Fibroblast growth factor-23 (FGF23) in patients with transient hypoparathyroidism: its important role in serum phosphate regulation

Vitamin D: Dosing, levels, form, and route of administration: Does one approach fit all?

The 4th International Conference on Controversies in Vitamin D was held as a virtual meeting in September, 2020, gathering together leading international scientific and medical experts in vitamin D. Since vitamin D has a crucial role in skeletal and extra-skeletal systems, the aim of the Conference was to discuss improved management of vitamin D dosing, therapeutic levels and form or route of administration in the general population and in different clinical conditions. A tailored approach, based on the specific mechanisms underlying vitamin D deficiency in different diseases that were discussed, was recommended. Specifically, in comparison to healthy populations, higher levels of vitamin D and greater amounts of vitamin D were deemed necessary in osteoporosis, diabetes mellitus, obesity (particularly after bariatric surgery), and in those treated with glucocorticoids. Emerging and still open issues were related to target vitamin D levels and the role of vitamin D supplementation in COVID-19 since low vitamin D may predispose to SARS-CoV-2 infection and to worse COVID-19 outcomes. Finally, whereas oral daily cholecalciferol appears to be the preferred choice for vitamin D supplementation in the general population, and in most clinical conditions, active vitamin D analogs may be indicated in patients with hypoparathyroidism and severe kidney and liver insufficiency. Parenteral vitamin D administration could be helpful in malabsorption syndromes or in states of vitamin D resistance.Specific guidelines for desired levels of vitamin D should be tailored to the different conditions affecting vitamin D metabolism with the goal to define disease-specific normative values.

Risk of Cardiovascular Conditions in Patients with Chronic Hypoparathyroidism: A Retrospective Cohort Study

Introduction

In patients with chronic hypoparathyroidism disordered calcium homeostasis has been associated with risk of cardiovascular diseases, including cardiomyopathy, congestive heart failure, and arrhythmia; however, larger-scale studies are needed to examine these risks. This study evaluated the risk of cardiovascular conditions among patients with chronic hypoparathyroidism.

Methods

Adults with and without chronic hypoparathyroidism were selected from a medical insurance claims database in the USA from January 2007 to June 2017, and were followed for up to 5 years. Associations between chronic hypoparathyroidism and incident atrial fibrillation (AF), tachyarrhythmia, myocardial infarction (MI), coronary artery disease (CAD), heart failure (HF), stroke, cerebrovascular disease, peripheral vascular disease (PVD), and a combined cardiovascular endpoint of cerebrovascular disease, CAD, HF, and PVD were compared between cohorts using Kaplan–Meier analyses and unadjusted and adjusted Cox proportional hazards models.

Results

In 8097 patients with chronic hypoparathyroidism compared with 40,485 patients without, respectively, mean ± SD ages were 58.6 ± 16.3 and 47.3 ± 18.0 years, 76.2% and 54.4% were female, and 19.4% and 9.5% had the combination of cardiovascular findings at baseline. In adjusted analyses, patients with chronic hypoparathyroidism had significantly higher risk (adjusted hazard ratio and 95% confidence interval) of incident AF (1.72; 1.51–1.97), tachyarrhythmia (1.68; 1.32–2.14), MI (1.18; 1.01–1.38), CAD (1.39; 1.26–1.54), HF (1.64; 1.46–1.84), stroke (1.45; 1.31–1.62), cerebrovascular disease (1.48; 1.34–1.62), PVD (1.66; 1.51–1.81), and combined cardiovascular endpoint (1.63; 1.52–1.75), all P < 0.001 except P = 0.036 for MI, compared with patients without chronic hypoparathyroidism.

Conclusions

This large retrospective cohort study showed that chronic hypoparathyroidism was associated with increased risk of incident cardiovascular conditions and arrhythmias. Results should be evaluated in light of limitations inherent to claims database analyses. Further studies are warranted to investigate reasons for these risks and to develop strategies for reducing cardiovascular conditions in patients with chronic hypoparathyroidism.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12325-021-01787-7.

Alfacalcidol vs Calcitriol in the Management of Patient With Hypoparathyroidism: A Randomized Controlled Trial

CONTEXT

Alfacalcidol and calcitriol are commonly used for managing hypoparathyroidism. Their relative merits have not been systematically assessed.

OBJECTIVE

We compared the effect of alfacalcidol and calcitriol on phosphatemic control, hypercalciuria, and associated factors in idiopathic-hypoparathyroidism (IH).

DESIGN AND SETTING

Open-label randomized controlled trial, tertiary care center.

SUBJECTS AND METHODS

IH patients with optimal calcemic control on alfacalcidol were continued on the same (n = 20) or switched to calcitriol (n = 25) at half of the ongoing alfacalcidol dose. The dose was adjusted during follow-up to maintain serum total calcium between 8.0 and 9.5 mg/dL. Serum calcium, phosphorus, 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D, 24-h urine calcium-to-creatinine ratio, and fractional excretion of phosphorus (FEPh) were measured at baseline and 6 months. Plasma intact-FGF23 was measured at final follow-up.

RESULT

Patients receiving alfacalcidol and calcitriol had comparable serum calcium at 6 months (8.7 ± 0.4 vs 8.9 ± 0.4 mg/dL, P = 0.13). Their median [interquartile range (IQR)] dose at 6 months was 2.0 (1.0-2.5) and 0.75 (0.5-1.0) µg/d, respectively. Serum 1,25(OH)2D levels were physiological in both (35.3 ± 11.6 and 32.3 ± 16.9 pg/mL). Serum phosphate and calcium excretion were comparable in 2 arms. A majority had hyperphosphatemia (75% vs 76%), hypercalciuria (75% vs 72%), and elevated FGF23 (116 ± 68 and 113 ± 57 pg/mL). Age showed significant independent association with plasma FGF23 (β = 1.9, P = 0.001). Average FEPh was low despite high FGF23.

CONCLUSION

At optimal calcium control, both alfacalcidol and calcitriol lead to comparable but high serum phosphate levels, hypercalciuria, physiological circulating 1,25(OH)2D, and elevated FGF23. Further studies are required to systematically investigate other treatment options.

Association of vitamin D and FGF23 with serum ferritin in hypoparathyroid thalassemia: a case control study

Background

FGF23 controls serum l,25(OH)₂D₃ levels and phosphate homeostasis. This study evaluates the effects of ferritin on intact PTH, FGF23, and l,25(OH)₂D₃ in patients with major thalassemia. It also evaluates FGF23 changes in patients with hypoparathyroidism to clarify the interaction between FGF23 and PTH in the absence of proper PTH functioning in human.

Methods

In this case-control study, 25 major-beta thalassemia patients with hypoparathyroidism were age- and gender-matched with major-beta thalassemia patients having normal parathyroid function. Biochemical studies assessed the serum calcium, albumin, phosphorus, alkaline phosphatase, PTH, FGF23, 25(OH) D, 1,25(OH)2D3, ferritin, and the fractional excretion of phosphorous.

Results

FGF23 was higher in the patients with hypoparathyroidism than the controls (P = 0.002). The fractional excretion of phosphorous was lower in patients with hypoparathyroidism, despite the high level of FGF23 (P = 0.001). There was a correlation between serum 1,25(OH)2D3 and FGF23 with ferritin in the controls (P = < 0.001and P = < 0.001, respectively).

Conclusions

The present study showed a strong positive correlation between serum ferritin and levels of FGF23 and 1,25(OH)2D3. We hypothesized that ferritin could have a stimulatory effect on the production of 1,25(OH)2D3. Moreover, a rise in FGF23 in patients with thalassemia, might be either associated with the stimulating effect of PTH and 1,25(OH)2D3, or directly related to the stimulating effect of ferritin.

Interaction between serum FGF-23 and PTH in renal phosphate excretion, a case-control study in hypoparathyroid patients

BACKGROUND

phosphate homeostasis is mediated through complex counter regulatory feed-back balance between parathyroid hormone, FGF-23 and 1,25(OH)2D. Both parathyroid hormone and FGF-23 regulate proximal tubular phosphate excretion through signaling on sodium- phosphate cotransporters II_a and II_c. However, the interaction between these hormones on phosphate excretion is not clearly understood. We performed the present study to evaluate whether the existence of sufficient parathyroid hormone is necessary for full phosphaturic function of FGF-23 or not.

METHODS

In this case-control study, 19 patients with hypoparathyroidism and their age- and gender-matched normal population were enrolled. Serum calcium, phosphate, alkaline phosphatase,parathyroid hormone, FGF-23, 25(OH)D, 1,25(OH)2D and Fractional excretion of phosphorous were assessed and compared between the two groups, using SPSS software.

RESULTS

The mean serum calcium and parathyroid hormone level was significantly lower in hypoparathyroid patients in comparison with the control group (P < 0.001 and P < 0.001, respectively). We found high serum level of phosphate and FGF-23 in hypoparathyroid patients compared to the control group (P < 0.001 and P < 0.001, respectively). However, there was no significant difference in Fractional excretion of phosphorous or 1,25OH2D level between the two groups. There was a positive correlation between serum FGF-23 and Fractional excretion of phosphorous just in the normal individuals (P < 0.001, r = 0.79).

CONCLUSIONS

Although the FGF-23 is a main regulator of urinary phosphate excretion but the existence of sufficient parathyroid hormone is necessary for the full phosphaturic effect of FGF-23.

Under-recognized Hypoparathyroidism in Thalassemia

OBJECTIVE

Symptomatic hypoparathyroidism [symptomatic hypocalcemia without elevated serum parathyroid hormone (PTH)] in patients with thalassemia is relatively rare. Asymptomatic mild hypocalcemia without elevated PTH, which is considered hypoparathyroidism, may be more common but under-recognized.

METHODS

Sixty-six transfusion-dependent thalassemic patients and 28 healthy controls were enrolled. Serum calcium (Ca), phosphate (P), creatinine (Cr), albumin, intact PTH, 25-hydroxyvitamin D (25-OHD), plasma intact fibroblast growth factor-23 (FGF-23), urinary Ca, P and Cr were measured. Tubular reabsorption of P was calculated.

RESULTS

Thalassemic patients had significantly lower median serum Ca levels than the controls [8.7 (7.8-9.7) vs 9.6 (8.7-10.1) mg/dL, p<0.001]. Hypoparathyroidism was found in 25 of 66 (38%) patients. Symptomatic hypoparathyroidism was not encountered. Thalassemic patients also had significantly lower median plasma FGF-23 levels than the controls [35.7 (2.1-242.8) vs 53.2 (13.3-218.6) pg/mL, p=0.01]. In patients with hypoparathyroidism, median plasma FGF-23 level was significantly lower than that of normoparathyroid patients [34.8 (2.1-120.0) vs 43.1 (3.2-242.8) pg/mL, p=0.048]. However, serum P, Cr, intact PTH and 25-OHD levels were not significantly different in the two groups.

CONCLUSION

Hypoparathyroidism was not uncommon in patients with transfusion-dependent thalassemia treated with suboptimal iron chelation. Plasma intact FGF-23 level in hypoparathyroid patients was lower than that of normoparathyroid patients.

Long-term outcome of cataract surgery in patients with idiopathic hypoparathyroidism and its relationship with their calcemic status

Cataract is a cardinal manifestation of hypoparathyroidism. Although patients with hypoparathyroidism require cataract surgery at a younger age than individuals without hypoparathyroidism, there is limited information on the outcome of this surgery. We assessed long-term complications of cataract surgery in patients with idiopathic hypoparathyroidism (IH) and its relationship with their clinical and biochemical parameters. Twenty-seven patients with IH and 25 nonhypoparathyroid controls with a minimum follow-up of 2 years after cataract surgery were assessed for visual acuity, intraocular pressure, lens centricity, Nd:YAG laser capsulotomy, and the severity of posterior capsular opacification (PCO) and anterior capsular opacification. High-resolution optical slit-lamp images were analyzed by an ophthalmologist. Patients with IH had cataract surgery at a younger age than controls (34.0 ± 16.4 years vs 58.0 ± 11.2 years, P < 0.001). A higher proportion of IH patients had dense white PCO (75.0 % vs 39.4 %, P = 0.004), Nd:YAG laser capsulotomy (44.2 % vs 10.0 %, P = 0.001), anterior capsular opacification (97.7 % vs 84.2 %, P = 0.03), and a decentric lens (28.3% vs 2.6 %, P = 0.001) at a comparable time after surgery (8.6 ± 6.1 years vs 8.7 ± 6.8 years, P = 0.85). On regression analysis, the severity of PCO in IH correlated only with male sex and not with other factors, including serum total calcium and inorganic phosphorus levels at the baseline and during follow-up. To conclude, patients with IH are likelier than individuals without IH to develop PCO and to require Nd:YAG laser capsulotomy after cataract surgery. Proper precautions should be taken during surgery to minimize this complication in IH.

Generation of mice encoding a conditional null allele of Gcm2

Glial cells missing homolog 2 (GCM2) is a transcription factor that is expressed predominately in the pharyngeal pouches and, at later stages, in the developing and mature parathyroid glands. In humans, loss of GCM2 function, either through recessive apomorphic mutations or dominant inhibitor mutations in the human GCM2 gene, leads to isolated hypoparathyroidism. In mice, homozygous disruption of Gcm2 by conventional gene targeting results in parathyroid aplasia and hypoparathyroidism. In this study, we report the generation and functional characterization of mice encoding a conditional null allele of Gcm2. We demonstrate the functional integrity of the conditional Gcm2 allele and report successful in vivo deletion of exon 2 using Cre recombinase. The mice with conditional deletion of Gcm2 displayed phenotypes similar to those previously described for a conventional Gcm2 knockout, including perinatal lethality, hypocalemia, low or undetectable serum levels of parathyroid hormone, and absent parathyroid glands. The production of a conditional mutant allele for Gcm2 represents a valuable resource for the study of the temporal- and spatial-specific roles for Gcm2, and for understanding the postnatal activities of GCM2 protein.

Insights from genetic disorders of phosphate homeostasis

The molecular identification and characterization of genetic defects leading to a number of rare inherited or acquired disorders affecting phosphate homeostasis has added tremendous detail to our understanding of the regulation of phosphate balance. The identification of the key phosphate-regulating hormone, fibroblast growth factor 23 (FGF23), as well as other molecules that control its production, such as the N-acetylgalactosaminyltransferase 3 GALNT3, the endopeptidase phosphate-regulating protein with homologies to endopeptidases on the X chromosome, and the matrix protein dentin matrix protein 1, and molecules that function as downstream effectors of FGF23, such as the longevity factor Klotho and the phosphate transporters NPT2a and NPT2c, has permitted us to understand the elegant and complex interplay that exists between the kidneys, bone, parathyroid, and gut. Such insights from genetic disorders have allowed not only the design of potent targeted therapies for some of these rare genetic disorders, such as using anti-FGF23 antibodies for treatment of X-linked hypophosphatemic rickets, but also have led to clinically relevant observations related to the dysregulation of mineral ion homeostasis in chronic kidney disease. Thus, we are able to leverage our knowledge of rare human disorders affecting only a few individuals, to understand and potentially treat disease processes that affect millions of patients.

FGF-23 and secondary hyperparathyroidism in chronic kidney disease

The metabolic changes that occur in patients with chronic kidney disease (CKD) have a profound influence on mineral and bone metabolism. CKD results in altered levels of serum phosphate, vitamin D, calcium, parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF-23); the increased levels of serum phosphate, PTH and FGF-23 contribute to the increased cardiovascular mortality in affected patients. FGF-23 is produced by osteocytes and osteoblasts and acts physiologically in the kidney to induce phosphaturia and inhibit the synthesis of 1,25-dihydroxyvitamin D3. PTH acts directly on osteocytes to increase FGF-23 expression. In addition, the high levels of PTH associated with CKD contribute to changes in bone remodelling that result in decreased levels of dentin matrix protein 1 and the release of low-molecular-weight fibroblast growth factors from the bone matrix, which stimulate FGF-23 transcription. A prolonged oral phosphorus load increases FGF-23 expression by a mechanism that includes local changes in the ratio of inorganic phosphate to pyrophosphate in bone. Other factors such as dietary vitamin D compounds, calcium, and metabolic acidosis all increase FGF-23 levels. This Review discusses the mechanisms by which secondary hyperparathyroidism associated with CKD stimulates bone cells to overexpress FGF-23 levels.

Direct and indirect effects of parathyroid hormone on circulating levels of fibroblast growth factor 23 in vivo

Fibroblastic growth factor 23 (FGF23) is a bone-derived hormone that has a pivotal role in the pathogenesis of mineral disorders in chronic kidney disease. To study the effect of parathyroid hormone (PTH) on FGF23, rats were parathyroidectomized for a week and then implanted with constant-delivery infusion pumps to provide vehicle, a physiological, or a threefold supraphysiological dose of parathyroid hormone. Parathyroidectomy resulted in a significant decrease in blood ionized calcium, FGF23, and calcitriol along with an increase in phosphorus concentrations. PTH replacement produced a dose-dependent increase in ionized calcium and FGF23 with decreased phosphorus. Calcitriol was also increased but there was no dose effect of PTH treatment. To maintain normal plasma calcitriol levels, two additional groups of parathyroidectomized rats were given calcitriol and temporarily treated with vehicle or the supraphysiological dose of PTH. FGF23 was significantly increased by calcitriol in the vehicle-treated rats but was not further increased above that in rats given the supraphysiological dose of PTH in the absence of calcitriol. Klotho expression in the kidney decreased after parathyroidectomy but was restored by hormone supplementation. Hence, our results show a direct and an indirect effect of PTH on FGF23 secretion, the latter through changes in calcitriol concentrations.

A nonphosphaturic mesenchymal tumor mixed connective tissue variant of the sacrum

Tumor-induced or oncogenic osteomalacia is a rare paraneoplastic syndrome characterized by overproduction of fibroblast growth factor-23 as a phosphaturic agent and renal phosphate wasting. A range of predominantly mesenchymal neoplasms have been associated with tumor-induced osteomalacia and classified as phosphaturic mesenchymal tumor mixed connective tissues. However, phosphaturic mesenchymal tumor mixed connective tissues could be nonphosphaturic in the first stage of the disease, either because the tumors are resected early in the clinical course or because the patient's osteomalacia was attributed to another cause. This article presents a case of a 42-year-old woman with a 2-year history of low back and right leg pain. Laboratory examinations including serum and urine calcium and phosphorous were within normal values. Imaging of the lumbar spine and pelvis showed an osteolytic lesion occupying the right sacral wing. Histology was unclear. Reverse-transcription polymerase chain reaction analysis for fibroblast growth factor-23 was positive and confirmed the diagnosis of phosphaturic mesenchymal tumor mixed connective tissues. Preoperative selective arterial embolization and complete intralesional excision, bone grafting, and instrumented fusion from L4 to L5 to the iliac wings bilaterally was performed. Postoperative recovery was uneventful. Neurological deficits were not observed. A lumbopelvic corset was applied for 3 months. At 12 months, the patient was asymptomatic. Serum and urine values of calcium and phosphorous were normal throughout the follow-up evaluation.

Fibroblast growth factor 23 concentrations in healthy term infants during the early postpartum period

Fibroblast growth factor 23 (FGF23) is a potent regulator of Pi and 1,25-(OH)(2)D homeostasis. Early postpartum infants show intriguing changes in serum levels of Ca, Pi, PTH and 1,25-(OH)(2)D. However, the role of FGF23 in the early neonatal mineral metabolism has not been clarified. In order to evaluate the significance of FGF23 during the early postpartum period, we examined the circulating FGF23 levels using an intact FGF23 ELISA and a C-terminal FGF23 ELISA either in 22 umbilical cord blood samples (the cord blood) or in 22 term infants at 5days of life (the 5-day-old infant). We also compared these ranges with those of 11 healthy adults. Data were expressed as mean+/-SD, and analyzed by two-way ANOVA, followed by the Tukey's test. C-terminal FGF23 in the cord blood, the 5-day-old infants and the healthy adults were 73.3+/-22.4, 81.0+/-28.2 and 39.0+/-7.8 RU/ml, respectively. Intact FGF23 in the cord blood, the 5-day-old infants and the healthy adults were 3.9+/-1.6, 21.8+/-17.6, and 27.6+/-7.3 pg/ml, respectively. Immunoprecipitation assays using anti-FGF23 antibodies demonstrated that the intact 32 kDa FGF23 was low and the fragmented FGF23 of 18kDa was abundant in the cord blood compared with those in the healthy adults. In conclusion, our observations indicated that the intact FGF23/C-terminal FGF23 ratio was very low due to the fragmentation of FGF23 during the early postpartum period and might have a considerable contribution to the Pi homeostasis in the healthy term infants.

Tumor-induced osteomalacia: a case report

Tumor-induced osteomalacia (TIO) is a rare paraneoplasic syndrome with overproduction of fibroblast growth factor 23 as a phosphaturic agent, leading to chronic hyperphosphaturia and hypophosphatemia, associated with inappropriately normal or low levels of 1,25-dihydroxyvitamin D. Diagnosis of this disease is often challenging. The following case report described a middle-aged man with symptoms of bone pain and severe muscle weakness, who was found to have TIO. The tumor responsible for the symptoms was localized on his thigh and its resection resulted in normalization of blood chemistry and complaints. Subsequent microscopic examination revealed a phosphaturic mesenchymal tumor, mixed connective tissue type. The authors reinforce the importance of recognition of this disease, as severe disability and even death can be avoided with the surgical removal of the causative tumor.

The journey from vitamin D-resistant rickets to the regulation of renal phosphate transport

In 1937, Fuller Albright first described two rare genetic disorders: Vitamin D resistant rickets and polyostotic fibrous dysplasia, now respectively known as X-linked hypophosphatemic rickets (XLH) and the McCune-Albright syndrome. Albright carefully characterized and meticulously analyzed one patient, W.M., with vitamin D-resistant rickets. Albright subsequently reported additional carefully performed balance studies on W.M. In this review, which evaluates the journey from the initial description of vitamin D-resistant rickets (XLH) to the regulation of renal phosphate transport, we (1) trace the timeline of important discoveries in unraveling the pathophysiology of XLH, (2) cite the recognized abnormalities in mineral metabolism in XLH, (3) evaluate factors that may affect parathyroid hormone values in XLH, (4) assess the potential interactions between the phosphate-regulating gene with homology to endopeptidase on the X chromosome and fibroblast growth factor 23 (FGF23) and their resultant effects on renal phosphate transport and vitamin D metabolism, (5) analyze the complex interplay between FGF23 and the factors that regulate FGF23, and (6) discuss the genetic and acquired disorders of hypophosphatemia and hyperphosphatemia in which FGF23 plays a role. Although Albright could not measure parathyroid hormone, he concluded on the basis of his studies that showed calcemic resistance to parathyroid extract in W.M. that hyperparathyroidism was present. Using a conceptual approach, we suggest that a defect in the skeletal response to parathyroid hormone contributes to hyperparathyroidism in XLH. Finally, at the end of the review, abnormalities in renal phosphate transport that are sometimes found in patients with polyostotic fibrous dysplasia are discussed.

Hypophosphatemia: the common denominator of all rickets

Rickets is a disease of the hypertrophic chondrocytes in the growth plate and is caused by hypophosphatemia-a derived defect in terminal chondrocyte apoptosis. This highlights the critical role of phosphorous in cartilage and bone metabolism. This review shows the role of phosphorous metabolism, transport and function in maintaining phosphorous supply to the growth plate, bone osteoblast and the kidney. Given that phosphorous is the common denominator of all rickets, this review proposes a new classification for the differential diagnosis of rickets, which is based on the mechanisms leading to hypophosphatemia-high PTH activity, high FGF23 activity or renal phosphaturia.

Development of tumor-induced osteomalacia in a subcutaneous tumor, defined by venous blood sampling of fibroblast growth factor-23

A 25-year-old man with severe lumbago was referred to our department for further evaluation. Serum phosphate and TmP/GFR levels were decreased. Physical examination revealed an elastic tumor in the instep of the right foot, which the patient reported having since the age of 10 years. He had no symptoms of osteomalacia at that time. Within the recent years, the tumor had grown in size and the patient developed lumbago. To examine the existence of a fibroblast growth factor-23 (FGF-23)-producing tumor, venous blood was collected from four main veins. FGF-23 levels were significantly increased in the right femoral vein, compared with other veins. After the resection of the tumor, the histopathology was consistent with a phosphaturic mesenchymal tumor (mixed connective tissue variant). Taken together, these results indicated that the development of osteomalacia in this patient was associated with the production of FGF-23 in the subcutaneous tumor.