Calcitriol (1,25-dihydroxyvitamin D) in dogs and cats
Calcitriol, or 1,25-dihydroxyvitamin D, is the most active form of vitamin D. The kidneys make it, and it raises the absorption of calcium and phosphate from the gut. It is measured infrequently — usually while a vet is working out why calcium is high or low, next to PTH and 25-hydroxyvitamin D.
What it shows
Vitamin D from food goes to the liver and becomes 25-hydroxyvitamin D (calcidiol). In the kidney tubules an enzyme, 1α-hydroxylase, turns part of it into calcitriol, which is far more active. Calcitriol increases the uptake of calcium and phosphate from the gut, helps the kidneys hold on to calcium, supports the effect of parathyroid hormone (PTH) on bone and signals the parathyroid glands to slow down.
The body adjusts calcitriol all the time. PTH, low ionized calcium and low phosphate switch production up; high calcium, high phosphate, the phosphate-regulating hormone FGF23 and calcitriol itself switch it down. This is the key difference from 25-hydroxyvitamin D: calcidiol reflects the animal’s vitamin D supply, while calcitriol shows what the kidneys and the calcium-control system are doing right now. A low calcidiol can even reflect brisk conversion into calcitriol.
Higher than reference — what it may be a sign of
- Granulomatous disease and some lymphomas. Macrophages contain the same activating enzyme and can make active vitamin D outside the kidneys’ control. This has been proposed as the cause of high calcium in dogs with granulomatous disease, such as some fungal infections, and in some dogs with lymphoma.
- Primary hyperparathyroidism. Excess PTH speeds up conversion, so calcitriol can be raised or inside the range.
- Vitamin D excess. Calcitriol can be raised. For poisoning with ordinary vitamin D (cholecalciferol, for example in some rodent baits) the specific test is 25-hydroxyvitamin D, not calcitriol.
- Young age. Kittens had higher calcitriol at 3 and 6 months of age.
- Calcitriol given as a medicine. It is used in some kidney and parathyroid conditions; a vet reads the value knowing this.
A high value is a sign, not a diagnosis: with high calcium the healthy response is to lower calcitriol, so the vet reads it together with calcium, phosphate and PTH.
Lower than reference
- Kidney disease. Low calcitriol is seen in some dogs and cats with kidney disease: the failing tubule cells make less of the enzyme, and high FGF23 is thought to be one of the main mechanisms in chronic kidney disease. Less calcitriol means less calcium absorbed and a push towards higher PTH.
- Primary hypoparathyroidism. Without enough PTH, too little vitamin D is activated, which adds to low calcium.
- High calcium from another cause. Lowering calcitriol is the expected response.
Look at together with
- Calcium, ideally ionized — calcitriol is read only against it.
- Phosphorus — calcium and phosphate high together fit vitamin D excess.
- PTH — the main driver of calcitriol production.
- 25-hydroxyvitamin D — the supply side; the usual test for vitamin D status and poisoning.
- Creatinine — kidney function, since the kidneys make calcitriol.
- PTHrP — the other candidate when calcium is high and PTH is low.
Units and conversion
Calcitriol is reported in pmol/L or pg/mL. One pg/mL equals 2.4 pmol/L, so 50 pg/mL is 120 pmol/L. In the app each row keeps the unit printed on the form; the chart converts to pmol/L so forms from different labs line up.
Reference range
The reference range is on your lab’s form. Assays for vitamin D metabolites differ, and liquid chromatography with tandem mass spectrometry is considered the gold standard, so values from different labs are not directly comparable. Age matters too — young animals can have higher values. Calcitriol has no meaning on its own: the vet reads it against calcium, phosphate, PTH and 25-hydroxyvitamin D from the same visit.
Cats and dogs
In cats, as in dogs, calcitriol is not a routine test. Kittens had higher values at 3 and 6 months of age. In idiopathic hypercalcaemia, the most common form of high calcium in cats, calcitriol is listed as variable, and vitamin D metabolites are part of ruling out other causes. In cats with kidney disease calcitriol can be low.
In dogs calcitriol helps mainly when calcium is high with a low PTH: granulomatous disease, for example fungal infections, and some lymphomas can produce it outside the kidneys. A high calcitriol was suspected to cause hereditary vitamin D-resistant rickets in a 5-month-old Pomeranian — a single case. In dogs with kidney disease calcitriol can be low.
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.
- eClinPath (Cornell University College of Veterinary Medicine). Minerals: overview (calcium, phosphate and their hormones).
- eClinPath (Cornell University College of Veterinary Medicine). Calcium — 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.
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.