. 2018 Nov;33(6):453–462. doi: 10.5001/omj.2018.85

Table 1. Actions of hormones regulating calcium.^6,7.

Hormones/prohormone (function)	Organ (action on calcium deprivation)
PTH (PTH’s function is to control calcium concentration in the extracellular fluid using their membrane receptor, which acts as a calcium sensor). The ionized calcium in the extracellular fluid is the principal signal regulating PTH secretion. PTH secretion is stimulated as calcium level decreases. Calcium also regulates PTH gene activity.	Kidney - Rapid effect on kidney causing reabsorption of calcium and excretion of phosphorus to maintain calcium level and prevent adverse effects of elevated phosphate from bone resorption. Calcium loss through urine is reduced. Bones - Increased rate of calcium and phosphate release from the bones by PTH binding to the osteoblasts causing release of cytokines and increased osteoclast activity. There is net loss of bone and rise in blood calcium levels.
1,25-(OH)2 vitamin D (calcitriol) (Vitamin D must be metabolically activated by PTH stimulation before it can regulate calcium levels and influence skeletal remodelling. 1,25-(OH)2 vitamin D decreases PTH gene expression creating a negative feedback loop for the regulation of PTH. Activation through hydroxylation stimulated as calcium levels decrease due to increased PTH secretion). Calcitonin (which is produced by the parafollicular (or C) cells of the thyroid, has the opposite effect on blood calcium levels as PTH. Very low-level secretion when calcium levels are low).	Small intestine - 1,25-(OH)2 vitamin D promotes enhanced absorption of dietary calcium from the small intestine through the intestinal epithelial cell. Bones - 1,25-(OH)2 vitamin D activates osteoblasts/osteoclasts leading to calcium and phosphate release from the bone. Kidney - Renal excretion of calcium is decreased due to enhanced tubular reabsorption stimulated by elevated PTH and vitamin D. Although calcitonin has a pharmacologic role in calcium disorders, its physiologic role in calcium homeostasis is unclear. When serum calcium increases, calcitonin decreases blood calcium levels by inhibiting osteoclasts, stimulating osteoblasts, inhibiting bone resorption, and stimulating calcium excretion by the kidneys. It does not play a major role in humans at least in adults.

Hormones/prohormone (function)

Organ (action on calcium deprivation)

PTH (PTH’s function is to control calcium concentration in the extracellular fluid using their membrane receptor, which acts as a calcium sensor). The ionized calcium in the extracellular fluid is the principal signal regulating PTH secretion. PTH secretion is stimulated as calcium level decreases. Calcium also regulates PTH gene activity.

Kidney - Rapid effect on kidney causing reabsorption of calcium and excretion of phosphorus to maintain calcium level and prevent adverse effects of elevated phosphate from bone resorption. Calcium loss through urine is reduced.
Bones - Increased rate of calcium and phosphate release from the bones by PTH binding to the osteoblasts causing release of cytokines and increased osteoclast activity. There is net loss of bone and rise in blood calcium levels.

1,25-(OH)2 vitamin D (calcitriol) (Vitamin D must be metabolically activated by PTH stimulation before it can regulate calcium levels and influence skeletal remodelling. 1,25-(OH)2 vitamin D decreases PTH gene expression creating a negative feedback loop for the regulation of PTH. Activation through hydroxylation stimulated as calcium levels decrease due to increased PTH secretion).
Calcitonin (which is produced by the parafollicular (or C) cells of the thyroid, has the opposite effect on blood calcium levels as PTH. Very low-level secretion when calcium levels are low).

Small intestine - 1,25-(OH)2 vitamin D promotes enhanced absorption of dietary calcium from the small intestine through the intestinal epithelial cell.
Bones - 1,25-(OH)2 vitamin D activates osteoblasts/osteoclasts leading to calcium and phosphate release from the bone.
Kidney - Renal excretion of calcium is decreased due to enhanced tubular reabsorption stimulated by elevated PTH and vitamin D.
Although calcitonin has a pharmacologic role in calcium disorders, its physiologic role in calcium homeostasis is unclear. When serum calcium increases, calcitonin decreases blood calcium levels by inhibiting osteoclasts, stimulating osteoblasts, inhibiting bone resorption, and stimulating calcium excretion by the kidneys. It does not play a major role in humans at least in adults.

PTH: parathyroid hormone; 1,25-(OH)2 vitamin D: 1,25-dihydroxy vitamin D.

Table 1. Actions of hormones regulating calcium.6,7.

Table 1. Actions of hormones regulating calcium.^6,7.