Total T4 (thyroxine)
Thyroid ·Also called: T4, TT4, total thyroxine, thyroid hormone
What it is
Thyroxine sets the body's metabolic pace. Almost all of it in blood is bound to carrier proteins, with only a tiny free fraction active in tissue. Total T4 measures the whole pool, which makes it a good screening test but a slightly blunt one.
Why your vet measures it
The two species pull this test in opposite directions. In cats over ten, a high total T4 with weight loss despite a good appetite is the signature of hyperthyroidism. In dogs, a low total T4 raises the question of hypothyroidism — but many other illnesses lower it too, so a low result alone is not a diagnosis.
What a high result can mean
- Hyperthyroidism in cats, usually from a benign overgrowth of thyroid tissue
- Over-supplementation in a pet already on thyroid medication
- Rarely, a thyroid tumour in dogs
What a low result can mean
- Hypothyroidism in dogs
- Any unrelated illness, which suppresses T4 — this is called sick euthyroid syndrome and is the commonest reason for a low result
- Medications including steroids, phenobarbital, sulfonamide antibiotics and some anti-inflammatories
- Sighthound breeds, which naturally run lower than other dogs
Dogs and cats differ
Hyperthyroidism is a disease of older cats and is one of the most common feline endocrine conditions; naturally occurring hypothyroidism in cats is rare. In dogs it is exactly reversed. A greyhound's T4 sits below the standard canine range in perfect health, and an ill dog's T4 drops for reasons that have nothing to do with the thyroid — which is why TSH and free T4 are added before treatment starts.
Questions worth asking your vet
- Is my pet on any medication that could lower this reading?
- Should we add free T4 and TSH before deciding?
- If my cat is hyperthyroid, how will treatment affect the kidney values?
Related terms
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