TSH (thyroid-stimulating hormone) in dogs and cats
TSH is the pituitary hormone that tells the thyroid to make T4. When the thyroid fails, TSH usually rises, so in dogs a high TSH next to a low T4 supports hypothyroidism. In dogs the test is canine TSH (cTSH); in cats its use is limited, and in either species it is never read on its own.
What it shows
The pituitary gland at the base of the brain releases thyroid-stimulating hormone (TSH), and the thyroid answers by making T4. The two are linked by feedback: when T4 falls, the pituitary releases more TSH; when T4 is high, TSH is held down. If the thyroid gland itself is failing, the pituitary keeps pushing, and TSH climbs.
TSH is measured with a veterinary test; on forms for dogs it appears as cTSH — canine TSH. On its own it is a weak test: a high value suggests hypothyroidism, but a normal or low value does not rule it out. Its real value is in combination with total T4 and free T4.
Higher than reference — what it may be a sign of
- Primary hypothyroidism in dogs — high TSH with low total T4 and low free T4 is the classic pattern of a failing thyroid gland.
- Another illness — TSH is increased in 8–10% of dogs with nonthyroidal illness and a healthy thyroid, so a high TSH with a normal free T4 is not proof.
- An underactive thyroid in a cat — cats that show signs of low thyroid function after radioiodine generally have high TSH; the same pattern, with low or low-normal T4, is used to diagnose hypothyroidism in cats.
Lower than reference
In cats with hyperthyroidism TSH is generally low — the feedback described above at work. Even so, TSH is not sensitive enough to diagnose hyperthyroidism and is read only together with total T4 and free T4.
In dogs a normal or low TSH does not exclude hypothyroidism: TSH stays inside the reference range in 20–40% of dogs with confirmed disease. Most of these dogs still have a problem in the thyroid itself; failure of the pituitary is extremely rare. A normal TSH next to a low total T4 and a normal free T4 fits a dog that is ill with something else rather than a thyroid problem.
Look at together with
- Total T4 and free T4 — TSH means little without them; normal TSH, free T4 and total T4 together effectively rule out hypothyroidism in a dog.
- Cholesterol — raised in about 80% of hypothyroid dogs, a useful background clue.
- Hematocrit — a mild non-regenerative anaemia appears in 40–50% of hypothyroid dogs.
- Thyroglobulin autoantibodies — found in up to half of hypothyroid dogs; they point to inflammation of the gland but say nothing about how well it works.
Units and conversion
Canine TSH is reported in ng/mL; some laboratories write the same quantity as µg/L. The two are equal — 1 ng/mL is 1 µg/L — so nothing needs recalculating. Because the assay is species-specific, a result is read against the range of the same test. In the app each row keeps the unit printed on the form.
Reference range
The reference range is on your lab’s form. For TSH it is especially tied to the test, because assays are made for a particular species. The upper limit matters most: a value above it supports hypothyroidism when T4 is low. A value inside the range is common in hypothyroid dogs, so “in range” here does not mean “the thyroid is fine”. A vet reads TSH together with the history, signs, routine blood tests and total or free T4 measured in the same sample.
Cats and dogs
TSH testing is offered for both dogs and cats, but in cats its role is limited. A feline-specific assay exists, and the canine assay has also been used in cats. It helps confirm an underactive thyroid, which in cats usually follows radioiodine, thyroid surgery or antithyroid medication, when TSH rises. In hyperthyroidism TSH is generally low but not sensitive enough for a diagnosis, so total T4 and free T4 remain the main tests.
In dogs canine TSH is a useful addition to T4 tests, not a stand-alone answer. A high TSH with low T4 supports hypothyroidism, which is most common at 4–10 years in mid- to large-size breeds, yet a normal TSH is found in 20–40% of hypothyroid dogs. Because sick dogs can also show raised TSH, thyroid testing is postponed until another illness has settled, when that is possible.
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
- Merck Veterinary Manual — Hypothyroidism in Animals
- Merck Veterinary Manual — Hyperthyroidism in Animals
- Cornell Animal Health Diagnostic Center — Canine Thyroid Testing
- Cornell Animal Health Diagnostic Center — Feline Thyroid Tests
- Cornell Animal Health Diagnostic Center — Thyroid Testing Interpretation
- VCA Animal Hospitals — Thyroid Hormone Testing in Dogs
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.