Filtered freely at glomerulus
80% reabsorbed in proximal tubule
Distal tubules 10%
2-3% collecting ducts

regulation

PTH - ↑ excretion/ ↓ reabsorption at proximal tubule
vitamin D - ↑ reabsorption in distal tubule
acidosis, glucocorticoids, calcitonin - ↑ excretion