Journal of Indian Society of Pedodontics and Preventive Dentistry
Journal of Indian Society of Pedodontics and Preventive Dentistry
                                                   Official journal of the Indian Society of Pedodontics and Preventive Dentistry                           
Year : 2011  |  Volume : 29  |  Issue : 3  |  Page : 278--279

Familial hypophosphatemic rickets


S Vivekanandan 
 Clinical Biochemistry Department, Chennai Institute of Biomedical Sciences, Vadapalani, Chennai, India

Correspondence Address:
S Vivekanandan
Consultant Clinical Biochemist, Chennai Institute of Biomedical Sciences, 54/3-3 pillaiyar Koil Street, Vadapalani, Chennai - 600 026
India




How to cite this article:
Vivekanandan S. Familial hypophosphatemic rickets.J Indian Soc Pedod Prev Dent 2011;29:278-279


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Vivekanandan S. Familial hypophosphatemic rickets. J Indian Soc Pedod Prev Dent [serial online] 2011 [cited 2019 Dec 10 ];29:278-279
Available from: http://www.jisppd.com/text.asp?2011/29/3/278/85842


Full Text

Sir,

Recent article "Familial Hypophosphatemic Rickets" (FHR) published in your Journal of Indian Society of Pedodontic and Preventive Dentistry, Oct-Dec issue 2010 volume 28 by commendable. [1] However, a few additional pieces of information might have been beneficial.

FHR, a group of disorders and X-linked hypophosphatemia (XLH) is the most common, results from mutations in PHEX, causing abnormalities of the osteocyte, osteoblast, and odontoblast function, [2] including increased bone expression of the fibroblast growth factor 23 (FGF-23). [3] FGF23 inhibits renal tubule phosphate reabsorption and down-regulates 1-hydroxylase. [3] Hence, XLH is biochemically characterized by hypophosphatemia due to renal phosphate (Pi) wasting, with inappropriately low or normal 1,25-dihydroxyvitamin D (calcitriol) concentrations despite hypophosphatemia with consequent mild gastrointestinal phosphate malabsorption, contributing to the evolution of the hypophosphatemia.

Though the laboratory hallmark of FHR is hypophosphatemia, renal Pi wasting is a major pathogenic abnormality in FHR and forms the basis of medical therapy. Biochemical measurement of TmP/GFR (quantitates maximum tubular capacity for phosphate reabsorption per glomerular filtration rate) together with plasma calcitriol is required for diagnosis (including differential) of X-linked hypophosphatemic rickets. [4] "Renal phosphate leak" is better reported by TmP/GFR, the most widely accepted/used index normal values are higher in children than in adults. The TmP/GFR correlates positively with height Z-score in pediatric patients [5] and its decrease reflects the degree of bowing deformity in affected adults. The TmP/GFR is a better measure than serum phosphate concentration alone for the quantitative monitoring of the recovery of the renal tubular function in response to therapy (including growth hormone) in tubular damage due to metabolites, drugs or toxins. [4] The TmP/GFR is calculated by measuring phosphate and creatinine in paired collections of urine and blood and the protocol is available in any approved standard clinical biochemistry text book (available upon request). The authors have not provided the TmP/GFR results, a simple lab assay to confirm their presumed diagnosis of renal phosphate loss and for further management.

Secondly, measurement of either ionized calcium or the albumin corrected calcium might have thrown more clinically useful information on the exact functional status of calcium-parathyroid axis rather than simple serum calcium measurement in the absence of calcitriol (at least 25-hydroxy vitamin D or) result.

But saito et al. (2009) indicates the difficulty of proper differential diagnosis of hypophosphatemic rickets/osteomalacia by clinical analysis alone in their recent case report emphasizes the importance of genetic studies. [6]

References

1Sattur A, Naikmasur VG, Shrivastava R, Babshet M. Familial hypophosphatemic rickets, J Indian Soc Pedod Prev Dent 2010;28:302-6.
2Berndt T, Kumar R. Novel Mechanisms in the Regulation of Phosphorus Homeostasis. Physiology (Bethesda) 2008;24:17-25.
3Jonsson KB, Zahradnik R, Larsson T, White KE, Sugimoto T, Imanishi Y, et al. Fibroblast growth factor 23 in oncogenic osteomalacia and X-linked hypophosphatemia. N Engl J Med 2003;348:1656-63.
4 Payne RB. Renal tubular reabsorption of phosphate (TmP/GFR): Indications and interpretation. Ann Clin Biochem 1998;35:201-6.
5Weinstein RS, Harris RL. Hypercalcemic hyperparathyroidism and hypophosphatemic osteomalacia complicating neurofibromatosis. Calcif Tissue Int 1990;46:361-6.
6Saito T, Nishii Y, Yasuda T, Ito N, Suzuki H, Igarashi T, et al. Familial hypophosphatemic rickets caused by a large deletion in PHEX gene. Eur J Endocrinol 2009;161:647-51.