The Parathyroid Glands
Parathyroid hormone (PTH) is the principal regu-lator of calcium homeostasis. It increases serum calcium concentrations by promoting resorption of bone and teeth, limiting renal excretion of calcium, and indirectly enhancing gastrointestinal absorp-tion by its effect on vitamin D metabolism. PTH decreases serum phosphate by increasing renal excretion. The effects of PTH on calcium serum levels are countered in lower animals by calcito-nin, a hormone excreted by parafollicular C-cells in the thyroid, but a physiological calcium-lowering effect for calcitonin has not been demonstrated in humans (Table 34–6). Of total body calcium, 99% is in the skeleton. Of the calcium in the blood, 40% is bound to proteins and 60% is ionized or complexed to organic ions. Unbound ionized calcium is physi-ologically the most important fraction.