Vitamin D for Dogs: What Does 25(OH)D Show and When Should Supplementation Be Considered?

Vitamin D is an essential nutrient for dogs, but this is one of those cases where “more is better” simply does not apply. Dogs need vitamin D, among other things, for normal calcium and phosphorus metabolism and to support bone health. Vitamin D receptors are also found in cells of the immune system, skin, intestines and other tissues, which is why researchers are studying vitamin D status in relation to a wide range of health problems.

This is where the subject quickly becomes more complicated than simply asking whether a dog gets enough vitamin D from food.

Lower vitamin D status has been associated in studies with conditions including chronic enteropathy, heart and kidney disease, some cancers, atopic dermatitis, pancreatitis and gallbladder mucocele. That does not automatically mean that vitamin D deficiency causes these diseases or that adding vitamin D treats them. A low vitamin D level may be a cause, a consequence, a marker of disease severity, or some combination of several factors.

One of the most important principles with vitamin D is therefore this: do not look only at the supplement bottle or a single blood-test number. Look at the whole dog.

Dogs do not get vitamin D from sunlight in the same way humans do

When people talk about vitamin D, sunlight usually enters the conversation very quickly. Dogs are different. Their skin cannot produce enough vitamin D through UVB exposure to meet the body’s needs. A dog’s vitamin D status therefore depends primarily on vitamin D obtained from the diet.

Natural sources include fatty fish and egg yolk. Cod liver oil can also provide vitamin D, but then the whole product matters because cod liver oil also contains a significant amount of vitamin A. Complete dog foods are supplemented with vitamin D according to their formulation. For a dog eating a complete diet, it therefore makes little sense to assume that extra vitamin D should be added “just in case”.

Before using a supplement, you also need to know how much vitamin D the dog is already receiving from the main diet, treats, oils and other supplements.

D3, 25(OH)D and calcitriol are not the same thing

A lot of confusion around vitamin D comes from placing several different compounds under one label. Vitamin D3, or cholecalciferol, is the form a dog receives from food or a supplement. The body then processes vitamin D3, and the liver forms 25-hydroxyvitamin D, or 25(OH)D.

This is the most abundant vitamin D metabolite in the blood and is commonly used to assess vitamin D status. The body can then convert it into the active form, 1,25-dihydroxyvitamin D, or calcitriol, which has hormone-like effects in the body.

These three are connected, but they are not the same number.

That distinction also matters when supplements are involved. The amount of D3 listed on a food label, the 25(OH)D result on a blood test and the dose of a vitamin D supplement given to a dog cannot simply be converted one-to-one.

Why is 25(OH)D measured in blood?

25(OH)D is the marker most often used to evaluate vitamin D status because it reflects the body’s vitamin D stores better than active calcitriol does. The body regulates calcitriol very tightly, and its concentration can remain normal in some situations even when vitamin D stores are reduced. Measuring the active form therefore does not automatically give a better picture of the dog’s overall vitamin D status.

Even 25(OH)D should not be interpreted in a vacuum. Different laboratories, analytical methods and studies may use different target and cut-off values. Some veterinary laboratory protocols, for example, have used 100–150 ng/ml as an optimal 25(OH)D range, while several studies have treated values below 40 ng/ml as low. These numbers should not be turned into a universal rule for every dog. It matters which laboratory performed the test, which method was used, and what reference or target range that laboratory applies.

A single blood test can be a map, not a verdict.

What does a low vitamin D level actually tell us?

This is where the vitamin D topic becomes especially interesting. A low 25(OH)D level can provide useful information, but the next question should not be only “how do we add more vitamin D?”. The more important question is: why is the level low?

Vitamin D status can be influenced by diet composition, nutrient absorption, gastrointestinal health, age, concurrent disease, kidney function, inflammatory processes, medications and probably many other individual factors. Chronic enteropathy is a good example. Several studies have found lower 25(OH)D concentrations in dogs with chronic intestinal disease, and lower values have also been associated with more severe disease and poorer prognosis.

At the same time, chronic intestinal disease itself can reduce the absorption of fat-soluble nutrients. That creates a classic chicken-and-egg question: does low vitamin D contribute to disease, or does the disease itself lower vitamin D status? The answer is probably not the same for every dog.

The same principle is worth remembering when looking at studies involving other diseases.

Vitamin D and chronic enteropathy

In dogs with chronic enteropathy, 25(OH)D is one of the better studied links between vitamin D status and disease. Studies have found that lower 25(OH)D may be associated with stronger inflammation, lower albumin and a poorer prognosis. That makes vitamin D status a clinically interesting marker, but it still does not prove that supplementation automatically improves the course of the disease.

A 2025 randomized, double-blind, placebo-controlled clinical trial in dogs with protein-losing enteropathy illustrated this well. Dogs receiving cholecalciferol had increased 25(OH)D concentrations, but the study found no clinical benefit compared with placebo. In some dogs, vitamin D levels became too high, although ionized calcium did not increase.

This is an excellent example of why correcting a laboratory number and treating a disease are not automatically the same thing.

If a dog has chronic digestive problems, it makes sense to assess digestion, nutrient absorption, the main diet and possible deficiencies as one picture. I discuss this in more detail in the article “Digestive problems in dogs don’t develop overnight”.

Vitamin D and the health of the heart, kidneys and other organs

Lower 25(OH)D levels have also been reported in dogs with heart disease. Dogs with congestive heart failure, for example, have been found to have lower 25(OH)D concentrations than healthy control dogs. Again, this does not tell us that low vitamin D caused the heart disease. Disease severity, nutrition, inflammation and altered metabolism can all affect the result.

A similar question arises with kidney disease. The kidneys participate in vitamin D metabolism, and changes in several vitamin D metabolites have been described in dogs with chronic kidney disease. Vitamin D therefore should not simply be added to a dog with kidney disease because “the number is low”. Kidney function, calcium and phosphorus balance, diet, and the form and dose of the supplement all need to be considered.

Lower 25(OH)D levels have also been reported in dogs with gallbladder mucocele. The studies themselves emphasize that it is still unknown whether low vitamin D is a cause or a consequence of the disease.

Vitamin D and the immune system

Vitamin D receptors are found on several immune cells, including white blood cells and mast cells. They are also present in skin and intestinal cells. This provides a biological reason to study connections between vitamin D, inflammation, skin disease and immune-mediated conditions.

Lower vitamin D status has been described in various inflammatory and immune-mediated diseases, and vitamin D has also been studied in canine atopic dermatitis.

There is, however, one easy trap to avoid: finding a receptor, or finding a lower vitamin D level, does not automatically mean that adding vitamin D treats the disease.

A nutrient can be an important part of a much larger system without being the whole solution.

What about cancer?

Researchers have been studying vitamin D and cancer in dogs for years. Some tumor types have been associated with lower 25(OH)D levels or altered vitamin D metabolism. Lymphoma, osteosarcoma and mast cell tumors are among those that have been studied. The results are not identical across all tumor types, and an observational study cannot by itself tell us which came first.

For that reason, “lower vitamin D has been found in cancer” should not turn into the conclusion that “vitamin D supplementation prevents or treats cancer”. An association is valuable information because it gives us the next question, not a ready-made answer.

When might it make sense to test vitamin D?

Integrative veterinarian Dr Judy Morgan recommends monitoring vitamin D status regularly in her educational materials and has even suggested annual testing.

I would not treat that as an automatic screening test that every healthy dog must have.

Testing can nevertheless be especially interesting when a dog has long-term digestive or absorption problems, chronic disease, changes in kidney function, recurring inflammatory issues or another reason to suspect that nutrient absorption or metabolism may not be typical. A blood test is also very sensible when a dog receives vitamin D in a therapeutic amount as a supplement.

In those situations, the goal is not merely to obtain a baseline value. There should also be a plan for when to reassess it.

Dr Morgan’s material uses roughly 8–10 weeks as a guide for the time in which a new balance may develop after a dose change. The actual timing of a repeat test should still depend on the product being used, the dose and the dog’s health status.

Should you simply give a dog extra vitamin D?

Vitamin D is one of those nutrients where “a little extra just in case” is not a particularly good strategy. It is fat-soluble. Unlike many water-soluble vitamins, the body does not simply get rid of excess vitamin D in urine.

Too much can accumulate and disturb calcium and phosphorus balance and damage the kidneys. The FDA lists vomiting, reduced appetite, increased thirst and urination, excessive drooling and weight loss among signs of vitamin D toxicity. In severe cases, excessive vitamin D can cause kidney failure and can be life-threatening.

For the same reason, I would not take a human vitamin D product and simply scale the human dose down according to the dog’s body weight.

The active ingredient name is not enough. The product strength, form of vitamin D, amount already supplied by the main diet, other supplements, calcium and phosphorus balance, the dog’s health status and medications all matter.

A natural source does not mean an unlimited amount

Fatty fish and eggs can be excellent sources of vitamin D and can fit well into a varied diet. That still does not mean that a suspected vitamin D deficiency should be treated by guessing with larger amounts of fish, egg yolks or cod liver oil. Cod liver oil in particular also delivers vitamin A along with vitamin D.

A food can be an excellent nutrient source, but a nutrient source is not automatically a therapeutic dose. The same principle applies to vitamins and minerals in general.

With an older dog, look at the whole picture

As dogs age, their energy needs, muscle mass, digestion, risk of disease and often the number of medications they take can change. That means the same diet or supplement routine that suited a dog five years ago may not remain optimal for the rest of life.

Vitamin D is only one part of that picture.

For an older dog, it is worth looking at protein, energy, muscle mass, fatty acids, digestion, minerals, vitamins and actual body condition together. I discuss this in more detail in the article “Senior dog food: what should change in an older dog’s bowl?”.

Ask seven questions before adding vitamin D

1. Why do I want to give my dog extra vitamin D?
2. How much vitamin D is the dog already receiving from the main diet and other supplements?
3. Has the dog’s 25(OH)D level been measured?
4. Which laboratory and method were used, and how does that laboratory interpret the result?
5. Does the dog have diseases or medications that may affect vitamin D metabolism or the safety of supplementation?
6. Which form and amount of vitamin D am I actually giving?
7. When and how will we check whether the chosen amount is suitable for this dog?

If you do not have answers to those questions, buying the next bottle is not yet a solution.

In summary: vitamin D matters, but one number does not tell the whole story

Dogs need vitamin D and, unlike humans, depend mainly on their diet to obtain it. Vitamin D status is usually assessed by measuring 25(OH)D in blood. Lower 25(OH)D has been associated with several diseases, but an association does not automatically show causation, and it does not mean that supplementation will magically improve the disease.

A low result should still raise useful questions. Is the dog getting enough vitamin D from food? How well is it being absorbed? Is there gastrointestinal, kidney or another disease that may influence the level? How do medications or supplements affect the number and the broader picture? Is low vitamin D part of the problem, or a consequence of disease?

The same kind of question matters when the level is high. Vitamin D is fat-soluble and excessive amounts can be dangerous. Neither ignoring deficiency nor adding vitamin D blindly is a good strategy. The approach that makes the most sense is the same one I use in dog nutrition generally: look at the whole picture, look for the reason, change one thing thoughtfully and watch what the dog’s body tells you in response.

When you are trying to connect laboratory results, symptoms and everyday observations, the owner’s own information matters a great deal too. You can read more about that in “Veterinarian and pet owner working together: why your observations matter”.

Frequently asked questions

Can dogs get vitamin D from sunlight?

A dog’s skin does not produce enough vitamin D through UVB exposure to meet its needs. Dogs therefore obtain the vitamin D they require mainly from food.

Which vitamin D value is measured in a dog’s blood test?

Vitamin D status is usually assessed using blood 25-hydroxyvitamin D, or 25(OH)D. It should not be confused with vitamin D3 in food or supplements, or with active calcitriol.

Can a dog be given a human vitamin D supplement?

Sometimes a human product may be used after veterinary or nutritional assessment, but a human dose should not simply be recalculated according to a dog’s body weight. Product strength, the full formulation, the dog’s diet and health, and other products being used all need to be considered.

Can too much vitamin D be dangerous for a dog?

Yes. Vitamin D is fat-soluble, and excessive amounts can cause dangerous disturbances in calcium and phosphorus metabolism and damage the kidneys. Vitamin D supplements should therefore not be used on a “just in case” basis.