Elevated Fibroblast Growth Factor 23 Levels Research Articles

FGF-23 and cardiovascular disease: review of literature

This review examines associations between fibroblast growth factor 23 (FGF-23) and cardiovascular disease. FGF-23 is a hormone produced by osteocytes and osteoblasts that aids with phosphate excretion by the kidney and acts as a negative feedback regulator for activated vitamin D synthesis. Recent studies have found associations between elevated FGF-23 levels and a number of cardiovascular diseases, including hypertension, left ventricular hypertrophy, endothelial dysfunction, cardiovascular events and mortality. Recent studies have explored the possible effects of FGF-23 on the cardiovascular system. In animal and observational human studies, there is a link between elevated FGF-23 levels and multiple cardiovascular outcomes, including hypertension, left ventricular hypertrophy and cardiovascular events and mortality. Further studies are required to evaluate whether decreasing FGF-23 levels improves cardiovascular outcomes.

Fibroblast growth factor 23 actions in inflammation: a key factor in CKD outcomes.

During chronic kidney disease (CKD), bone mineral metabolism is disturbed owing in part to the endogenous hormone fibroblast growth factor 23 (FGF23). Elevated FGF23 levels are seen in CKD patients. Current research has demonstrated that FGF23 directly modulates the immune response and host defense to bacterial infections. FGF23 also impairs the activation and recruitment of neutrophils, which are the main immune effector cells required for host defense against bacterial infections. In addition, while FGF23 levels reduce leukocyte recruitment and functions, inflammatory conditions may also-in a reverse fashion-contribute to elevated FGF23 levels in the circulation. In this context, altered hypoxia inducible factor 1α signaling and iron metabolism may contribute to intact FGF23 (iFGF23) production. This review examines evidence on the role of FGF23 in inflammation, immune cell function and recruitment as well as the regulation of FGF23 during inflammation and the clinical implications of this process for the immune system in individuals with CKD. Clinical observations and laboratory investigations indicate an important role of FGF23 in directly modulating leukocyte activation and recruitment behavior with consequences on host defense against bacterial infections. This novel observation may in part explain the increased infectious risk among patients with CKD. However, studies of FGF23 neutralization also revealed increased mortality after sustained administration over several weeks in rats. Thus, therapeutic interventions targeting FGF23 must be carefully evaluated.

The Three Sisters of Fate in Multiple Sclerosis: Klotho (Clotho), Fibroblast Growth Factor-23 (Lachesis), and Vitamin D (Atropos)

Background: The klotho (Klt)-fibroblast growth factor-23 (FGF-23)-vitamin D axis is the main component of calcium (Ca) and phosphorus (P) metabolisms; on the contrary, it is also secreted from the choroid plexus (CP). Purpose: This study is aimed at evaluating serum soluble Klt (sKlt), FGF-23, and 25-(OH)-vitamin D levels in multiple sclerosis (MS) patients. Methods: Thirty-two relapsing-remitting MS patients (11 males and 21 females; mean age 38.3 years) and 31 age-sex matched healthy controls (12 males and 19 females; median age 38.5 years) were included in this study. All patients were diagnosed with MS according to the criteria of McDonald. Results: Serum sKlt, FGF-23, and P levels were significantly higher in MS patients compared to the control group (p < 0.01, p < 0.01, and p = 0.02, respectively). Serum 25-(OH)-vitamin D and Ca levels were significantly lower in MS patients (p < 0.01 and p = 0.04, respectively). Conclusion: Klt, which is secreted from CP, could be a response to the inflammatory condition in MS. Elevated FGF-23 levels suppress 1α-hydroxylase and upregulates 24α-hydroxylase, which results in a decrease in 1,25-(OH)₂D₃ levels. Thus, the neuroprotective and immunomodulatory effects of vitamin D might not be seen in MS patients.

Association of Fibroblast Growth Factor 23 With Atrial Fibrillation in Chronic Kidney Disease, From the Chronic Renal Insufficiency Cohort Study.

Levels of fibroblast growth factor 23 (FGF23) are elevated in chronic kidney disease (CKD) and strongly associated with left ventricular hypertrophy, heart failure, and death. Whether FGF23 is an independent risk factor for atrial fibrillation in CKD is unknown. To investigate the association of FGF23 with atrial fibrillation in CKD. Prospective cohort study of 3876 individuals with mild to severe CKD who enrolled in the Chronic Renal Insufficiency Cohort Study between June 19, 2003, and September 3, 2008, and were followed up through March 31, 2013. Baseline plasma FGF23 levels. Prevalent and incident atrial fibrillation. The study cohort comprised 3876 participants. Their mean (SD) age was 57.7 (11.0) years, and 44.8% (1736 of 3876) were female. Elevated FGF23 levels were independently associated with increased odds of prevalent atrial fibrillation (n = 660) after adjustment for cardiovascular and CKD-specific factors (odds ratio of highest vs lowest FGF23 quartile, 2.30; 95% CI, 1.69-3.13; P < .001 for linear trend across quartiles). During a median follow-up of 7.6 years (interquartile range, 6.3-8.6 years), 247 of the 3216 participants who were at risk developed incident atrial fibrillation (11.9 events per 1000 person-years). In fully adjusted models, elevated FGF23 was independently associated with increased risk of incident atrial fibrillation after adjustment for demographic, cardiovascular, and CKD-specific factors, and other markers of mineral metabolism (hazard ratio of highest vs lowest FGF23 quartile, 1.59; 95% CI, 1.00-2.53; P = .02 for linear trend across quartiles). The results were unchanged when further adjusted for ejection fraction, but individual adjustments for left ventricular mass index, left atrial area, and interim heart failure events partially attenuated the association of elevated FGF23 with incident atrial fibrillation. Elevated FGF23 is independently associated with prevalent and incident atrial fibrillation in patients with mild to severe CKD. The effect may be partially mediated through a diastolic dysfunction pathway that includes left ventricular hypertrophy, atrial enlargement, and heart failure events.

THE ROLE OF FBROBLAST GROWTH FACTOR 23 (FGF 23) IN THE MINERAL AND BONE DISORDER (MBD) OF THE PATENTS WITH CHRONIC KIDNEY DISEASE

Fibroblast growth factor 23 (FGF23) is a key regulator of phosphorus metabolism whose effects in patients with chronic kidney disease (CKD) have only recently begun to be appreciated. Recent study of this phosphaturic hormone has revealed new path-ways of mineral regulation in both individuals with normal kidney function and in patients with CKD. While the effects of FGF23 on mineral metabolism in CKD appears to be similar to its effects in individuals with normal kidney function, elevated levels of the protein in the CKD population have also been linked to kidney disease progression, altered skeletal histology, and increased mortality rates, relationships that have not been examined in the general population.Thus, potential differences in FGF23 metabolism accompany the elevated levels found in CKD patients and, although the exact pathophysiological consequences remain mostly unknown, elevated FGF23 levels appear to contribute to major complications of CKD that plague both adults and children. Key words: FGF23, chronic kidney

Inflammation regulates fibroblast growth factor 23 production.

Fibroblast growth factor 23 (FGF23) is a hormone secreted by osteocytes and osteoblasts that regulates phosphorus and vitamin D homeostasis. FGF23 levels increase progressively in chronic kidney disease (CKD), and FGF23 excess might be a causal factor of left ventricular hypertrophy, CKD progression and death. Therefore, understanding the molecular mechanisms that control FGF23 production is critical to design therapies to lower FGF23 levels. The present review focuses on the role of inflammatory stimuli on FGF23 regulation and summarizes recent studies that support a novel framework linking inflammation to FGF23 regulation. Inflammation and iron deficiency, which are common occurrences in CKD, have emerged as novel FGF23 regulators. Recent findings show that inflammation increases FGF23 production in bone through direct and iron-related indirect mechanisms. In these settings, hypoxia-inducible factor (HIF)-1α orchestrates FGF23 transcription in response to inflammation and is primarily responsible for coordinating FGF23 production and cleavage. We demonstrate that inflammation increases FGF23 production and may contribute to elevated FGF23 levels in CKD. Osseous HIF-1α may represent a therapeutic target to lower FGF23 levels in CKD patients and minimize the negative consequences associated with FGF23 excess.

Tumour-induced osteomalacia: An emergent paraneoplastic syndrome

Endocrine paraneoplastic syndromes are distant manifestations of some tumours. An uncommon but increasingly reported form is tumour-induced osteomalacia, a hypophosphatemic disorder associated to fibroblast growth factor 23 (FGF-23) secretion by tumours. The main biochemical manifestations of this disorder include hypophosphatemia, inappropriately low or normal tubular reabsorption of phosphate, low serum calcitriol levels, increased serum alkaline phosphatase levels, and elevated or normal serum FGF-23 levels. These tumours, usually small, benign, slow growing and difficult to discover, are mainly localized in soft tissues of the limbs. Histologically, phosphaturic mesenchymal tumours of the mixed connective tissue type are most common. Various imaging techniques have been suggested with variable results. Treatment of choice is total surgical resection of the tumour. Medical treatment includes oral phosphorus and calcitriol supplements, octreotide, cinacalcet, and monoclonal antibodies.

Fibroblast growth factor 23 signaling in hippocampal cells: impact on neuronal morphology and synaptic density

Endocrine fibroblast growth factor 23 (FGF23) is predominantly secreted by osteocytes and facilitates renal phosphate excretion. However, FGF23 is also present in cerebrospinal fluid. In chronic kidney disease, FGF23 serum levels are excessively elevated and associated with learning and memory deficits. Structural plasticity of the hippocampus such as formation of new synapses or an altered dendritic arborization comprises a cellular and morphological correlate of memory formation. Therefore, we hypothesize that FGF23 alters hippocampal neuron morphology and synapses. To address this, we prepared primary murine hippocampal cultures and incubated them with recombinant FGF23 alone or together with a soluble isoform of its co-receptor α-Klotho. Neuronal expression of a fluorescent reporter allowed for a detailed evaluation of the neuronal morphology by Sholl analysis. Additionally, we evaluated synaptic density, identified by stainings, for synaptic markers. We show an enhanced number of primary neurites combined with a reduced arborization, resulting in a less complex morphology of neurons treated with FGF23. Moreover, FGF23 enhances the synaptic density in a FGF-receptor (FGF-R) dependent manner. Finally, we addressed the corresponding signaling events downstream of FGF-R employing a combination of western blots and quantitative immunofluorescence. Interestingly, FGF23 induces phospholipase Cγ activity in primary hippocampal neurons. Co-application of soluble α-Klotho leads to activation of the Akt-pathway and modifies FGF23-impact on neuronal morphology and synaptic density. Compared with other FGFs, this alternative signaling pattern is a possible reason for differential effects of FGF23 on hippocampal neurons and may thereby contribute to learning and memory deficits in chronic kidney disease patients. In this study, we show that fibroblast growth factor 23 inhibits neuronal ramification and enhances the synaptic density in primary hippocampal cultures accompanied by phospholipase Cγ-activation. Co-application of the co-receptor α-Klotho leads to an Akt-activation and further modifies neuronal morphology and number of synapses. Those effects provide a mechanistic basis for memory deficits in patients suffering from chronic kidney disease (CKD) characterized by excessively elevated FGF23 levels as well as memory deficits.

Cord Blood Ferritin and Fibroblast Growth Factor-23 Levels in Neonates.

Elevated levels of the phosphate-regulating hormone, fibroblast growth factor-23 (FGF-23) are associated with skeletal and cardiovascular disease. Levels of FGF-23 are elevated in neonates, but the mechanisms are poorly understood. Iron deficiency is a recently described stimulus for FGF-23 production. To test the hypothesis that lower fetal iron status, as measured by lower cord blood ferritin, is independently associated with elevated FGF-23 levels in neonates. This is a cross-sectional study of 64 full-term, healthy neonates. This study took place in a university-based, tertiary care center. Plasma levels of second generation C-terminal FGF-23 (cFGF-23) and intact FGF-23 (iFGF-23). Levels of cFGF-23 ranged from 108 to 7508 reference units (RU)/ml (median, 824 RU/ml), and iFGF-23 from undetectable (<8.5) to 135.4 pg/ml (median, <8.5 pg/mL). Ferritin ranged from 58 to 719 ng/ml (mean, 203 ng/ml). Lower cord blood ferritin levels were associated with higher cFGF-23 (r = −0.320; P = .014), but not iFGF-23 levels (r = −0.222; P = .082). In multivariate analyses adjusted for glycemic indices, maternal race, and parity, lower ferritin levels remained independently associated with higher cFGF-23 levels (B = −0.261, P = .01). In the full models, higher cord blood glucose and C-peptide levels were also independently associated with higher cFGF-23 levels. cFGF-23, but not iFGF-23 levels, are elevated in cord blood of healthy term neonates and independently associated with lower serum ferritin and higher glycemic indices.

Expression of osteoclastogenic factor transcripts in osteoblast-like UMR-106 cells after exposure to FGF-23 or FGF-23 combined with parathyroid hormone.

As a bone-derived hormone, fibroblast growth factor-23 (FGF-23) negatively regulates phosphate and calcium metabolism, while retaining growth-promoting action for mesenchymal cell differentiation. Elevated FGF-23 levels, together with hyperparathyroidism, are often observed in chronic kidney disease, which is associated with impaired bone mineralization and enhanced bone resorption. Although overexpression of osteoblast-derived osteoclastogenic cytokines might contribute to this metabolic bone disease, whether FGF-23 alone and FGF-23 plus parathyroid hormone (PTH) directly modulated the expression of osteoblast-derived osteoclastogenic genes remained elusive. Herein, we demonstrated the direct effects of FGF-23 on proliferation and mRNA expression of osteoblast-specific differentiation and osteoclastogenic markers in rat osteoblast-like UMR-106 cells in the presence or absence of PTH. FGF-23 was found to suppress UMR-106 cell proliferation, while increasing FGF-23 expression, the latter of which suggested the presence of positive feedback regulation of FGF-23 expression in osteoblasts. FGF-23 also upregulated the mRNA expression of osteoblast differentiation markers (e.g., Runx2, osterix, AJ18, Dlx5, alkaline phosphatase, and osteopontin), osteoclastogenic factors (e.g., MCSF, MCP-1, IL-6, and TNF-α), and bone resorption regulators (RANKL and osteoprotegerin). However, combined PTH and FGF-23 exposure did not alter the levels of FGF-23-induced transcripts, suggesting that both hormones had no additive effect. In conclusion, FGF-23 directly suppressed osteoblast proliferation, while inducing osteoclastogenic gene expression in UMR-106 cells, and the FGF-23-induced transcripts were not altered by long-standing PTH exposure.

Association of fibroblast growth factor 23, parathyroid hormone, and vitamin D with acute kidney injury

Introduction Fibroblast growth factor 23 (FGF23) plays an important role in regulating phosphate and vitamin D homeostasis. Elevated levels of FGF23 are independently associated with mortality in patients with chronic kidney disease and End stage renal disease (ESRD). Whether FGF23 levels are elevated and associated with adverse outcomes in patients with acute kidney injury (AKI) has not been studied so far. Objective The aim of this study was to determine the relationship between FGF23 levels in patients with AKI and morbidity, mortality, and/or the need for renal replacement therapy. Patients and methods The study included two groups: group 1, which included 30 AKI patients from the general medical ward and ICUs [identified in accordance with the criteria established by Acute Kidney Injury Network (AKIN) grading of AKI]; and group 2, which included 30 healthy controls matched with the patients as regards age and sex. Plasma levels of C-terminal FGF23, 1,25-dihydroxy vitamin D [1,25(OH)2D], and intact parathyroid hormone (iPTH) were measured within 24 h of AKI onset and 5 days later. The composite end point was death or need for renal replacement therapy. Results FGF23 levels on day 1 were significantly higher among participants with AKI than among controls (mean level: 278.20 ± 220.58 vs. 14.60 ± 9 pg/ml). There was a statistically significant negative correlation between FGF23 and vitamin D on day 1, with a P-value of less than 0.023, whereas there was no statistically significant negative correlation between FGF23 and vitamin D on day 5, with a P-value of 0.102. There was a statistically significant positive correlation between FGF23 on day 1 and both Acute Physiology and Chronic Health Evaluation and Sequential Organ Failure Assessment scores, with a P-value of less than 0.001. FGF23 proved to be a good predictor of mortality (sensitivity: 100%, specificity: 85%) at a cutoff value of 280 pg/ml. Conclusion FGF23 levels are elevated in AKI patients and are associated with increased mortality. AKI is also associated with significant reduction in the level of 1,25(OH)2D and with significant elevation of PTH.

Tumor-induced Osteomalacia Due to Phosphaturic Mesenchymal Tumor, Mixed Cell Type, of the Sphenoid Bone

ABSTRACT Objective: Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome caused by small endocrine tumors that secrete fibroblast growth factor 23 (FGF23), a phosphaturic hormone. Methods: A 44-year-old male was evaluated following complaints of progressive leg pain, difficulty walking, and muscle pain over the previous 9 years. Biochemical evaluation showed low serum phosphorus, high urine phosphorus, and elevated FGF23 levels. These findings were suggestive of TIO. A whole-body (68)Gallium-labeled (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid)-1-NaI(3)-octreotide positron emission tomography–computed tomography scan revealed a large vascular lesion in the anterior squamous portion of the right temporal lobe and right greater wing of the sphenoid bone, involving the posterolateral wall of the right orbit. Axial T1-weighted magnetic resonance imaging revealed isointense lesions containing multiple vascular channels. The histomorphologic feature of the excised tumor was compatible...

Osteomalacia tumoral: un síndrome paraneoplásico emergente

Endocrine paraneoplastic syndromes are distant manifestations of some tumours. An uncommon but increasingly reported form is tumour-induced osteomalacia, a hypophosphatemic disorder associated to fibroblast growth factor23 (FGF-23) secretion by tumours. The main biochemical manifestations of this disorder include hypophosphatemia, inappropriately low or normal tubular reabsorption of phosphate, low serum calcitriol levels, increased serum alkaline phosphatase levels, and elevated or normal serum FGF-23 levels. These tumours, usually small, benign, slow growing and difficult to discover, are mainly localized in soft tissues of the limbs. Histologically, phosphaturic mesenchymal tumours of the mixed connective tissue type are most common. Various imaging techniques have been suggested with variable results. Treatment of choice is total surgical resection of the tumour. Medical treatment includes oral phosphorus and calcitriol supplements, octreotide, cinacalcet, and monoclonal antibodies.

Fibroblast Growth Factor 23: Mechanisms of Action and Regulation

Fibroblast growth factor 23 (FGF23) is a central regulator in mineral metabolism. It is produced mainly by osteocytes in the bone and exerts its effects on distant organs in an endocrine manner. FGF23 generally requires a transmembrane protein named aKlotho to evoke its signal via an FGF receptor (FGFR). In the kidney, FGF23 increases the renal phosphate excretion and decreases the production of 1,25-dihydroxyvitamin D [1,25(OH)2D]. In the parathyroid gland, it suppresses the secretion of parathyroid hormone (PTH). The placenta also expresses both FGFR1 and aKlotho, and an elevated level of maternal FGF23 induces the placental expression of 25-hydroxyvitamin D-24-hydroxylase, affecting fetal vitamin D metabolism. Pathologically elevated levels of FGF23 may exert its effects even on the tissues without aKlotho expression, such as myocardium. Excessive action of FGF23 of various causes leads to hypophosphatemic rickets/osteomalacia, while its impaired action results in hyperphosphatemia and ectopic calcification. Some of the molecules responsible for hereditary hypophosphatemic rickets/osteomalacia reside in the osteocytes and function as local regulators of the production and/or activity of FGF23. The FGF23 expression is controlled by systemic factors also, among which 1,25(OH)2D appears to be a principal regulator. In chronic kidney disease (CKD), FGF23 levels begin to increase from the early stages, although the underlying mechanism still remains unclear. The elevated FGF23 levels in CKD have been shown to be associated with poor outcomes. Elucidation of the mechanism for action and regulation of FGF23 will contribute to the development of new strategies for diagnosis and treatment of the diseases with impaired mineral metabolism.

Elevated Fibroblast Growth Factor 23 Concentration: Prediction of Mortality among Chronic Kidney Disease Patients

Background: The osteocyte-derived hormone, fibroblast growth factor 23 (FGF23), regulates the phosphorus metabolism and suppresses 1,25-dihydroxyvitamin D production, thereby mitigating hyperphosphatemia in patients with renal disorders. An elevated FGF23 level is suggested to be an early biomarker of altered phosphorus metabolism in the initial stages of chronic kidney disease (CKD) and acts as a strong predictor of mortality in dialysis patients. In the Saudi population, there is no report on the FGF23 level in CKD patients to date. This study aims to estimate the plasma FGF23 levels in the Saudi population and to correlate it with its clinical manifestations in order to ascertain its role in the pathogenesis of CKD patients. Methods: The FGF23 level in the plasma samples was determined using ELISA in a diverse cohort of 89 cases with stage 3-5 CKD and 100 healthy subjects. The plasma FGF23 level was correlated with other biochemical parameters. Results: The results revealed that the FGF23 level was markedly elevated among CKD patients compared to the control group, and a significant inverse correlation was observed between the FGF23 level and glomerular filtration rate. FGF23 elevation was approximately 40-fold among stage 5 patients compared to the control, while the elevation of phosphate, parathyroid hormone (PTH) and alkaline phosphatase was 2-, 3- and 8-fold in this stage, respectively. Conclusion: Elevated FGF23 levels may have a strong correlation with the disease pathogenesis. In addition, FGF23 might be a future therapeutic target to intervene against the progression of CKD as well as to increase patient survivability.

Where is the link between mineral bone markers and cardiovascular disease in CKD?

Where is the link between mineral bone markers and cardiovascular disease in CKD?

Mineral (Mal)Adaptation to Kidney Disease--Young Investigator Award Address: American Society of Nephrology Kidney Week 2014.

In the short time since its initial discovery as the cause of rare hypophosphatemic disorders, fibroblast growth factor-23 (FGF-23) has emerged as a major regulator of mineral metabolism and critical component of the bone and mineral adaptation to CKD. However, because elevated FGF-23 levels are also a novel biomarker and possible molecular mediator of increased risks of cardiovascular disease and death in CKD, the initially adaptive response to increase FGF-23 levels to maintain neutral phosphate balance in CKD may ultimately become maladaptive. Incorporating FGF-23 into understanding the complex physiology that governs normal bone and mineral metabolism and its alterations in CKD has filled critical knowledge gaps and opened a new landscape of exciting hypotheses and novel therapeutic strategies to be tested in the continued quest to alleviate the burden of CKD.

A Novel PHEX Mutation in Japanese Patients with X-Linked Hypophosphatemic Rickets.

X-linked hypophosphatemic rickets (XLH) is a dominant inherited disorder characterized by renal phosphate wasting, aberrant vitamin D metabolism, and abnormal bone mineralization. Inactivating mutations in the gene encoding phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX) have been found to be associated with XLH. Here, we report a 16-year-old female patient affected by hypophosphatemic rickets. We evaluated her serum fibroblast growth factor 23 (FGF23) levels and conducted sequence analysis of the disease-associated genes of FGF23-related hypophosphatemic rickets: PHEX, FGF23, dentin matrix protein 1, and ectonucleotide pyrophosphatase/phosphodiesterase 1. She was diagnosed with XLH based on her clinical features and family history. Additionally, we observed elevated FGF23 levels and a novel PHEX exon 9 mutation (c.947G>T; p.Gly316Val) inherited from her father. Although bioinformatics showed that the mutation was neutral, Gly316 is perfectly conserved among humans, mice, and rats, and there were no mutations in other FGF23-related rickets genes, suggesting that in silico analysis is limited in determining mutation pathogenicity. In summary, we present a female patient and her father with XLH harboring a novel PHEX mutation that appears to be causative of disease. Measurement of FGF23 for hypophosphatemic patients is therefore useful for the diagnosis of FGF23-dependent hypophosphatemia.

Elevated Fibroblast Growth Factor-23 levels is Associated with Diastolic Dysfunction among No-CKD Patients with Preserved Left Ventricular Ejection Function

Fibroblast growth factor-23 (FGF23) plays a crucial role in phosphate metabolism via suppressing renal tubular phosphate re-absorption. Recent studies showed that patients with higher serum FGF23 more likely to have cardiac hypertrophy, which is independent of renal function, and blood pressure.

Abnormalities in renal tubular phosphate handling in children with sickle cell disease.

The mechanisms responsible for the hyperphosphatemia in patients with sickle cell disease (SCD) and preserved glomerular filtration rate (GFR) are not fully understood. The role of fibroblast growth factor 23 (FGF23), a phosphaturic hormone has not been investigated in SCD. Hence, we evaluated parameters of renal tubular phosphorus handling and their relation to prevailing FGF23 levels in a cohort of young SCD patients. Renal tubular phosphate handling and circulating levels of various analytes including FGF23 and parathyroid hormone (PTH) were measured in 24 children with SCD and normal estimated GFR in a cross sectional study. Correlation and regression analysis were employed to derive relationships between serum phosphorus and several variables. Most children showed elevated age- adjusted serum phosphorus (5.1 ± 0.7 mg/dl) levels. Tubular re-absorption of phosphorus(TRP) (96.3 ± 2.1%) and tubular maximum re-absorption of phosphorus per unit volume of GFR (TMP/GFR) (4.9 ± 0.6 mg/dl) were both elevated. Plasma intact FGF23 concentrations were elevated (81 ± 38 pg/ml) while the average PTH values were normal in most patients (50 ± 27 pg/ml). Univariate analysis showed significant correlations of serum phosphorus with TMP/GFR, alkaline phosphatase, age, lactate dehydrogenase (LDH), and log intact FGF23. TMP/GFR correlated with log intact FGF23 (r = 0.5, P< or = 0.01) but not with PTH. Multiple regression analysis yielded an independent relationship of serum phosphorus with TMP/GFR. The elevated serum phosphorus concentrations with simultaneously increased TMP/GFR and elevated FGF23 levels collectively suggest that patients with SCD display proximal tubular resistance to the action of FGF23 before any decline in GFR.