Final answer:
The formation of 1,25-dihydroxyvitamin D in the kidney is stimulated by parathyroid hormone (PTH), not by calcitonin or left unregulated.
Step-by-step explanation:
The formation of 1,25-dihydroxyvitamin D in the kidney is stimulated by parathyroid hormone (PTH). When calcium levels in the blood are low, PTH is secreted, which then stimulates the transformation of vitamin D into its active form, calcitriol (1,25-dihydroxycholecalciferol), in the kidneys. PTH's role is to increase the concentration of blood calcium by promoting the release of calcium from bones, the reabsorption of calcium in the kidneys, and enhancing the digestion system's absorption of calcium through the synthesis and secretion of calcitriol. Calcitonin, on the other hand, has functions that generally oppose those of PTH, thus its role is not in stimulating the production of calcitriol.