Vitamin D (25-hydroxy)
Investigation : Vitamin D (25-hydroxy)
Specimen type : Serum
Spec container : Serum Gel 
Volume required : 5 ml
Reference range : see below for interpretation
Turnaround : <5 days

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

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 nameICE page nameColumnComment\Advice
Chem / Haem
Nutrit & Metab


Laboratory Services Home Page