Vitamin D is synthesized from cholesterol by the action of sunlight on the skin. It is also found in oily fish, in some plants and may be added to some foodstuffs e.g. breakfast cereals. In the northern hemisphere low sunlight levels, especially in winter, make it difficult to synthesize sufficient vitamin D, and deficiency is common. Those whose exposure to sunlight is limited and those with darker skin are at increased risk of deficiency. Recommended use of sunscreen is a further barrier to vitamin D synthesis. Current advice is for adults and children over 1 year to consider taking 10microg (400 IU) of vitamin D daily in order to protect their musculoskeletal health.

Vitamin D deficiency causes osteomalacia in adults. Lesser deficiency may be symptomless, but diverse health problems ranging from asthma to heart disease, from autoimmune diseases to cancers have been associated with low levels of vitamin D though causal links are yet to be established.

Vitamin D toxicity is serious but rare. Hypercalcaemia, demineralisation of bone, soft tissue calcification and renal damage are unlikely to be caused by over the counter supplements taken as advised.

Measuring vitamin D is often unnecessary dependent upon patient symptoms and other abnormal biochemistry. Deficient patients may have low serum calcium though secondary hyperparathyroidism may sustain calcium within the normal range. Serum phosphate tends to decrease and alkaline phosphatase to increase.

Clinical situations in which vitamin D requesting is required are:

• Investigation of primary hyperparathyroidism
• Prior to bisphosphonate therapy
• Symptomatic hypocalcaemia/osteomalacia
• Failure to respond to Vitamin D3 replacement
• Patients with complex nutritional needs e.g. long term TPN, following bariatric surgery malabsorptive conditions etc.

Widespread vitamin D requesting is not warranted and for most patients suspected of having, or who are at risk of vitamin D deficiency, a trial of over-the-counter vitamin D is appropriate with further investigations only required for those who fail to respond (clinically or biochemically) after 10-12 weeks.

It is unnecessary to measure Vitamin D after initiation of treatment unless symptoms persist. For those with e.g. complex nutritional requirements, annual testing is recommended.

1. The Scottish Government [Internet]. Scotish Government. Vitamin D: advice for all age groups; 2022 Feb 20 [cited 2022 Oct 04]. Available from: https://www.gov.scot/publications/vitamin-d-advice-for-all-age-groups/
2. Francis R, Aspray T, Fraser W, Macdonald H, Patel S, Mavroeidi A, et al. Vitamin D and Bone Health: A Practical Clinical Guideline for Patient Management [Internet]. 2nd ed. Royal Osteoporosis Society; 2018 Dec [cited 2022 Oct 04]. Available from: https://theros.org.uk/media/ef2ideu2/ros-vitamin-d-and-bone-health-in-adults-february-2020.pdf
3. Vitamin D supplements and tests [Internet]. Royal Osteoporosis Society; 2022 Jan [cited 2022 Oct 04]. Available from: https://strwebprdmedia.blob.core.windows.net/media/grija5r1/ros-vitamin-d-supplements-and-tests.pdf