The kidney is the primary site for the final activation step of vitamin D to 1,25-dihydroxyvitamin D.

The kidney is the primary site for the final activation step of vitamin D to 1,25-dihydroxyvitamin D.

• A. True
• B. False
• C. Not sure
• D. It is the skin.

Answer: (A)

The final activation of vitamin D to its hormonally active form happens mainly in the kidney. Vitamin D obtained from skin exposure to UV light or from diet is first hydroxylated in the liver to 25-hydroxyvitamin D, which is then transported to the kidney. In the proximal tubule, the enzyme 1-alpha-hydroxylase converts 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D (calcitriol), the form that acts as a hormone to regulate calcium and phosphate metabolism. This step is tightly regulated by parathyroid hormone and other factors, and is the step most responsible for the systemic activation of vitamin D. The skin is where vitamin D synthesis begins, but the final activation to calcitriol does not occur there. Therefore the statement is true.

The final activation of vitamin D to its hormonally active form happens mainly in the kidney. Vitamin D obtained from skin exposure to UV light or from diet is first hydroxylated in the liver to 25-hydroxyvitamin D, which is then transported to the kidney. In the proximal tubule, the enzyme 1-alpha-hydroxylase converts 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D (calcitriol), the form that acts as a hormone to regulate calcium and phosphate metabolism. This step is tightly regulated by parathyroid hormone and other factors, and is the step most responsible for the systemic activation of vitamin D. The skin is where vitamin D synthesis begins, but the final activation to calcitriol does not occur there. Therefore the statement is true.