Vitamin D is called a vitamin, but in your body it works like a hormone. Your skin makes it from sunlight. Your liver and kidneys then turn it into its active form, which works inside your cells by switching genes on and off. Thyroid and sex hormones work in the same way. This is why we look after vitamin D in our hormone service.¹

 

How your body makes it, in three steps

 

1. Skin. Sunlight turns a substance in your skin into vitamin D3. Long hours in the sun do not lead to too much vitamin D, because extra sunlight breaks down the excess.
2. Liver. Your liver changes vitamin D into a storage form called 25-OH vitamin D. This is the form we measure in your blood.
3. Kidneys. Your kidneys change the storage form into the active hormone, as your body needs it.¹

 

D3 or D2?

 

Vitamin D comes in two forms. Vitamin D3 is the form your skin makes, and it is also found in oily fish, liver and egg yolk. Vitamin D2 comes from mushrooms exposed to UV light. Studies that pooled many randomised trials found that D3 raises blood levels more than D2 and keeps them up for longer. We use vitamin D3 in every vitamin D product we give.²

 

When people buy vitamin D online, it is easy to pick D2 by mistake. The form matters.

 

From our doctor One of our patients came to us for other care. She had always taken vitamin D bought from a pharmacy, and her levels had been good. Then she started buying vitamin D online. After three bottles, her blood test showed a low level. We suspect the product did not contain the amount written on the label. She also told us that while her level was low, her teeth felt slightly loose. Some doctors have described this in people who are low in vitamin D. Because she was tested, she found out within months rather than years.^3,4

Most of us learned that our skin makes vitamin D in sunlight, so it seems logical that people who spend a lot of time outdoors must have plenty. Research shows this is not always true.

 

In a well-known study, researchers measured vitamin D in 93 healthy young adults in Honolulu, Hawaii, one of the sunniest places in the United States. Their average age was 24. They spent about 29 hours a week in the sun, more than 22 of those hours without sunscreen. Even so, about half of them (51%) had low vitamin D. The researchers concluded that people respond to sunlight differently: two people can spend the same hours in the sun, and one makes much more vitamin D than the other.⁵

 

Time outdoors is good for you, but it cannot tell you your vitamin D level. You cannot feel or see it. A blood test is the only way to know.

Medical guidelines list these groups as more likely to be low:^6,7

 

• You spend most of the day indoors, or always use sunscreen or cover your skin outdoors
• You are 70 or older
• Your BMI is over 30. Vitamin D is stored in fat cells, so less of it stays in your blood.
• You have darker skin
• You have a condition or surgery that affects fat absorption, such as Crohn’s disease or weight-loss surgery
• You eat a very low-fat diet, a strict vegetarian diet, or no dairy
• You take medicines that lower vitamin D, such as orlistat (a weight-loss medicine) or a statin (a cholesterol medicine)
• You have liver or kidney disease
• You have broken a bone from a minor fall, or have been told you have thin bones

Physicians who focus on optimal hormone levels look for a wider range of signs than the standard checklist:

 

• Bone thinning: a stooped back, or bones that break easily
• Bone softening: bone pain, bones that hurt when pressed, or swollen joints
• Muscle aches: weakness, aching muscles, and tender spots along the tendons
• Loose teeth, when the shortage is severe and the bones are thin

 

These signs have many other causes. Only a blood test tells you whether vitamin D is the reason.

We don’t ask you to take our word for it. This is what large studies have found:

 

• Bones. In people who are low, vitamin D improves bone density and lowers the risk of fractures and falls. In older adults, vitamin D taken with calcium reduces fractures.⁶
• Autoimmune disease. The VITAL trial followed 25,871 adults aged 50 and over for about five years. People who took a daily vitamin D supplement had 22% fewer autoimmune diseases.⁸
• Cancer. A combined analysis of large trials found that vitamin D did not prevent cancer, but deaths from cancer were 12–13% lower.⁶
• Mood. An analysis of 41 trials with more than 53,000 people found a clear benefit in people diagnosed with depression, alongside their main treatment. There was no effect in people who were not depressed.⁹
• Blood sugar. In adults with prediabetes, about 10% fewer went on to develop diabetes.¹⁰
• Later life. In people aged 75 and over, vitamin D slightly lowered the overall death rate.¹⁰
• Infections. Vitamin D clearly reduced chest and throat infections in children.¹¹

Many lab reports set the bar for “sufficient” quite low. We aim a little higher, for the optimal range.

 

• Why not lower. At the lower edge of our range, your parathyroid hormone settles. When vitamin D is low, this hormone rises and pulls calcium out of your bones. Waiting at a low level for a long time is not what we want for you.⁷
• Why not higher. More is not better. Very high vitamin D raises the calcium in your blood, which can be harmful. In one 3-year trial, people who took high doses every day ended up with thinner wrist bones than people who took a small dose.¹²

 

Standard medical guidelines set our safety limits. Within those limits, we use the optimal-range approach to aim for the level where you benefit most. That is also why we keep checking your level.

We measure two things together:

 

• Total 25-OH vitamin D. This shows the vitamin D from sunlight and from food or supplements combined. Results can differ between testing methods, so we use the same laboratory and the same method every time.
• Blood calcium. Many people have high calcium without knowing it. Vitamin D raises calcium, so we check it before you start.¹³

 

Both tests are included in your Annual Panel.

Vitamin D is safe for most people, but it is not one size fits all. Some people need special consideration before they start, because their body handles vitamin D or calcium differently, or because the same dose would not suit them. These people need closer care. Before you start, our doctor checks whether any of these apply to you:

 

• High blood calcium
• Kidney or liver disease that a specialist is treating
• Sarcoidosis, tuberculosis or lymphoma
• Bowel conditions or surgery that affect absorption
• Certain medicines, such as seizure medicines, steroid tablets, antifungals, HIV medicines and the heart medicine digoxin
• Pregnancy, or planning a pregnancy
• Past kidney stones (we skip the stronger starting dose)
• Other vitamin D or calcium supplements you already take

 

For some of these, we adjust your plan. For others, the specialist who already looks after you knows your results and medicines best, so we refer you back to them rather than start treatment ourselves.¹⁴

Your doctor sets your dose from your blood test.

 

• If your level is low, you may start with a stronger loading dose for a set number of weeks, from its own bottle. When that bottle is empty, you move on to your regular dose from a second bottle.
• If your level is in the optimal range, you keep the same dose.
• If your level is too high, you pause vitamin D, and we check again in 3 months.

 

The two bottles are labelled clearly, and you take one bottle at a time, never both together. Small details like this prevent mistakes. We look after every detail.

The capsule leaves your blood within about a day, but the storage form it becomes lasts for weeks. That has three practical meanings:¹⁵

 

• One capsule a day is enough. There is no need to split doses.
• Missing a day or two is not a worry. Your level falls very slowly.
• We wait about 3 months before retesting. Your level takes that long to settle. Testing sooner would show a number that is still rising, and could lead to the wrong dose change. Once you are on target, a test every 6 months is enough.¹⁵

If you are taking vitamin D and your level is still below our target, we first check how you take it. Are you taking it every day, with a main meal that contains some fat? Vitamin D needs fat to be absorbed well.⁶

 

• Not every day, or not with a meal: we keep the same dose, go over how to take it, and retest in 3 months.
• Every day, taken correctly: your doctor may raise your dose, up to the highest dose we use for long-term care. If you are already on it, we keep your dose the same. More is not better (see “Why not higher”).
• Still clearly low after a starting course, even though you take it correctly every day: we refer you to a specialist or hospital to look for the cause, such as a problem absorbing fat from food.⁷

1. Bikle DD. Vitamin D: Production, Metabolism, and Mechanism of Action. In: Feingold KR, et al., editors. Endotext. MDText.com; updated 2025.

2. Tripkovic L, Lambert H, Hart K, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. Am J Clin Nutr. 2012;95:1357–1364.

3. LeBlanc ES, Perrin N, Johnson JD, et al. Over-the-counter and compounded vitamin D: is potency what we expect? JAMA Intern Med. 2013;173(7):585–586.

4. Wan M, Patel A, Patel JP, et al. Quality and use of unlicensed vitamin D preparations in primary care in England. Br J Clin Pharmacol. 2021;87:1338–1346.

5. Binkley N, et al. Low vitamin D status despite abundant sun exposure. J Clin Endocrinol Metab. 2007;92(6):2130–2135.

6. National Institutes of Health, Office of Dietary Supplements. Vitamin D — Fact Sheet for Health Professionals. Updated 2025.

7. Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. Evaluation, Treatment, and Prevention of Vitamin D Deficiency: an Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2011;96:1911–1930.

8. Hahn J, Cook NR, Alexander EK, et al. Vitamin D and marine omega 3 fatty acid supplementation and incident autoimmune disease: VITAL randomized controlled trial. BMJ. 2022;376:e066452.

9. Mikola T, Marx W, Lane MM, et al. The effect of vitamin D supplementation on depressive symptoms in adults: a systematic review and meta-analysis of randomized controlled trials. Crit Rev Food Sci Nutr. 2023;63(33):11784–11801.

10. Demay MB, Pittas AG, Bikle DD, et al. Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2024;109(8):1907–1947.

11. Jolliffe DA, Camargo CA, Sluyter JD, et al. Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis of aggregate data from randomised controlled trials. Lancet Diabetes Endocrinol. 2021;9(5):276–292.

12. Burt LA, Billington EO, Rose MS, et al. Effect of High-Dose Vitamin D Supplementation on Volumetric Bone Density and Bone Strength: A Randomized Clinical Trial. JAMA. 2019;322(8):736–745.

13. Goltzman D. Hypercalcemia. In: Feingold KR, et al., editors. Endotext. MDText.com; updated 2026.

14. Colonis Pharma Ltd. Colecalciferol 1,000 IU Capsules — Summary of Product Characteristics. emc, UK; 2021.

15. National Osteoporosis Society. Vitamin D and Bone Health: A Practical Clinical Guideline for Patient Management. 2018.