Phosphate homeostasis and endocrine regulators.

Phosphate homeostasis is essential for skeletal integrity, cellular metabolism, and endocrine regulation. In humans, circulating phosphate is typically maintained between 2.5 and 4.5 mg/dL by coordinated processes that balance gastrointestinal uptake from the diet, storage in cells and bone matrix, and renal reclamation that regulates excretion. Dietary phosphate enters primarily through the small intestine via a low‑affinity paracellular diffusion and a high‑affinity, sodium‑coupled transcellular route mediated by transporters such as NPT2b, PIT1, and PIT2. After absorption, phosphate is incorporated into hydroxyapatite within the skeleton and continuously shuttled between extra‑ and intracellular compartments mediated by transporters such as PIT1, PIT2, and XPR1 as metabolic demands change. At the kidney, proximal tubular transporters-especially NPT2a and NPT2c-set the tone for systemic phosphate balance. Endocrine regulators, including parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23), and 1,25‑dihydroxyvitamin D (calcitriol), adjust intestinal uptake, skeletal turnover, and renal handling to avert hyper‑ or hypophosphatemia. Beyond these hormones, phosphate appears to signal directly to cells, engaging pathways such as ERK1/2 and thereby modulating secretion of PTH and FGF23. Perturbations in these circuits manifest as rickets, osteomalacia, and vascular calcification. Understanding the interplay between transport systems, hormonal control, and cellular phosphate sensing is crucial for preventing and treating phosphate‑related disease.