Chapter 58 - Calcium and Phosphate Physiology

Abstract

In healthy subjects, the calcium and phosphate balance may be positive, normal, or negative in the absence of any overt abnormality in either serum calcium or phosphate concentration. Therefore simply measuring serum calcium and phosphate concentrations is of poor help in predicting calcium and phosphate balance. Anyway, if treatment is initiated assuming an initial condition of low serum calcium or high serum phosphate levels, a subsequent increase in parathyroid hormone (PTH) synthesis and secretion, accompanied by a rapid parathyroid gland hyperplasia, occurs. Target tissues for PTH are bone, kidney, and gut. The effects on bone are to enhance bone resorption to increase serum calcium and phosphate levels. The effects on kidneys are to increase calcium reabsorption but produce phosphate excretion, with an enhancement in active vitamin D. Active vitamin D increases calcium and phosphate reabsorption from the gut. Finally, higher calcium levels suppress PTH secretion through negative feedback.