Vitamin D 25 hydroxy vs Dihydroxy: What's the Difference?

Vitamin D is more than just one vitamin. It’s a family of nutrients that shares similarities in chemical structure. Vitamin D helps your body absorb calcium and maintain strong bones throughout your entire life. Your body produces vitamin D when the sun’s UV rays contact your skin. Other good sources of the vitamin include fish, eggs, and fortified dairy products. It’s also available as a dietary supplement.

In your diet, the most commonly found members are vitamin D2 and D3. While both types help you meet your vitamin D requirements, they differ in a few important ways.

Research even suggests that vitamin D2 is less effective than vitamin D3 at raising blood levels of vitamin D.

Dietary vitamin D comes in two main forms:
  • Vitamin D2 (ergocalciferol)
  • Vitamin D3 (cholecalciferol)
The two forms of vitamin D differ depending on their food sources.

Vitamin D3 is only found in animal-sourced foods, whereas D2 mainly comes from plant sources and fortified foods.

Your skin makes vitamin D3 when it’s exposed to sunlight.

Vitamin D2 and D3 are also not equal when it comes to raising your vitamin D status.

Both are effectively absorbed into the bloodstream. However, the liver metabolizes them differently.

vitamin D and COVID-19

What is 25-hydroxy vitamin D?

Vitamin D must go through several processes in your body before your body can use it. The first transformation occurs in the liver. 

Here, the liver converts vitamin D2 into 25-hydroxyvitamin D2 and vitamin D3 into 25-hydroxyvitamin D3. These two compounds are collectively known as calcifediol (also known as calcidiol).

Calcifediol is the main circulating form of vitamin D, and its blood levels reflect your body’s stores of this nutrient.

For this reason, your health care provider can estimate your vitamin D status by measuring your levels of calcifediol (Trusted Source).

However, vitamin D2 seems to yield less calcifediol than an equal amount of vitamin D3.

Most studies show that vitamin D3 is more effective than vitamin D2 at raising blood levels of calcifediol (Trusted SourceTrusted Source).

What is the 25-hydroxy vitamin D test?

The 25-hydroxy vitamin D test is the best way to monitor vitamin D levels. The amount of 25-hydroxyvitamin D in your blood is a good indication of how much vitamin D your body has. The test can determine if your vitamin D levels are too high or too low.

The test is also known as the 25-OH vitamin D test and the calcidiol 25-hydroxycholecalciferol test. It can be an important indicator of osteoporosis (bone weakness) and rickets (bone malformation).

What is the 25 Dihydroxy vitamin D1 test?

There is another blood test for normal vitamin D levels, called Vitamin D 1,25 Dihydroxy level or 1,25(OH)(2)D.

The Vitamin D, 1,25 Dihydroxy level does not have a major change until normal Vitamin D levels drop to levels that are considered a VERY severe Vitamin D deficiency.

Calcidiol is converted to calcitriol, or 1,25(OH)2D, mostly in your kidneys. This is the active, steroid-hormone form of vitamin D.

Calcitriol interacts with the vitamin D receptor (VDR), which is found in almost every single cell in your body (Trusted SourceTrusted Source).

When the active form of vitamin D binds to this receptor, it turns genes on or off, leading to changes in your cells. This is similar to how most other steroid hormones work (Trusted SourceTrusted Source).

1,25 dihydroxy-vitamin D or 1.25 Dihydroxy level has normal values that are usually listed as 20-76 pg/ml.

Normal Vitamin D Lab Values for 1,25 Dihydroxy Vitamin D
  • Adults in good health (age 20-50) : 17 - 53 pg/ml
  • Children up to 12: ca. 40% higher values
  • Pregnant women (8-42 week): ca. 60% higher values
  • Persons older than 70: ca. 40% lower values
  • The normal range is independent of the season.
Most considered the most potent form of vitamin D. It stimulates calcium absorption from our small intestines and bones (along with PTH) and increases reabsorption of calcium by the kidneys.

The level of serum 1,25-dihydroxy-vitamin D is not typically used to know vitamin D status. It has a short half-life of only 15 hours. It is regulated by parathyroid hormone, calcium, and phosphate. So a decrease will not be noted until vitamin D deficiency is severe.

Why is a 25-hydroxy vitamin D test done?

Your doctor may request a 25-hydroxy vitamin D test for several different reasons. It can help them figure out whether too much or too little vitamin D is causing bone weakness or other abnormalities. It can also monitor people who are at risk for having a vitamin D deficiency.

Those who are at high risk of having low levels of vitamin D include:
  • people who don’t get much exposure to the sun
  • older adults
  • people with obesity
  • babies who are breastfed only (formula is usually fortified with vitamin D)
  • people who have had gastric bypass surgery
  • people who have a disease that affects the intestines and makes it difficult for the body to absorb nutrients, such as Crohn’s disease
Your doctor may also want you to do a 25-hydroxy vitamin D test if they’ve already diagnosed you with a vitamin D deficiency and want to see if treatment is working.

How is the 25-hydroxy vitamin D test performed?

Your doctor will tell you not to eat anything for four to eight hours before the test.

The 25-hydroxy vitamin D test requires a common blood test. Your healthcare provider will draw blood from a vein in your arm using a needle. A quick finger prick will more than likely provide enough for a blood sample in children and infants.

Evaluating the results of a 25-hydroxy vitamin D test

Results will depend on your age, sex, and the testing methods used. Results can also slightly vary from lab to lab.

According to the Office of Dietary Supplements (ODS), levels of vitamin D are measured by the 25-hydroxy level in nanomoles/liter (nmol/L) or nanograms/milliliter (ng/mL). The results can indicate the following:
  • deficiency: less than 30 nmol/L (12 ng/mL)
  • potential deficiency: between 30 nmol/L (12 ng/mL) and 50 nmol/L (20 ng/mL)
  • normal levels: between 50 nmol/L (20 ng/mL) and 125 nmol/L (50 ng/mL)
  • high levels: higher than 125 nmol/L (50 ng/mL)
If your vitamin D levels are low and you’re having symptoms of bone pain, a doctor may recommend a special scan to check bone density. Doctors use this painless scan to evaluate a person’s bone health.

Low blood levels of 25-hydroxy vitamin D usually mean one (or more) of the following:
  • you aren’t eating a balanced, complete diet
  • your intestines aren’t absorbing the vitamin properly
  • you’re not spending enough time outside to absorb adequate vitamin D levels through sun exposure
Some evidence links vitamin D deficiency to a higher risk of certain cancers, immune diseases, and cardiovascular disease.

High vitamin D blood levels generally result from taking too many vitamin pills and other nutritional supplements.
High doses of vitamin D can result in a condition called hypervitaminosis D. 

Hypervitaminosis is a rare but serious condition that could put you at risk for liver or kidney problems.

High levels are rarely due to consuming too much of the vitamin through foods or sun exposure.

Your doctor will help explain the results of your test and determine if you have a vitamin D deficiency.

Risks of a 25-hydroxy vitamin D test

As with any routine blood test, risks of the 25-hydroxy vitamin test are minimal and include excessive bleeding, lightheadedness and a slight chance of infection where the needle pierces your skin.

Outlook

Vitamin D is vital to the body. Deficiencies at any age can cause problems. Your doctor may recommend supplements or other treatment options if you’re very deficient. Eating foods that contain vitamin D in addition to adding supplements to your regimen can help keep your vitamin D levels stable.

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