Vitamin D Deficiency and Related Disorders Workup

Updated: Sep 08, 2022
  • Author: Vin Tangpricha, MD, PhD; Chief Editor: Romesh Khardori, MD, PhD, FACP  more...
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Approach Considerations

Serum 25(OH)D

This is the best test to determine vitamin D status (see Vitamin D3 25-Hydroxyvitamin D). The circulating half-life of 25(OH)D is 2 weeks. A 25(OH)D level of less than 30 ng/mL is considered vitamin D insufficient. [1] A 25(OH)D level of less than 15 or 20 ng/mL have been used to define vitamin D deficiency. Intestinal calcium absorption is optimized at levels above 30-32 ng/mL. [8, 39] Parathyroid hormone levels start to rise at 25(OH)D levels below 31 ng/mL, [71] which is another marker of vitamin D insufficiency. [72]

Parathyroid hormone

Although not always required for the diagnosis of vitamin D insufficiency, a serum PTH may be used to help establish the diagnosis of vitamin D insufficiency. Often, patients with vitamin D insufficiency have a corresponding elevated PTH, indicating secondary hyperparathyroidism. An inverse relationship exists between PTH and 25(OH)D levels. [71] Usually, PTH levels decrease after the correction of a vitamin D insufficiency.