Which organ converts 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D?

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Multiple Choice

Which organ converts 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D?

Explanation:
The kidney is where 25-hydroxyvitamin D is converted to the active form, 1,25-dihydroxyvitamin D. In the proximal tubule cells, the enzyme 1-alpha-hydroxylase adds a hydroxyl group to produce calcitriol, the hormonally active vitamin D. This step is carefully regulated: parathyroid hormone stimulates it when calcium is low, while factors like high phosphate and fibroblast growth factor 23 inhibit it. The liver handles the initial 25-hydroxylation to form 25-hydroxyvitamin D, skin makes vitamin D3 with UV exposure, and the parathyroid gland doesn’t perform this conversion. Thus, the organ responsible for this activation is the kidney.

The kidney is where 25-hydroxyvitamin D is converted to the active form, 1,25-dihydroxyvitamin D. In the proximal tubule cells, the enzyme 1-alpha-hydroxylase adds a hydroxyl group to produce calcitriol, the hormonally active vitamin D. This step is carefully regulated: parathyroid hormone stimulates it when calcium is low, while factors like high phosphate and fibroblast growth factor 23 inhibit it. The liver handles the initial 25-hydroxylation to form 25-hydroxyvitamin D, skin makes vitamin D3 with UV exposure, and the parathyroid gland doesn’t perform this conversion. Thus, the organ responsible for this activation is the kidney.

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