25-hydroxyvitamin D in cats and dogs
25-hydroxyvitamin D, written 25(OH)D or calcidiol, is the form of vitamin D that labs measure to judge an animal’s vitamin D status. It is not a routine test: a vet usually orders it when blood calcium is high and the cause is not yet clear, or when vitamin D poisoning is suspected.
What it shows
Unlike people, dogs and cats make very little vitamin D in their skin, so they rely on food for it. Vitamin D from food is carried to the liver and turned into 25-hydroxyvitamin D (calcidiol). The kidneys then turn part of it into calcitriol — the most active form, which raises calcium and phosphorus absorption from the gut and helps bones form.
Calcidiol is the most commonly measured form and is used to assess vitamin D status. It is not the active hormone, though: calcitriol is a separate test. Values in healthy animals vary widely, largely with diet, so a vet reads 25(OH)D together with calcium rather than on its own.
Higher than reference — what it may be a sign of
- Vitamin D poisoning. Eating rodent bait with cholecalciferol (vitamin D3), vitamin D supplements, or vitamin D analogues such as calcipotriene. A high 25(OH)D is the specific confirmation of such poisoning. Laboratory changes often appear within 12–48 hours of eating it. This is an emergency — the vet decides what to do.
- Too much vitamin D in food. Excess vitamin D added to commercial pet food by mistake has been linked to vitamin D toxicity.
- After the episode. Calcidiol can stay above the reference range for weeks or months after calcium has come back down, so a high result may reflect an earlier exposure.
Excess vitamin D raises calcium and phosphorus and can lead to mineral deposits in the kidneys, heart and other soft tissues — this is why a vet takes it seriously.
Lower than reference
A low 25(OH)D has been described with gut disease such as inflammatory bowel disease (poorer absorption), with inflammation (calcidiol falls as part of the inflammatory response — shown in people), and in dogs and cats with various cancers, where it is unclear whether it is a cause or a result. A diet without enough vitamin D can cause rickets in young animals, but this is rare on complete, balanced commercial food and is seen mainly with unsupplemented home-made diets. A low value does not always mean low vitamin D overall, because the kidneys keep converting calcidiol into the active form.
Look at together with
- Calcium, ideally ionized calcium — 25(OH)D is interpreted relative to calcium.
- Phosphorus — rises together with calcium in vitamin D excess.
- Parathyroid hormone (PTH) and PTH-related protein — in a high-calcium workup vitamin D is usually checked after these point away from a parathyroid tumour or cancer.
- Calcitriol — the active form, ordered separately.
- Creatinine — the kidneys make calcitriol and are also damaged by mineral deposits.
Units and conversion
Labs report 25(OH)D in nmol/L or ng/mL. One ng/mL equals 2.496 nmol/L, so 30 ng/mL is about 75 nmol/L. In the app each row keeps the unit printed on the form; the chart converts to nmol/L so forms from different labs line up.
Reference range
The reference range is on your lab’s form. For 25(OH)D it depends strongly on the lab: methods differ (immunoassays and liquid chromatography with mass spectrometry, which is considered the most accurate), and healthy animals show a wide spread largely because of diet. A value is read against the interval printed next to it and against the animal’s calcium, not against a number from another lab.
Cats and dogs
In cats the most common form of high calcium is idiopathic hypercalcaemia — a diagnosis made once other causes are excluded, and vitamin D metabolites are among the tests considered on the way; in this condition calcidiol is normal or low. Vitamin D poisoning from rodent bait, supplements or vitamin D analogues is possible in cats as well. In growing kittens calcidiol was found lowest at three months of age.
In dogs 25(OH)D is mostly ordered in a high-calcium workup or when rodent bait with cholecalciferol may have been eaten. In some dogs with granulomatous disease or lymphoma, high calcium is linked to calcitriol made by immune cells rather than to 25(OH)D. In studies, dogs with lower vitamin D concentrations had a higher cancer risk — an association, not proof of cause.
Add a lab form — Pet Tracker Space recognises the lines, keeps the form’s units and draws each indicator over time with your lab’s reference band.
Sources
- eClinPath (Cornell University College of Veterinary Medicine). Vitamin D — chemistry / minerals.
- Ham K. Physiology and Calcium-Regulating Hormones in Dogs and Cats. Merck Veterinary Manual, professional version, full review June 2023.
- Ham K. Hypercalcemia in Dogs and Cats. Merck Veterinary Manual, professional version, full review June 2023.
- Tauer D. Cholecalciferol (Vitamin D3) Poisoning in Animals. Merck Veterinary Manual, professional version, full review November 2025.
- Michigan State University Veterinary Diagnostic Laboratory. Spotlight on Endocrinology at the MSU VDL (newsletter, winter 2020).
- Sanderson SL. Nutritional Requirements of Small Animals. Merck Veterinary Manual, professional version, full review November 2023.
This page explains what a lab value means. It is not a diagnosis and not a substitute for your veterinarian — only they can interpret results for your pet.