1. A simplified schema for the regulation of serum phosphate by PTH, 1,25(OH)2D and fibroblast growth factor 23 (FGF23). FGF23 is produced predominantly by osteocytes buried deep in the bone matrix. Its actions are mediated by FGF receptors, differing in their expression in different tissues, and by the membrane co-receptor molecule alpha klotho. The alpha klotho/FGF receptor complex mediates high-affinity activation of signal transduction by FGF23 in key target tissues. FGF23 has potent effects on calcium and phosphate homeostasis as shown. FGF23 reduces 1,25(OH)2D production by reducing 1-alpha hydroxylase activity and expression and increases its catabolism by enhancing 24-hydroxylase activity and expression. Together this reduces levels of 1,25(OH)2D, which has downstream effects to reduce intestinal calcium and phosphate absorption. FGF23 also reduces the expression of key phosphate transporters in the kidney (NaPi2a and NaPi2c) thereby facilitating the excretion of phosphate. The reciprocal interaction between PTH and FGF23 is unclear and may depend on the physiologic circumstances, but PTH independently also stimulates phosphate excretion via the same NaPi transporters. The combined actions of FGF23 lower serum phosphate concentrations. Not displayed here are the negative feedback control mechanisms including: the ability of 1,25(OH)2D and phosphate to stimulate FGF23 production in bone and the suppression of PTH secretion by 1,25(OH)2D. Excessive FGF23 production and/or action (hypophosphatemia, chronic phosphate wasting) can result in abnormal bone matrix mineralization, causing rickets or osteomalacia.
Source: Metabolic Bone Disease, Greenspan's Basic & Clinical Endocrinology, 10e
Citation: Gardner DG, Shoback D. Greenspan's Basic & Clinical Endocrinology, 10e; 2017 Available at: Accessed: October 12, 2017
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