Which lab finding would differentiate Vitamin D dependent rickets type 2 from vitamin D deficiency rickets?

Which lab finding would differentiate Vitamin D dependent rickets type 2 from vitamin D deficiency rickets?

• A. Elevated 1,25(OH) vitamin D levels in VDDR type 2
• B. Low 1,25(OH) vitamin D levels in VDDR type 2
• C. Elevated calcium in VDDR type 2
• D. Decreased PTH in VDDR type 2

Answer: (A)

End-organ resistance to active vitamin D is the key idea. In vitamin D dependent rickets type 2, mutations in the vitamin D receptor mean tissues don’t respond to calcitriol (1,25-dihydroxy vitamin D). The body senses low calcium and raises PTH, which drives the kidney to convert more 25(OH)D to calcitriol. Even though calcitriol levels rise, the receptors can’t respond, so calcium and phosphate absorption remain poor and rickets develops. That’s why the lab finding you’d see is elevated 1,25(OH)2D despite the deficiency state. In vitamin D deficiency rickets, calcitriol is low or inappropriately normal because there isn’t enough substrate and production is limited. So a high 1,25(OH)2D level helps differentiate VDDR type 2 from vitamin D deficiency rickets.

End-organ resistance to active vitamin D is the key idea. In vitamin D dependent rickets type 2, mutations in the vitamin D receptor mean tissues don’t respond to calcitriol (1,25-dihydroxy vitamin D). The body senses low calcium and raises PTH, which drives the kidney to convert more 25(OH)D to calcitriol. Even though calcitriol levels rise, the receptors can’t respond, so calcium and phosphate absorption remain poor and rickets develops. That’s why the lab finding you’d see is elevated 1,25(OH)2D despite the deficiency state. In vitamin D deficiency rickets, calcitriol is low or inappropriately normal because there isn’t enough substrate and production is limited. So a high 1,25(OH)2D level helps differentiate VDDR type 2 from vitamin D deficiency rickets.