Vitamin D (25-hydroxyvitamin D) deficiency is defined as a serum level less than:

Vitamin D (25-hydroxyvitamin D) deficiency is defined as a serum level less than:

• A. 100 ng/mL
• B. 50 ng/mL
• C. 120 ng/mL
• D. 20 ng/mL

Answer: (D)

Understanding vitamin D status relies on the serum level of 25-hydroxyvitamin D as the best overall indicator of total vitamin D stores. Clinically, deficiency is defined as a level below twenty nanograms per milliliter, which reflects insufficient substrate for optimal calcium absorption and bone mineralization. When this level falls, parathyroid hormone often rises to maintain calcium homeostasis, leading to potential bone demineralization and higher risk of osteomalacia in adults and rickets in children. Levels above this threshold but below about thirty nanograms per milliliter are commonly labeled insufficiency, while generally higher levels are considered sufficient for bone health. Twenty nanograms per milliliter is equivalent to about fifty nanomoles per liter, and some guidelines use slightly different cutoffs, but this threshold is widely used in nutrition and clinical practice.

Understanding vitamin D status relies on the serum level of 25-hydroxyvitamin D as the best overall indicator of total vitamin D stores. Clinically, deficiency is defined as a level below twenty nanograms per milliliter, which reflects insufficient substrate for optimal calcium absorption and bone mineralization. When this level falls, parathyroid hormone often rises to maintain calcium homeostasis, leading to potential bone demineralization and higher risk of osteomalacia in adults and rickets in children. Levels above this threshold but below about thirty nanograms per milliliter are commonly labeled insufficiency, while generally higher levels are considered sufficient for bone health. Twenty nanograms per milliliter is equivalent to about fifty nanomoles per liter, and some guidelines use slightly different cutoffs, but this threshold is widely used in nutrition and clinical practice.