Dr Toni Hazell provides top tips on assessing, diagnosing, and treating vitamin D deficiency in primary care

Dr Toni Hazell

Dr Toni Hazell

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Read this article to learn more about

  • the difference between vitamin D insufficiency and deficiency
  • when a vitamin D test is required
  • pharmacological and lifestyle management.

Vitamin D is the most common nutritional deficiency worldwide. Around one-quarter of the UK population is deficient in vitamin D, rising to around one-third in the winter months.1 The main source of vitamin D is from sunlight. It is also present in specific foods (e.g. cod liver oil, oily fish, egg yolk, milk, mushrooms), but it is very difficult to get enough vitamin D from diet alone. Vitamin D promotes absorption of calcium from the gastrointestinal tract and helps with bone mineralisation. The main consequence of vitamin D deficiency in adults is osteomalacia, where bone is broken down by osteoclasts to increase serum calcium. Various claims have been made for the importance of vitamin D in chronic fatigue syndrome, reproductive health, various cancers, and macular degeneration, but there is limited evidence to support these claims.

1. Know the causes and risk factors

Sunlight is important, but do not forget about other causes and risk factors. Vitamin D obtained from diet or sunlight is inactive, and undergoes conversion in both the liver and the kidney to form the active metabolite, 1,25-dihydroxyvitamin D. Renal conversion is promoted by parathyroid hormone. Knowing about conversion makes it easy to remember the following categories of risk factors for vitamin D deficiency (see Box 1).

Box 1: Risk factors for vitamin D deficiency

Reduced intake due to diet or lack of sun exposure

  • restricted diet (e.g. vegan)
  • malabsorption (e.g. coeliac, post bariatric surgery)
  • reduced sun exposure (e.g. rarely leaving the house or covering skin when outside)
  • age over 65 years (the amount of substrate in the skin from which vitamin D is synthesised reduces with age)
  • darker skin (melanin absorbs some ultraviolet B light so people with dark skin need more sunlight than people with lighter skin to produce the same amount of vitamin D)

Reduced liver conversion

  • chronic liver disease
  • induction of liver enzymes

Reduced renal conversion

  • chronic renal failure
  • nephrotic syndrome
  • reduced parathyroid hormone

Increased demand of vitamin D

  • pregnancy
  • breastfeeding.

2. Recommend vitamin D supplementation

Public Health England (PHE) has advised that nearly everyone in the UK should take a 10 mcg (400 IU) supplement of vitamin D in the autumn and winter months,2,3 as there is not enough sunlight at this time of year for sufficient production of vitamin D. The only exception is babies under the age of 1 year who are consuming at least 500 ml of formula daily, because formula is fortified with vitamin D. Babies under the age of 1 year who are breastfed should consume a 8.5–10 mcg supplement.3 People who have very little exposure to the sun (e.g. people in care homes or who cover all of their skin when outside) should take a supplement all year round.3

3. Do not offer routine tests

Most people do not need a vitamin D test. NICE recommends that practitioners should not routinely test for vitamin D levels. Tests should only be performed if there is an indication to do so, including (in adults aged over 18 years):1

  • a suspicion of osteomalacia (bony pain, muscle aches and weakness, impaired physical function, waddling gait, symmetric lower back pain)
  • chronic widespread pain
  • the individual has had a fall
  • symptoms of hypocalcaemia (e.g. muscle cramps, numbness)
  • a bone disease that may be improved with vitamin D treatment (e.g. osteoporosis, Paget’s disease).

Indications to test children include:4

  • a suspicion of rickets (bow legs, knock knees, delayed tooth eruption, painful wrist swelling)
  • unexplained bony pain or muscular weakness
  • a chronic disease that may increase the risk of vitamin D deficiency (e.g. anything that causes malabsorption).

4. Recognise deficiency from insufficiency

NICE acknowledges that there is no clear consensus on when to diagnose vitamin D deficiency or insufficiency, and thresholds may vary between laboratories; however, the National Osteoporosis Society (NOS),5 NICE1 and the Institute of Medicine6 have agreed reasonable thresholds for defining vitamin D status (see Table 1).

Table 1: Vitamin D thresholds1,5,6

Serum 25OHD levels (nmol/l)

Vitamin D status




Insufficient (may be inadequate in some people)



The Scientific Advisory Committee on Nutrition, which informed the PHE report on vitamin D supplementation,2 advises that the risk of skeletal ill-health increases with a vitamin D level less than 25 nmol/l,3 but this variance is unlikely to make a huge difference to the management of vitamin D deficiency in the real world.

5. Prescribe a loading dose to treat vitamin D deficiency

People with vitamin D deficiency should be prescribed a loading dose of approximately 300,000 IU vitamin D.1 This can be achieved in several ways, including:1

  • 50,000 IU once a week for 6 weeks
  • 20,000 IU twice a week for 7 weeks
  • 4000 IU daily for 10 weeks.

Note that these high doses are not available over the counter and need to be prescribed.

6. Repeat blood tests after initial treatment

Calcium levels should be checked 1 month after the end of the treatment course. Hypercalcaemia should raise suspicion that the patient has primary hyperparathyroidism; the patient should not take any more vitamin D and appropriate investigations should be instigated.

The frequency of repeat vitamin D testing depends on which guideline is being followed:

  • NOS recommends routine monitoring is unnecessary unless the patient has malabsorption, remains symptomatic, or there is a likelihood of poor adherence5
  • NICE recommends re-checking vitamin D levels within 3–6 months of a loading dose (no sooner as it takes at least 3 months for the vitamin D level to stabilise).1 If vitamin D levels do not rise despite good adherence, or the patient remains symptomatic despite normalisation of vitamin D, then consideration should be given to other diagnoses and referral may be indicated.1

It is up to the individual GP to decide which guideline they need to follow; however, it is clear that regular ongoing monitoring is not needed.

7. Direct patients to over-the-counter supplements

Most people can buy vitamin D supplements over the counter and do not require routine monitoring. People who have vitamin D insufficiency, and people with deficiency who have completed their loading dose, should take a maintenance dose of 800 IU per day (or 2000 IU, occasionally up to 4000 IU, for people with malabsorption disorders).1 Vitamin D is very cheap to buy over the counter and some CCGs recommend that healthcare professionals should refrain from prescribing vitamin D maintenance preparations, and encourage patients to buy their dose instead.7 For the children of families on a low income, Healthy Start vouchers (see: www.healthystart.nhs.uk) provide a means of acquiring vitamin D for free, without the need for a prescription.

Patients should be advised that the maintenance dose should be taken for life.1 Regular testing of people on a maintenance dose of vitamin D is not necessary.

8. Remind patients about sunscreen

Exposure to the sun is a significant risk factor for melanoma, particularly if the patient has been sunburnt in the past, and there have been large public health campaigns to increase awareness of the need for sunscreen.8 This should not be forgotten in the patient’s efforts to increase exposure to the sun to get more vitamin D. Patients should be advised that sufficient vitamin D can be obtained by spending short periods of time in the sun without sunscreen during the summer months;9 however, sunscreen is still advised for prolonged exposure. People who are unable to expose their skin to the sun should be taking supplements all year round. Sunbeds increase the risk of developing melanoma by around 20% and should therefore not be used.10

9. Be aware of the symptoms of vitamin D toxicity

Vitamin D toxicity is rare and is usually only seen in people who are taking very high doses for prolonged periods, but it is important that healthcare professionals are able to recognise the symptoms. Symptoms include hypercalcaemia (e.g. nausea, vomiting, altered bowel habit, weight loss, thirst, headache). Vitamin D toxicity is associated with serum levels of at least 300 nmol/l, and usually above 600 nmol/l; it is very unlikely that a 300,000 IU loading dose, or subsequent maintenance dose would cause toxicity.3

10. Do not forget lifestyle advice

So far this article has concentrated on how GPs can treat patients with vitamin D deficiency or insufficiency, but many patients are willing to help themselves by making lifestyle changes. Patients should be advised to optimise their dietary intake of vitamin D and calcium—downloadable information sheets (such as the British Dietetic Association’s Food Fact sheet on vitamin D, see: www.bda.uk.com/foodfacts/VitaminD.pdf) can be helpful.11 Obesity is a risk factor for vitamin D deficiency, possibly because vitamin D (which is fat-soluble) is sequestered in adipose tissue and so is less bioavailable. It would therefore be reasonable to advise weight loss, particularly in people with a body mass index of over 30 kg/m2.

Dr Toni Hazell

Part-time GP, Greater London


  1. NICE. Vitamin D deficiency in adults—treatment and prevention. NICE Clinical Knowledge Summary. NICE, 2016. cks.nice.org.uk/vitamin-d-deficiency-in-adults-treatment-and-prevention (accessed 11 April 2018).
  2. Public Health England (PHE) press office. PHE publishes new advice on vitamin D. PHE, 2016. gov.uk/government/news/phe-publishes-new-advice-on-vitamin-d (accessed 11 April 2018).
  3. Powers H, Cashman K, Francis R, the Scientific Advisory Commission on Nutrition (SACN) Working Group on Vitamin D. Vitamin D and health. SACN, 2016. Available at: www.gov.uk/government/uploads/system/uploads/attachment_data/file/537616/SACN_Vitamin_D_and_Health_report.pdf
  4. NICE. Vitamin D deficiency in children. NICE Clinical Knowledge Summary. NICE, 2016. cks.nice.org.uk/vitamin-d-deficiency-in-children#!diagnosissub (accessed 11 April 2018).
  5. Francis R, Aspray T, Fraser W et al. Vitamin D and bone health: a practical clinical guideline for patient management. Bath: National Osteoporosis Society, 2013. Available at: nos.org.uk/media/2073/vitamin-d-and-bone-health-adults.pdf
  6. Dawson-Hughes B, Cooper C. IOF statement on new IOM dietary reference intakes for calcium and vitamin D. Switzerland: International Osteoporosis Foundation, 2010. iofbonehealth.org/iof-statement-new-iom-dietary-reference-intakes-calcium-and-vitamin-d (accessed 11 April 2018).
  7. NHS Hertfordshire. Guidance on vitamin D deficiency/insufficiency—November 2011 (updated February 2012). NHS Hertfordshire, 2012. Available at: psnc.org.uk/kentlpc/wp-content/uploads/sites/106/2016/03/Herts-Guidance-on-Vitamin-D-Deficiency-201111HMMC-Updated-Feb-2012-Long.pdf
  8. Cancer Research UK (CRUK). Let’s stay safe in the sun. CRUK, 2017. Available at: publications.cancerresearchuk.org/sites/default/files/publication-files/CRUK%20-%20Cut%20Your%20Cancer%20Risk%20-%20Sunsmart%20Leaflet.pdf
  9. NHS Choices. How to get vitamin D from sunlight. NHS Choices, 2015. nhs.uk/Livewell/Summerhealth/Pages/vitamin-D-sunlight.aspx (accessed 11 April 2018).
  10. Cancer Research. Sunbeds and cancer. Available at: cancerresearchuk.org/about-cancer/causes-of-cancer/sun-uv-and-cancer/sunbeds-and-cancer (accessed 11 April 2018).
  11. British Dietetic Association (BDA). Food fact sheet—vitamin D. BDA, 2016. Available at: www.bda.uk.com/foodfacts/VitaminD.pdf