Calcium metabolism

3 tissues : bone, kidney and intestine

3 hormones: PTH, calcitonin, and vit. D

The two hormones which primarily control calcium metabolism are:

• parathyroid hormone (PTH)
• 1,25-dihydroxycholecalciferol (calcitriol, the active form of vitamin D)

Other hormones include

• calcitonin: secreted from the parafollicular cells (C-cells) of the thyroid gland
• thyroxine
• growth hormone

Actions of parathyroid hormone

• increases plasma calcium, decreases plasma phosphate
• increases renal tubular reabsorption of calcium
• increases osteoclastic activity in bones
• increases renal conversion of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol
• decreases renal phosphate reabsorption

Actions of 1,25-dihydroxycholecalciferol

• increases plasma calcium and plasma phosphate
• increases renal tubular reabsorption and gut absorption of calcium
• increases osteoclastic activity