Different Forms of Vitamin D and How To Diagnose Vitamin D
Vitamin D itself is inactive and needs to get converted to the liver to 25-hydroxy vitamin D (25-OH vitamin D) and then in the kidney to 1, 25-hydroxy vitamin D. It is only the 1, 25- OH vitamin D which is biologically active. This form of vitamin D acts to allow for absorption of calcium from the intestinal tract. Therefore, patients with low vitamin D levels will have low calcium and in severe cases get rickets (in children) or osteomalacia (in adults) which is when the bone bows out and is poorly formed. In mild cases of vitamin D deficiency, osteoporosis occurs, but low calcium is rare.
The conversion from the 25-OH vitamin D to the 1, 25-OH vitamin D that occurs in the kidney is catalyzed by parathyroid hormone, also called PTH. Therefore, patients with very low vitamin D levels will have relatively high PTH levels often with low calcium levels.
This is similar to patients with primary hypothyroidism having elevated TSH levels while having normal thyroid hormone levels. Additionally, the 25-OH vitamin D form which is the storage form and is much more abundant that the 1, 25-OH vitamin D form which, although is active, is less abundant. Therefore, in states of vitamin D deficiency, low levels of 25-OH vitamin D are found, but the 1, 25-OH vitamin D levels are either normal or actually slightly high. They are slightly high because the excess PTH that is stimulated by the low 25-OH vitamin D levels stimulates the conversion up to 25-OH vitamin D to the 1, 25-OH vitamin D. Thus, patients that are vitamin D deficient usually have a low 25-OH vitamin D level, a high PTH level, a low normal calcium, and a normal or an elevated 1, 25- OH vitamin D level. However some patients may actually have a high normal calcium as
the elevated PTH and 1, 25-OH vitamin D may cause increased calcium absorption from the GI track and reabsorption from the kidney. If 25-OH vitamin D levels were not measured, these patients might have been incorrectly diagnosed with mild hyperparathyroidism as they have a high normal calcium and a high PTH.