Parathyroid hormone (PTH) in dogs and cats
Parathyroid hormone (PTH) is made by tiny glands next to the thyroid and keeps blood calcium steady. A PTH value only makes sense next to ionized calcium: the same number can be fitting in one situation and clearly wrong in another. It is a specialised test, usually ordered to find out why calcium is high or low.
What it shows
The parathyroid glands sense free ionized calcium. When it drops, they release PTH; when it rises, secretion stops. The link is steep — very small changes in calcium cause large changes in PTH. PTH raises calcium in three ways: it releases calcium from bone, makes the kidneys keep calcium and lose phosphate, and switches on the kidney step that makes active vitamin D (calcitriol), which increases absorption from the gut. The net effect of high PTH is more calcium and less phosphate in the blood.
The hormone lasts only 3–5 minutes in the blood, and enzymes in a sample break it down. Labs use assays for the intact molecule (on forms often iPTH); assays for one end fragment of the hormone read falsely high when the kidneys filter poorly.
Higher than reference — what it may be a sign of
- Primary hyperparathyroidism — a parathyroid gland (for example, a benign adenoma) makes PTH regardless of calcium. The tell-tale pattern is high ionized calcium with PTH that is high or even inside the range: in an animal with high calcium, a PTH in the range is already inappropriate.
- Kidney disease (renal secondary hyperparathyroidism) — in chronic kidney disease of dogs and cats, low active vitamin D and lower ionized calcium push PTH up. Ionized calcium is low or normal, while total calcium can be normal or even high. In some animals the bone loss is severe enough to soften the jaw (“rubber jaw”).
Lower than reference
Low PTH is the expected, healthy answer to high calcium. When calcium is high and PTH is low or undetectable, the cause lies outside the parathyroid glands: cancer that releases a PTH-like protein (PTHrP) — for example anal sac adenocarcinoma, some lymphomas or squamous cell carcinoma — or too much vitamin D. A vet reads low PTH together with low calcium as confirming primary hypoparathyroidism, where the glands do not make enough hormone. A sample that thawed on the way can also give a falsely low result, because the hormone breaks down.
Look at together with
- Ionized calcium (see calcium) — indispensable: PTH is always read against it.
- Phosphorus — low in primary hyperparathyroidism; high or normal in chronic kidney disease and high in vitamin D excess.
- Creatinine and urea — to see whether the kidneys explain a high PTH.
- PTHrP and 25-hydroxyvitamin D — the usual partners in a calcium work-up: PTHrP is high in cancer-related high calcium, 25-hydroxyvitamin D in vitamin D intoxication.
- Calcium × phosphorus product — a calculated row that shows calcium and phosphate rising together.
Units and conversion
PTH is reported in pmol/L or pg/mL. One pg/mL equals about 0.106 pmol/L, so 50 pg/mL is about 5.3 pmol/L. Intact PTH (iPTH) uses the same units. In the app each row keeps the unit printed on the form, and the chart converts results to one unit.
Reference range
The reference range is on your lab’s form — but for PTH the range alone does not answer the question. The same value inside the range can be fitting with normal calcium and clearly inappropriate with high ionized calcium. In one study of dogs with confirmed primary hyperparathyroidism, 75% had PTH inside the reference range, which is why PTH is always paired with ionized calcium. Handling matters too: the sample is frozen and has to stay frozen in transit, because enzymes degrade the hormone.
Cats and dogs
Primary hyperparathyroidism is less common in cats than in dogs; in cats the gland change is most often an adenoma, sometimes with large cysts. In cats with chronic kidney disease, raised PTH is part of a disturbed calcium–phosphate balance, and high levels of FGF23, another phosphate-regulating hormone, are seen in these cats too. In idiopathic hypercalcaemia of cats PTH is low or normal, and the diagnosis is made only after other causes are excluded.
Most dogs with primary hyperparathyroidism are older than 7 years, with no sex predilection, and in Keeshonds the disease is inherited. Affected dogs have high total and ionized calcium, low phosphate and urine that is not concentrated. In dogs with chronic kidney disease, bone changes from renal secondary hyperparathyroidism — loose teeth, a soft jaw — are uncommon and seen mainly in young dogs with severe congenital kidney disease.
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) — Minerals: overview (calcium, phosphate and their hormones)
- eClinPath (Cornell University) — Calcium
- eClinPath (Cornell University) — Phosphate
- Merck Veterinary Manual — Hypercalcemia in Dogs and Cats
- Merck Veterinary Manual — Physiology and Calcium-Regulating Hormones in Dogs and Cats
- Merck Veterinary Manual — Renal Dysfunction in Dogs and Cats
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.