Hormones March 01, 2026

Vitamin D: The Sunshine Hormone

Why Endocrinologists Care About It

Vitamin D is far more than a dietary supplement. It is a steroid hormone essential for calcium balance, skeletal integrity, immune regulation, and multiple metabolic processes. In endocrinology, vitamin D status is one of the most common and clinically important laboratory concerns.

Geography plays a major role in vitamin D sufficiency. In regions above approximately 37° north latitude, such as the Pacific Northwest, ultraviolet B (UVB) radiation during fall and winter months is insufficient for adequate skin production of vitamin D. Even individuals who spend time outdoors may not generate enough vitamin D, particularly with appropriate and recommended sunscreen use.

Dietary sources are limited. Only a few foods naturally contain meaningful amounts of vitamin D (such as fatty fish and egg yolks), and fortified foods contribute variably.

According to the 2017–2018 National Health and Nutrition Examination Survey (NHANES), approximately 21% of the U.S. population had suboptimal vitamin D levels, highlighting how common this issue remains.

Vitamin D Chemistry and Activation

Vitamin D synthesis begins in the skin following exposure to UVB radiation (not UVA, and not reliably from tanning beds). The molecule then undergoes two critical activation steps:

  1. Liver: Conversion to 25-hydroxyvitamin D [25(OH)D]
  2. Kidneys: Conversion to the active form, 1,25-dihydroxyvitamin D [1,25(OH)₂D]

This process depends on:

Because vitamin D functions as a hormone, its production and activation are tightly regulated by the endocrine system.

Why We Measure 25(OH)D — Not the Active Form

A common misconception is that the active hormone, 1,25-dihydroxyvitamin D, should be measured to assess vitamin D status. From an endocrinology perspective, this is usually incorrect.

We measure 25(OH)D because:

In contrast, 1,25(OH)₂D:

There are specific clinical scenarios, such as advanced kidney disease or hypoparathyroidism, where measuring 1,25(OH)₂D may be appropriate, but it is not a routine test for assessing vitamin D stores.

Interpreting Vitamin D Levels

Serum 25(OH)D is considered the gold standard test.

General reference ranges:

(Reference ranges may vary slightly depending on laboratory standards and clinical guidelines.)

Physiological Roles of Vitamin D

1. Calcium and Bone Metabolism (Classic Role)

Vitamin D increases intestinal calcium and phosphate absorption.

When levels are low:

Chronic deficiency can lead to:

Vitamin D deficiency is one of the most modifiable risk factors for metabolic bone disease.

2. Muscle Strength and Fall Prevention

Adequate vitamin D levels support:

Even mild deficiency may contribute to muscle weakness and instability.

3. Metabolic Regulation

Vitamin D receptors are present in pancreatic beta cells and adipose tissue. Research suggests vitamin D may influence:

While associations exist, supplementation beyond correcting deficiency has not consistently shown dramatic metabolic benefits in clinical trials.

4. Immune System Modulation

Vitamin D plays an important role in immune balance:

Low levels have been associated with certain autoimmune and inflammatory conditions, though supplementation outcomes vary depending on the condition.

5. Cellular Growth and Cancer Research

Vitamin D influences:

Low vitamin D levels are associated with certain malignancies; however, large clinical trials have not consistently demonstrated that supplementation prevents cancer in the general population.

Who Should Be Screened?

Screening should be individualized based on risk factors rather than performed universally in low-risk individuals.

Higher-risk populations include:

Vitamin D deficiency is often silent and discovered incidentally during laboratory evaluation.

Supplementation: Evidence-Based Approach

Vitamin D supplementation must be individualized. More is not better.

Excessive supplementation can lead to toxicity, resulting in:

Typical Dosing

Vitamin D2 vs D3

Vitamin D2 (ergocalciferol) remains an acceptable alternative in certain cases, particularly in prescription formulations.

Key Takeaways

In endocrinology, optimizing vitamin D status represents one of the most accessible and modifiable interventions to support long-term skeletal and metabolic health.