Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1297 page)

   Vitamin D–dependent rickets, types 1 and 2

Normal In

   HPT

   Humoral hypercalcemia of malignancy

   Limitations

   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.

VITAMIN D, 25-HYDROXY

   Other names: 25-hydroxy D2; 25-hydroxy D3; 5-hydroxy vitamin D; 25-hydroxycholecalciferol; 25-hydroxyergocalciferol; 25-OH vitamin D; calcidiol.

   Definition

   A steroid hormone that has long been known for its important role in regulating body levels of calcium and phosphorus and in the mineralization of bone. The term “vitamin D” specifically refers to two biologically inert precursors, vitamin D
₃
(cholecalciferol) or D
₂
(ergocalciferol). Neither vitamin D
₃
nor vitamin D
₂
has significant biologic activity; rather they must be metabolized within the body to the hormonally active form. Vitamin D
₃
is generated in the skin when light energy is absorbed (UV radiation in the UVB spectrum 290–320 nm) by a precursor molecule 7-dehydrocholesterol (7-DHC; provitamin D
₃
). However, cutaneous vitamin D
₃
production after single prolonged UVB exposure is capped at approximately 10–20% of the original epidermal 7-DHC concentration, a limit achieved with suberythemogenic UV exposures. Vitamin D
₂
is plant derived, produced exogenously by irradiation of ergosterol, and enters the circulation through diet. Vitamin D
₃
from the skin and vitamin D
₃
and D
₂
from the diet enter the blood and are metabolized to their 25-hydroxy counterparts. Once formed, 25-hydroxyvitamin D (25-OHD) is metabolized in the kidney to 1,25-dihydroxyvitamin D (1,25-OHD).

   
Normal range:
see Table 16.84.

TABLE 16–84. Normal Ranges of 25-OH Vitamin D

   Use