Figure 6.

The regulation of mineral homeostasis by parathyroid hormone (PTH) and 1α,25(OH)₂D3. The physiological functions of PTH and 1α,25(OH)₂D3 are activated when serum calcium levels drop. The hormones act in conjunction with each other and exert coordinated effects on the kidneys, bones, and intestine to increase Ca²⁺ levels to normal. There is bone resorption, increased calcitriol formation by the kidneys and decreased calcium excretion from urine, and increased Ca²⁺ absorption by the intestine. Upon achievement of homeostasis, the process is shut down by a negative feedback loop, which is initiated by calcitonin secreted by the thyroid gland. Thus, the combined effect of PTH and 1α,25(OH)₂D3 is necessary to maintain mineral homeostasis.