney damage or a reduced glomerular filtration rate (GFR < 60 mL/min/1.73 m²)2 persisting for at least 3 months, irrespective of the underlying etiology3. Hemodialysis (HD) serves as the primary renal replacement therapy (RRT) for patients with end-stage kidney disease (ESKD), facilitating the extracorporeal clearance of metabolic waste products, excess fluid, and electrolytes when endogenous renal function becomes inadequate4. Both pre-dialysis CKD and maintenance HD are closely associated with profound disturbances in mineral homeostasis, particularly alterations in parathyroid hormone (PTH) secretion, which adversely affect clinical outcomes. Secondary hyperparathyroidism, characterized by persistently elevated circulating PTH levels, represents a hallmark complication of progressive CKD and contributes to skeletal and cardiovascular morbidity5.

Disordered phosphate homeostasis is another critical determinant of cardiovascular risk and all-cause mortality in patients with CKD and those undergoing maintenance HD. As renal excretory capacity diminishes, impaired urinary phosphate clearance frequently leads to hyperphosphatemia6. Concurrently, vitamin D deficiency and insufficiency are highly prevalent among pre-dialysis CKD and HD populations. Because vitamin D plays an essential role in regulating calcium–phosphate balance and parathyroid function, optimizing vitamin D status is a cornerstone of clinical management; notably, high-dose cholecalciferol (vitamin D3) supplementation can safely restore serum 25-hydroxyvitamin D levels and attenuate PTH hypersecretion without inducing hypercalcemia7. Fibroblast growth factor-23 (FGF-23) is a bone-derived phosphaturic hormone synthesized predominantly by osteocytes and osteoblasts. It acts as a master regulator of systemic phosphate homeostasis and vitamin D metabolism by inhibiting renal tubular phosphate reabsorption and suppressing 1α-hydroxylase activity8.

Therefore, the present study aimed to investigate circulating intact FGF-23 levels and evaluate their correlations with renal function indices and key regulators of mineral metabolism—specifically PTH, vitamin D, calcium, and phosphorus—in an Iraqi cohort of pre-dialysis chronic renal failure (CRF) patients and maintenance HD patients.

Physiological aging is accompanied by a gradual decline in GFR, a primary parameter for diagnosing and staging CKD. Older adults are particularly susceptible to CKD due to cumulative genetic, environmental, and cellular senescence factors that accelerate nephron loss10. Previous comparative outcomes research in elderly patients with advanced CKD has suggested that conservative management may yield survival rates comparable or superior to dialysis initiation in patients with high comorbidity burdens11. Regarding sex differences, epidemiological data indicate that women generally have a lower risk of progressing to ESRD than men, with an adjusted hazard ratio of 0.84 after accounting for age and comorbidities2. Men are more likely to initiate RRT due to a more rapid decline in renal function, potentially mediated by androgen-related deleterious effects on renal hemodynamics and lifestyle risk factors12. In our cross-sectional cohort, female participants constituted a slightly higher proportion of the CRF (60.0%) and HD (56.7%) groups; however, sex distribution did not differ significantly across the three study groups, likely reflecting the modest sample size and single-center recruitment design.

With respect to adiposity, lower BMI in advanced CKD has been associated with increased progression to ESRD and higher mortality, whereas higher BMI has been linked to improved survival in certain advanced CKD and maintenance HD populations—a phenomenon termed the 'obesity paradox'—despite conferring an elevated risk of cardiovascular events13,14. Conversely, our cross-sectional findings demonstrated significantly higher mean BMI values and a high prevalence of obesity in both the pre-dialysis CRF (80.0%) and HD (86.7%) groups compared with healthy controls (33.4%). This observation aligns with established pathophysiological evidence that obesity is a major primary risk factor for the development and progression of CKD via its close association with metabolic syndrome, systemic hypertension, and type 2 diabetes mellitus, even though its prognostic association with survival may become complex in late-stage disease15.

In patients with CRF, declining nephron mass impairs the clearance of nitrogenous waste products, resulting in pronounced elevations in serum urea and creatinine concentrations that frequently exceed 200 mg/dL for urea and rise several-fold above reference intervals for creatinine16. Maintenance hemodialysis effectively reduces circulating urea and creatinine concentrations across individual dialysis sessions, as consistently demonstrated by prospective pre- versus post-dialysis comparisons17,18. Unlike longitudinal intradialytic clearance studies, our cross-sectional evaluation demonstrated a lower mean pre-dialysis serum creatinine level in the maintenance HD cohort (3.58 ± 0.34 mg/dL) than in the pre-dialysis CRF cohort (6.97 ± 0.43 mg/dL). This difference likely reflects variations in residual renal function, skeletal muscle mass and nutritional status, and steady-state interdialytic solute kinetics between patients managed conservatively and those established on regular thrice-weekly hemodialysis. Similarly, elevated serum uric acid (UA) is recognized as an independent risk factor for renal function decline, cardiovascular events, and all-cause mortality in early-stage CKD19, and maintenance HD plays an important role in clearing circulating UA to mitigate hyperuricemia20.

Chronic kidney disease–mineral and bone disorder (CKD-MBD) represents a pervasive systemic complication in advanced renal failure and maintenance HD populations. Previous clinical investigations have reported a high prevalence of hypocalcemia among regular HD patients—with up to 60% exhibiting serum calcium levels below 8.4 mg/dL—driven largely by phosphate retention rather than uremia alone21. Our findings corroborate this pattern, demonstrating significantly lower serum calcium concentrations alongside marked hyperphosphatemia in both the pre-dialysis CRF and HD groups compared with healthy controls. Other clinical studies of HD cohorts have likewise documented disturbed calcium–phosphorus homeostasis accompanied by a high prevalence (58.5%) of secondary hyperparathyroidism22. Furthermore, anthropometric and nutritional indices such as BMI and blood urea nitrogen (BUN) have been identified as significant predictors of hyperphosphatemia in HD patients, underscoring the necessity of dietary phosphate restriction and individualized nutritional management23. Because maintenance dialysis patients frequently exhibit higher serum phosphorus concentrations than non-dialysis CKD patients, integrated therapeutic strategies combining dietary counseling, adequate dialysis clearance, and pharmacological phosphate binders are generally required24.

Parathyroid hormone is a principal regulator of skeletal remodeling and mineral ion transport, and its sustained elevation in CKD drives high-turnover bone disease and extraskeletal calcification25. Beyond skeletal pathology, elevated PTH levels in HD patients have been linked to renal anemia through bone marrow fibrosis and consequent hyporesponsiveness to erythropoiesis-stimulating agents (ESAs)26. Concurrently, vitamin D insufficiency and deficiency are widespread among CKD and HD cohorts. For instance, a study conducted at the Tripoli Dialysis Center reported vitamin D insufficiency in 37% and frank deficiency in 30% of HD patients27, while an investigation in Abidjan found vitamin D deficiency (< 20 ng/mL) in 16% of supplemented HD patients28. Consistent with these reports, both our pre-dialysis CRF and HD cohorts exhibited severe vitamin D depletion (mean levels of 10.31 and 12.15 ng/mL, respectively), which directly contributes to impaired intestinal calcium absorption, secondary hyperparathyroidism, and the progression of CKD-MBD29.

Fibroblast growth factor-23 is central to the adaptive regulation of phosphate homeostasis in CKD. As functional nephron mass declines, osteocytic FGF-23 secretion rises early to augment fractional urinary phosphate excretion and counteract phosphate retention. Together with PTH, elevated FGF-23 participates in the complex pathogenesis of renal osteodystrophy and abnormal bone turnover30. Clinically, elevated circulating FGF-23 concentrations in CKD and HD patients are independently associated with left ventricular hypertrophy (LVH), incident heart failure, and overall cardiovascular morbidity and mortality, with residual kidney function modulating the magnitude of these risks31,32. In agreement with prior studies demonstrating marked elevations of biologically active intact FGF-23 in patients receiving maintenance HD33, our study found significantly higher serum FGF-23 levels in the HD group than in healthy controls. Systemic inflammation in CKD has also been shown to stimulate FGF-23 production and alter its cleavage dynamics, an association that is particularly pronounced for intact FGF-23 compared with C-terminal fragments34. Mechanistically, FGF-23 suppresses renal 1α-hydroxylase (CYP27B1) and induces 24-hydroxylase (CYP24A1), thereby reducing 1,25-dihydroxyvitamin D synthesis. Moreover, clinical evidence indicates that circulating FGF-23 is significantly elevated in CKD patients with concurrent vitamin D deficiency, reflecting a compensatory drive to offload phosphate despite low vitamin D reserves35. Although both FGF-23 and PTH rise progressively as GFR declines, FGF-23 fails to inhibit parathyroid PTH secretion in advanced kidney failure due to the downregulation of parathyroid α-Klotho–FGFR1c receptor complexes, resulting in maladaptive secondary hyperparathyroidism36.

Several limitations of the present study warrant consideration. First, the absolute serum FGF-23 concentrations observed in our pre-dialysis CRF and HD cohorts exhibited partial range overlap with healthy controls and were lower than the extreme elevations (often exceeding thousands of pg/mL) classically reported in end-stage renal disease. This discrepancy may be attributable to the specific epitope recognition profile and calibration range of the ELISA kit employed, pre-analytical sample handling and storage conditions, or differences in analytical cross-reactivity between intact FGF-23 and C-terminal FGF-23 fragments. Second, the cross-sectional, single-center design and modest sample size precluded causal inference and longitudinal evaluation of clinical endpoints. Finally, individual patient eGFR values and urinary fractional phosphate excretion measurements were not incorporated into the correlation models.

This study demonstrates a significant elevation in circulating intact FGF-23 concentrations among maintenance hemodialysis patients compared with healthy controls, alongside pronounced hyperphosphatemia, hypocalcemia, vitamin D depletion, and elevated PTH levels. Serum FGF-23 correlated moderately with PTH and weakly with serum creatinine across the study cohort, but showed no significant direct correlation with serum phosphorus or calcium, and did not differ significantly between pre-dialysis CRF patients and healthy controls. Collectively, these findings support a close association between circulating FGF-23 and markers of renal impairment and secondary hyperparathyroidism in advanced kidney disease, particularly among patients undergoing maintenance hemodialysis. Larger, multicenter longitudinal studies utilizing standardized intact and C-terminal FGF-23 assays are warranted to further delineate its diagnostic and prognostic utility across the spectrum of CKD.

ANOVA: Analysis of variance; AUC: Area under the curve; BMI: Body mass index; BUN: Blood urea nitrogen; CKD: Chronic kidney disease; CKD-MBD: Chronic kidney disease–mineral and bone disorder; CRF: Chronic renal failure; CT: Computed tomography; CVD: Cardiovascular disease; ECL: Electrochemiluminescence; eGFR: Estimated glomerular filtration rate; ELISA: Enzyme-linked immunosorbent assay; ESA: Erythropoiesis-stimulating agent; ESKD: End-stage kidney disease; ESRD: End-stage renal disease; FGF-23: Fibroblast growth factor-23; GFR: Glomerular filtration rate; HD: Hemodialysis; iFGF-23: Intact fibroblast growth factor-23; LVH: Left ventricular hypertrophy; PO4: Phosphate / Inorganic phosphorus; PTH: Parathyroid hormone; ROC: Receiver operating characteristic; RRT: Renal replacement therapy; SE: Standard error; SPSS: Statistical Package for the Social Sciences; SUA: Serum uric acid; UA: Uric acid.

The authors would like to thank the research consultants for their valuable guidance. The authors also express their gratitude to the University of Al-Maarif (Anbar, Iraq) for providing the laboratory facilities required to complete the experimental work of this study.

AAR contributed to conceptualization, methodology, validation, formal analysis, resources, visualization, supervision, project administration, writing–original draft, and writing–review and editing. HHA contributed to conceptualization, methodology, software, validation, investigation, resources, data curation, and supervision. SMM contributed to participant recruitment and selection, data curation, and writing–review and editing. All authors read and approved the final manuscript and agreed with the results and conclusions.

None. This work received no external funding.

The datasets supporting the findings and conclusions of this study are available from the corresponding author upon reasonable request.

This study was reviewed and approved by the Institutional Review Board of the College of Medicine at Al-Nahrain University (Approval No. 20231005). Written informed consent was obtained from all participants prior to enrollment.

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None. The authors declare that they have no competing interests.