Vitamin D deficiency is one of the most common nutritional deficiencies worldwide, affecting an estimated 1 billion people. Despite being called a "vitamin," vitamin D functions more like a hormone — it plays critical roles not just in bone health, but in immune function, muscle strength, cardiovascular health, and even mood regulation.
What Does Vitamin D Do?
Vitamin D has two primary forms: D2 (ergocalciferol, from plant sources) and D3 (cholecalciferol, from sunlight and animal sources). Both are converted in the liver and kidneys to the active hormone form (calcitriol). Its key functions include:
- Regulating calcium and phosphorus absorption from the gut
- Maintaining bone mineralization and strength
- Supporting immune system function
- Modulating muscle function
- Potential roles in cancer prevention, cardiovascular health, and insulin sensitivity
Causes of Deficiency
Vitamin D deficiency results from inadequate sunlight exposure, poor dietary intake, or impaired absorption/metabolism. Common risk factors include:
- Limited sun exposure (office work, northern climates, sunscreen use, dark skin pigmentation)
- Obesity (vitamin D is sequestered in fat tissue)
- Malabsorption conditions (Crohn's disease, celiac disease, gastric bypass surgery)
- Kidney or liver disease (impaired conversion to active form)
- Older age (reduced skin synthesis and dietary intake)
- Certain medications (antiepileptics, glucocorticoids, antifungals)
💡 Houston Note: Despite Texas sunshine, vitamin D deficiency is still very common in our patient population — particularly in those who spend most of their day indoors or who have darker skin pigmentation, which reduces UV absorption.
Symptoms
Mild to moderate deficiency is often completely asymptomatic and found only on routine bloodwork. When symptoms occur, they may include:
- Bone pain or aching, particularly in the back, hips, or legs
- Muscle weakness or fatigue
- Mood changes, depression, or "brain fog"
- Frequent infections (immune dysfunction)
- Severe deficiency causes rickets in children and osteomalacia (soft bones) in adults
Testing and Diagnosis
Vitamin D status is measured with a blood test: 25-hydroxyvitamin D [25(OH)D]. Reference ranges:
- Deficiency: Below 20 ng/mL
- Insufficiency: 20–29 ng/mL
- Sufficient: 30–100 ng/mL
- Toxicity risk: Above 150 ng/mL (rare, only from excessive supplementation)
Endocrinologists may also test parathyroid hormone (PTH), which rises in response to low vitamin D, as well as calcium and phosphorus levels.
Treatment
Treatment depends on the severity of deficiency and underlying cause:
- Mild insufficiency: Over-the-counter vitamin D3, typically 1,000–2,000 IU daily
- Moderate deficiency: Higher doses of vitamin D3 (2,000–4,000 IU daily) or prescription weekly high-dose therapy (50,000 IU vitamin D2 weekly for 8–12 weeks)
- Severe deficiency or malabsorption: Higher prescription doses with close monitoring
- Diet: Fatty fish, egg yolks, fortified dairy and cereals; dietary sources alone are rarely sufficient
- Sun exposure: 10–15 minutes of midday sun on arms and legs several days per week can help, though this varies significantly by skin type and geography
Follow-up testing after 3 months of supplementation is recommended to confirm adequate response.
Vitamin D and Bone Health
Vitamin D is essential for calcium absorption — without adequate vitamin D, even high calcium intake cannot prevent bone loss. This is why vitamin D supplementation is a cornerstone of osteoporosis prevention and treatment. Patients on bisphosphonates (like alendronate) or other osteoporosis therapies must have adequate vitamin D levels for these medications to work effectively.
Key Takeaways
- Vitamin D deficiency is extremely common and often has no symptoms
- A simple blood test (25-OH vitamin D) confirms diagnosis
- Supplementation is safe, effective, and often needed year-round in Texas
- Adequate vitamin D is essential for osteoporosis prevention and treatment
- Underlying causes (malabsorption, kidney disease) should be identified and treated
Our Team Sees Vitamin D & Calcium Patients
All five of our providers evaluate and manage vitamin d & calcium conditions. Book with any member of our team:
Book an Appointment → or call 832-968-7003