TSH (thyroid stimulating hormone)
Thyroid ·Also called: TSH, cTSH, thyrotropin, thyroid stimulating hormone
What it is
TSH is made in the brain, not the thyroid, and it regulates thyroid output through a feedback loop. The assay used in veterinary practice measures canine TSH, and it is far more informative in dogs than in cats.
Why your vet measures it
Pairing TSH with T4 separates true thyroid failure from the suppression that any illness causes. Low T4 with high TSH means the gland itself is failing. Low T4 with normal TSH usually means the dog is unwell for another reason.
What a high result can mean
- Hypothyroidism in dogs, the classic pairing with a low T4
- Recovery from a non-thyroidal illness
- Under-dosing in a dog already on thyroid supplementation
What a low result can mean
- Hyperthyroidism in cats, where the excess hormone suppresses the pituitary signal — an undetectable TSH supports the diagnosis
- Steroid treatment
- Around a quarter of genuinely hypothyroid dogs have a normal TSH, so a normal value does not rule the condition out
Dogs and cats differ
The test was developed for dogs and performs best there. In cats it is used the other way round: a low or undetectable TSH helps confirm early hyperthyroidism in a cat whose T4 is still borderline.
Questions worth asking your vet
- Do the T4 and TSH results agree with each other?
- If TSH is normal but my dog has the signs, is a trial of treatment or a repeat test reasonable?
- For a cat with borderline T4, does the TSH support hyperthyroidism?
Related terms
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