Clinical use: Vitamin D is a secosteroid that is made in the skin when 7-dehydrocholesterol is converted to previtamin D₃ during exposure to ultraviolet B radiation from sunlight. Vitamin D can also be ingested in the diet as vitamin D₂ (ergocalciferol) or vitamin D₃ (cholecalciferol). Although synthesis by the skin is the more important source of vitamin D, the normally low dietary intake of vitamin D₂ or vitamin D₃ maybe critical when exposure to sunlight is lacking. Once vitamin D₂ or vitamin D₃ enters the circulation, it is transported to the liver. In the liver both Vitamin D₂ and vitamin D₃ are hydroxylated to give 25-hydroxyvitamin D₂ (25-(OH)D₂) or 25-hydroxyvitamin D₃ (25-(OH)D₃) which is the main circulating form of vitamin D. The hepatic vitamin D 25-hydroxylase is not tightly regulated thus 25-OHD levels will reflect an increased/decreased cutaneous production or ingestion of vitamin D. Although 25-OHD is much less biologically active than 1,25(OH)₂D, the measurement of circulating 25-OHD provides the best information to determine if a patient is vitamin D deficient, vitamin D insufficient, vitamin D sufficient or vitamin D intoxicated. There are several reasons for this including the increased half-life of 25-OHD, thus it provides an indication of vitamin D stores obtained from UV light and also dietary intake over long periods. The half-life of 25-OHD is approximately 3 weeks, this is in contrast to the half-life of the parent vitamin D which is 24 hours and the half-life of 1,25(OH)₂D which is of the order 4-6 hours. In addition plasma concentrations of 1,25(OH)₂D but not 25-OHD are maintained normal or even elevated in mild to moderate osteomalacia due to secondary hyperparathyroidism.

Patient preparation: None

Sample requirements: Serum preferred although lithium heparin plasma may be used. For external users: Sample requirement: 0.5 mL of serum.

Clinical interpretation:

Total 25-OH vitamin D results are reported only and not 25 (OH) vitamin D2 and 25 (OH) vitamin D3

• 25-OH Vitamin D <25 nmol/L is deficient
• 25-OH Vitamin D 25 - 50 nmol/L is insufficient
• 25-OH Vitamin D >50 nmol/L is sufficient in the absence of bone loss
• 25-OH Vitamin D >150 nmol/L ? Over replaced, may need a reduction in dose.

Vitamin D levels of up to 500 nmol/L are safe in the short term but the target range for long term maintenance is 50-100 nmol/L in the general population and 75-125 nmol/L in patients with bone loss. When a maintenance dose of 300,000 IU is given over a short period, vitamin D levels overshoot the target temporarily and gradually fall to reach a steady state after about 4 to 5 months - so the usual cause for high levels of vitamin D is re-measuring too soon after a maintenance dose.

If the vitamin D is >150 nmol/L in a patient on a maintenance dose for >6 months, either reduce the frequency of the maintenance dose or stop for a few months depending on how high the result.

Please see link for PAN Mersey Cluster Guidance for Treatment of Vitamin D Deficiency in Adults Link.

Additional information on 25 hydroxyvitamin D and its clinical use can be found here: 25 hydroxyvitamin D.

ICE requesting info:	ICE panel name	ICE page name	Column	Comment\Advice
	Chem / Haem	Nutrit & Metab	1

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