ABSTRACT
Plasma calcium is sensed by a calcium-sensitive receptor (CaSR) in parathyroid and renal tubular cells, and maintained within a narrow physiological range by parathyroid hormone (PTH), vitamin D and calcitonin. The plasma calcium concentration is the major factor controlling PTH secretion and a reduction in calcium concentration stimulates PTH release. PTH raises plasma calcium and lowers phosphate concentration. It acts on kidney and bone. PTH causes phosphaturia and increases renal tubular reabsorption of calcium, which in association with mobilization of calcium from bone, increases the plasma calcium concentration. Effects of PTH on bone include stimulation of osteoclast activity, formation of new osteoclasts from progenitor cells and transient depression of osteoblast activity. PTH also plays a role in the regulation of vitamin D metabolism, indirectly increasing gut absorption of calcium by stimulating production of the active metabolite 1,25dihydroxycholecalciferol (1,25-DHCC or calcitriol, see below).
