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Subject: Vitamin D, 1,25-Dihydroxy
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It is the active form of vitamin D and is produced primarily in the kidney by the hydroxylation of 25-hydroxyvitamin D. Other names: calcitriol and 1,25-dihydroxycholecalciferol (1,25-OHD).
Normal range: 15–75 pg/mL.
As a second-order test in the assessment of vitamin D status, especially in patients with renal disease
Investigation of some patients with clinical evidence of vitamin D deficiency (e.g., vitamin D–dependent rickets due to hereditary deficiency of renal 1-alpha hydroxylase or end-organ resistance to 1,25-dihydroxyvitamin D)
Differential diagnosis of hypercalcemia
Sarcoidosis (synthesized by macrophages within granulomas).
Non-Hodgkin lymphoma (approximately 15% of cases). Returns to normal after therapy.
Vitamin D–dependent rickets, types 1 and 2
Humoral hypercalcemia of malignancy
The level of 1,25-OHD is maintained despite significant vitamin D depletion, because secondary hyperparathyroidism stimulates increased conversion of 25-OHD to 1,25-OHD in this situation.
Although 1,25-OHD is the biologically active form of vitamin D, its level in the body provides no useful information about a patient's vitamin D status. The kidney tightly controls serum 1,25-OHD levels, which are often normal or even elevated in vitamin D deficiency. Therefore, a patient with normal or high levels of 1,25-OHD is vitamin D deficient despite high serum levels of the active hormone. At this time, there is consensus that serum 1,25-OHD is a measure of only the endocrine function of vitamin D and not an indicator of the body stores or the ability of vitamin D to perform its pleiotropic autocrine functions.