Vitamin D Blood Test (25-OH)

A vitamin D blood test can help you:

• Detect vitamin D deficiency or insufficiency that may be weakening your bones and muscles.
• Explain symptoms such as bone pain, muscle weakness, frequent falls, or low mood.
• Assess your status before or during pregnancy, menopause, or intensive training blocks.
• Optimise supplementation dose rather than guessing, and avoid both under dosing and excess.
• Monitor vitamin D in people with gut, kidney, liver, or parathyroid conditions where regulation is disrupted.

Vitamin D is a fat soluble vitamin that acts more like a hormone in the body. There are two main forms:

• Vitamin D3 cholecalciferol, made in your skin when exposed to ultraviolet B sunlight and found in animal based foods.
• Vitamin D2 ergocalciferol, found in some plant sources and fortified foods and used in some supplements.

Both forms are converted in the liver to 25 hydroxyvitamin D 25 OH D, the main circulating storage form, and then in the kidneys and other tissues to the active hormone 1,25 dihydroxyvitamin D.

A standard vitamin D blood test measures 25 OH D, because this reflects your overall vitamin D supply from both sun and diet over the last few weeks to months.

Vitamin D plays several key roles:

• Regulates calcium and phosphate absorption from the gut, helping to mineralise bone and maintain teeth.
• Supports normal bone turnover and reduces the risk of osteomalacia in adults and rickets in children.
• Influences muscle strength and function, which affects balance and fall risk.
• Modulates immune responses, inflammation, and possibly susceptibility to some infections.

At a biochemical level, active vitamin D works with PTH to keep blood calcium within a narrow range, by adjusting absorption from the gut, release from bone, and excretion by the kidneys.

Vitamin D matters because:

• Low vitamin D reduces calcium absorption, which can lead to secondary hyperparathyroidism, bone demineralisation, and a higher risk of fractures over time.
• Deficiency is associated with muscle weakness, particularly around the hips and shoulders, contributing to falls and reduced performance.
• Severe deficiency in children causes rickets, and in adults causes osteomalacia with bone pain and tenderness.
• Emerging evidence links low vitamin D with mood changes, immune dysregulation, and cardiometabolic risk, especially when combined with other nutrient or lifestyle stresses.

These three markers are tightly linked but reflect different parts of mineral regulation:

• Vitamin D 25 OH D indicates your longer term vitamin D supply from sun and diet.
• Calcium reflects the current concentration of calcium in your blood, which the body defends very tightly.
• PTH parathyroid hormone is released by the parathyroid glands when calcium falls and acts to raise it by mobilising bone calcium and increasing vitamin D activation and kidney reabsorption.

Typical patterns include:

• Vitamin D deficiency with normal calcium and raised PTH early secondary hyperparathyroidism.
• More severe deficiency with low or low normal calcium, high PTH, and bone or muscle symptoms.

This is why vitamin D is best interpreted alongside calcium, ALP, and sometimes PTH.

Vitamin D levels are shaped by sunlight, diet, health conditions, and genetics. Key influences include:

1. Sunlight exposure

• In the UK, UVB is sufficient for vitamin D synthesis mainly from late spring to early autumn, around midday, when skin is uncovered.
• Darker skin, covering clothes, indoor lifestyles, and high latitude living reduce cutaneous vitamin D production.

2. Diet and supplements

• Few foods naturally contain much vitamin D oily fish, egg yolks, and some mushrooms are exceptions.
• Fortified foods and supplements can contribute significantly, particularly in winter months.
• Very high dose supplements can raise vitamin D to excessive levels if not monitored.

3. Body composition

• Vitamin D is fat soluble and stored in adipose tissue, so people with higher body fat may have lower circulating 25 OH D at a given intake.

4. Gut, liver, and kidney health

• Malabsorption syndromes, bariatric surgery, and chronic liver or kidney disease can reduce vitamin D absorption, activation, or handling.
• Certain medications such as anticonvulsants, glucocorticoids, and some HIV treatments can accelerate vitamin D breakdown.

5. Age and life stage

• Infants, older adults, and pregnant or breastfeeding women have higher risk of deficiency and different requirements.
• Older skin is less efficient at synthesising vitamin D from sunlight.

Yes. This is common in early deficiency.

You can have:

• Low vitamin D with normal blood calcium because PTH rises and increases calcium release from bone and reduces losses in urine.
• Normal calcium and near normal ALP early on, even though bone is already under strain.

Over time, if deficiency is not corrected, this compensation can lead to bone demineralisation, higher ALP, bone pain, and increased fracture risk, especially in older adults.

Reference ranges vary slightly between guidelines and laboratories, but in UK practice 25 OH vitamin D is often interpreted as:

• Deficiency: less than 25 nmol/L.
• Insufficiency: 25 to 50 nmol/L.
• Sufficiency or adequate for most bones: above 50 nmol/L.
• Potentially too high: typically above 125 to 150 nmol/L, with toxicity usually at much higher sustained levels.

For many adults looking at broader health and performance, aiming for a level comfortably above the deficiency and insufficiency thresholds, often somewhere in the 50 to 100 nmol/L range, is common, while avoiding unnecessary very high levels.

Fasting is not usually required for a vitamin D test. You can typically eat and drink as normal.

You may be advised to:

• Take your vitamin D supplement as usual if the goal is to understand your steady state level.
• Or, in some protocols, hold very high dose vitamin D for a short period before testing to avoid capturing a transient spike after a recent dose.

Follow the instructions given for your specific panel so results are easy to compare over time.

Optimising vitamin D involves raising low levels safely, avoiding excess, and supporting the wider bone and metabolic picture. Depending on your situation, clinician guided strategies may include:

• Adjusting supplements to a daily or weekly dose appropriate for your starting level, body weight, and risk factors.
• Increasing safe sunlight exposure where possible, balancing skin cancer risk and individual sensitivity.
• Supporting gut, liver, and kidney health to improve vitamin D handling.
• Coordinating vitamin D optimisation with calcium intake, physical activity, and strength training to support bones and muscles.
• Monitoring 25 OH D, calcium, and sometimes PTH to ensure levels move into and remain in a healthy range.