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Vitamin D is a Hormone

  • Writer: David S. Klein, MD FACA FACPM
    David S. Klein, MD FACA FACPM
  • Jan 24
  • 4 min read
Vitamins that are also minerals
Vitamins that are also Hormones

Why That Changes How We Diagnose, Treat, and Think About It


Below is a finished, physician-to-patient blog, aligned with your established template, tone, and educational depth.



Most people think of vitamin D as a simple nutrient—something you take for bone health or to “boost immunity.” From a medical standpoint, that framing is incomplete and, at times, misleading.


Vitamin D is not merely a vitamin. It functions as a steroid hormone, with widespread effects throughout the body. Understanding this distinction fundamentally changes how we evaluate deficiency, dosing, risk, and long-term health implications.


Clinical Roles of Vitamin D as a Hormone Bone and Mineral Metabolism
Soy-Free Vitamin D3 in Olive Oil


By definition, a vitamin is an essential nutrient that the body cannot manufacture in adequate amounts and must obtain from the diet. Vitamin D violates this definition in several important ways.


The human body:

  • Synthesizes vitamin D in the skin in response to ultraviolet B (UVB) radiation

  • Converts it through a two-step endocrine activation process

  • Uses the active form to regulate gene expression


These characteristics place vitamin D squarely in the category of hormones, not traditional vitamins.


The Hormonal Activation Pathway



Vitamin D undergoes a tightly regulated process similar to other steroid hormones:


  1. Skin: UVB exposure converts 7-dehydrocholesterol into cholecalciferol (vitamin D₃)

  2. Liver: Conversion to 25-hydroxyvitamin D [25(OH)D], the storage form measured in blood tests

  3. Kidney (and other tissues): Conversion to 1,25-dihydroxyvitamin D (calcitriol), the active hormone


Calcitriol binds to the vitamin D receptor (VDR), a nuclear receptor found in over 30 different tissues, influencing the transcription of hundreds of genes.



Differences between Vitamins and Minerals
Differences between Vitamins and Minerals

Vitamin D receptors are found far beyond bone and calcium pathways, including:


  • Brain and central nervous system

  • Immune cells (T cells, B cells, macrophages)

  • Cardiovascular tissue

  • Skeletal muscle

  • Pancreatic beta cells

  • Colon, breast, and prostate tissue


This widespread receptor distribution explains why vitamin D status has been linked to outcomes far beyond osteoporosis.


Clinical Roles of Vitamin D as a Hormone Bone and Mineral Metabolism
For Osteoporosis or Osteopenia, take Vitamin D3 with Vitamin K2

Clinical Roles of Vitamin D as a Hormone


Vitamin D regulates:

  • Intestinal calcium and phosphorus absorption

  • Bone remodeling

  • Parathyroid hormone (PTH) suppression


Deficiency leads not only to osteopenia and osteoporosis, but also increased fracture risk and impaired bone quality.


Immune Modulation


Vitamin D:

  • Enhances innate immune defense

  • Modulates inflammatory cytokine production

  • Helps regulate autoimmune responses


Low levels have been associated with increased susceptibility to respiratory infections and dysregulated immune activity.


Muscle Function and Falls


Adequate vitamin D levels support:


  • Muscle strength

  • Neuromuscular coordination

  • Reduced fall risk in older adults


Muscle weakness is often one of the earliest—and least recognized—signs of deficiency.


Brain and Cognitive Health


Vitamin D plays a role in:

  • Neuroprotection

  • Neurotransmitter regulation

  • Reduction of neuroinflammation


Emerging evidence links low vitamin D levels to cognitive decline and increased dementia risk, particularly in aging populations.


Why Deficiency Is So Common


Despite its importance, vitamin D deficiency is widespread—even in sunny climates.

Contributing factors include:


  • Limited sun exposure

  • Sunscreen use and indoor lifestyles

  • Aging skin (reduced synthesis)

  • Obesity (vitamin D sequestration in fat tissue)

  • Malabsorption syndromes

  • Chronic kidney or liver disease


Diet alone is rarely sufficient to maintain optimal levels.


What Blood Levels Really Mean


The standard test, 25-hydroxyvitamin D, reflects body stores—not active hormone levels.


From a clinical perspective:

  • Levels below 20 ng/mL indicate deficiency

  • 20–30 ng/mL is often insufficient

  • Many physicians aim for 30–50 ng/mL to support broader physiologic functions


Optimal targets may vary based on age, bone health, immune status, and comorbid disease.


Why Dosing Requires Medical Judgment


Because vitamin D is fat-soluble and hormonally active:

  • Excessive dosing can lead to toxicity

  • Calcium balance must be monitored

  • Individual response varies widely


Supplementation should be intentional, monitored, and personalized, not arbitrary.


The Takeaway

Calling vitamin D “just a vitamin” understates its importance and invites under-treatment. In reality, vitamin D functions as a master regulatory hormone influencing bone, immune, muscle, and brain health.


Recognizing this distinction allows patients and clinicians to move beyond casual supplementation toward evidence-based assessment, targeted dosing, and meaningful prevention.


In upcoming posts, we’ll explore how vitamin D interacts with calcium, magnesium, hormones, and inflammation, and why balance—not megadosing—is the key to long-term health.


References


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