Complete Guide to Vitamin D Deficiency: Symptoms, Causes & Testing

Vitamin D is a fat-soluble hormone precursor that your body produces in the skin in response to sunlight and absorbs in smaller amounts from food. It plays a central role in calcium absorption, bone mineralisation, immune regulation and muscle function. A deficiency occurs when blood levels fall below the threshold required to support these processes.¹

Clinicians measure vitamin D status using the serum concentration of 25-hydroxyvitamin D, abbreviated 25(OH)D. This marker reflects both sun exposure and dietary intake, and is considered the most reliable indicator of whether your body has adequate stores.²

An estimated 42% of adults may have insufficient vitamin D levels, based on pooled prevalence estimates across population studies in temperate and high-latitude regions.

Important: This article is for educational purposes only and is not a substitute for professional medical advice, diagnosis or treatment. Always consult a qualified clinician about your individual results.

Symptoms of Vitamin D Deficiency

Vitamin D deficiency can be deceptively quiet. Many people have low levels for years without obvious complaints, while others experience a cluster of non-specific symptoms that are easy to attribute to a busy lifestyle.³

Common early signs

• Persistent fatigue and low energy
• Bone pain, muscle weakness or aches
• Frequent colds and slower recovery from infection
• Low mood, particularly during darker months
• Hair thinning and slow wound healing

Because the symptoms overlap with so many everyday conditions, a blood test is the only way to know your vitamin D status with certainty.

What Causes It?

Deficiency develops when the body either makes too little vitamin D or cannot use what it has. The most frequent drivers are limited sun exposure, darker skin pigmentation, certain medical conditions and inadequate dietary intake.⁴

In sunny climates we often assume deficiency is rare, yet indoor lifestyles, sun-protective clothing and routine sunscreen use mean we see low vitamin D across all age groups.

Who Is Most at Risk?

Some groups carry a meaningfully higher risk of deficiency and may benefit from earlier and more regular testing.⁵

• Older adults: Skin synthesises less vitamin D with age, and time spent indoors tends to increase.
• Limited sun exposure: Office-based routines, veiled clothing and consistent sunscreen use all reduce production.
• Darker skin tones: Higher melanin reduces the skin's ability to synthesise vitamin D from sunlight.
• Malabsorption: Coeliac, Crohn's and bariatric surgery can impair absorption of fat-soluble vitamins.

Diagnosis: Reading Your Levels

Vitamin D status is graded by the serum 25(OH)D concentration. Understanding where your result sits helps you and your clinician decide whether action is needed.⁶

Recommended Tests

A single 25(OH)D measurement is sufficient for most people. If your clinician suspects an underlying cause, they may add calcium, phosphate, parathyroid hormone or kidney function to the panel.⁷

Treatment & Supplementation

Most deficiencies are corrected with oral vitamin D3 supplementation over several weeks to months, followed by a maintenance dose. Your clinician will tailor the regimen to your baseline level, body weight and any underlying conditions.⁸

1. Confirm deficiency with a baseline 25(OH)D test.
2. Begin a loading or correction dose as directed by your clinician.
3. Re-test after 8–12 weeks to confirm your levels have improved.
4. Continue a maintenance dose and review periodically.

Prevention

Sensible, regular sun exposure, a diet that includes oily fish, eggs and fortified foods, and routine monitoring for at-risk groups are the cornerstones of prevention.⁹