Vitamin D Blood Test: Normal Ranges, Causes & What Your Results Mean

What Is Vitamin D?

Vitamin D is a fat-soluble hormone precursor that the body produces when skin is exposed to UVB sunlight. It exists in two main forms: vitamin D2 (ergocalciferol, from plant sources) and vitamin D3 (cholecalciferol, from sunlight and animal sources). Both are converted in the liver to 25-hydroxyvitamin D (25(OH)D), the form measured in blood tests, and then to the active hormone 1,25-dihydroxyvitamin D in the kidneys.

Vitamin D is essential for calcium absorption, bone mineralisation, immune regulation, muscle function, and mood. Deficiency is extremely common in the UK, where limited sunlight between October and March means most people cannot produce adequate vitamin D through skin exposure alone. Public Health England recommends that everyone consider supplementing during autumn and winter.

Why Is Vitamin D Tested?

• Investigate symptoms of deficiency: bone pain, muscle weakness, fatigue, or frequent infections
• Assess bone health in patients with osteoporosis, osteopenia, or recurrent fractures
• Screen at-risk populations: elderly, housebound, dark-skinned individuals, veiled women, and those in northern latitudes
• Monitor supplementation effectiveness
• Evaluate patients with malabsorption (coeliac disease, Crohn's disease, cystic fibrosis)
• Investigate unexplained hypocalcaemia or elevated parathyroid hormone (PTH)
• Assess immune function - vitamin D plays a role in both innate and adaptive immunity

Normal Ranges

The NHS considers 50 nmol/L sufficient, but many experts recommend 75–100 nmol/L for optimal health. Toxicity is rare below 250 nmol/L.

Causes of Low Vitamin D

• Insufficient sunlight: The primary cause in the UK - UVB is too weak from October to March at UK latitudes (above 51°N)
• Dark skin: Higher melanin levels reduce UVB-induced vitamin D synthesis by up to 90%
• Indoor lifestyle: Office workers, shift workers, and housebound individuals get minimal sun exposure
• Obesity: Vitamin D is sequestered in fat tissue, reducing bioavailability
• Age: Skin produces 75% less vitamin D at age 70 compared to age 20
• Malabsorption: Coeliac disease, Crohn's disease, cystic fibrosis, and short bowel syndrome
• Liver or kidney disease: Impairs conversion to the active form
• Medications: Anticonvulsants, glucocorticoids, antiretrovirals, and cholestyramine
• Covered skin: Wearing full-body covering for cultural or religious reasons
• Sunscreen: SPF 30 reduces vitamin D synthesis by 95% (though sun protection remains important)

Causes of High Vitamin D

• Excessive supplementation: The only realistic cause of toxicity - impossible from sun exposure alone
• Granulomatous diseases: Sarcoidosis, tuberculosis, and some lymphomas produce extra active vitamin D
• Williams syndrome: A rare genetic condition with increased vitamin D sensitivity

How to Optimise Your Vitamin D Levels

• Supplement year-round (or at minimum Oct–Mar): NHS recommends 400 IU (10 µg) daily; many experts suggest 1,000–2,000 IU (25–50 µg) daily for adults
• Choose D3 over D2: Vitamin D3 (cholecalciferol) is more effective at raising blood levels than D2
• Take with fat: Vitamin D is fat-soluble - absorption improves when taken with a meal containing fat
• Get sensible sun exposure: 10–15 minutes of midday sun on bare arms and face in summer (without sunscreen) produces roughly 10,000 IU
• Eat vitamin D-rich foods: Oily fish, egg yolks, fortified milk, mushrooms exposed to UV light
• Test and adjust: If your level is below 50 nmol/L, a higher loading dose (e.g., 4,000 IU daily for 8–12 weeks) may be needed before switching to maintenance
• Pair with vitamin K2: Vitamin K2 helps direct calcium to bones rather than soft tissues when supplementing high-dose D3

When Should You Get Tested?

• You live in the UK and don't supplement - the majority of adults are deficient in winter
• You experience bone pain, muscle weakness, or recurrent stress fractures
• You have dark skin and live in northern latitudes
• You are elderly, housebound, or have limited sun exposure
• You have a malabsorption condition or have had gastric surgery
• You want to optimise your immune function
• You are already supplementing and want to check your levels are in the optimal range

Which Lola Health Tests Include Vitamin D?

• Peak Insights - includes 25(OH)D plus 70+ biomarkers for a complete health picture
• Vital Check — includes vitamin D as part of a comprehensive health screen
• Core Health — essential health screening including vitamin D