Thyroxine (T4)

What it measures

T4 is the main hormone released by the thyroid gland and affects the body's metabolic rate. The most common thyroid problem in cats is hyperthyroidism, which usually affects middle-aged and older cats, often alongside other illnesses. Total T4 on a blood panel is the most common screening test for it.

What a high value may mean

The main cause is hyperthyroidism. In older cats that already show typical signs (for example, losing weight despite a good appetite), more than 90% of hyperthyroid cats have a clearly high total T4, so the diagnosis is usually straightforward. Hyperthyroid cats also often have raised ALT and ALP.

What a low value may mean

When a cat with a normal thyroid has another serious illness, total T4 can be pushed below the reference range; this does not mean the thyroid itself is the problem. Cats treated for hyperthyroidism (for example with radioactive iodine) can develop an underactive thyroid; otherwise, naturally occurring hypothyroidism is rare in cats.

Usually read together with

Free T4 (fT4), creatinine, SDMA, ALT, and ALP. Hyperthyroidism speeds up kidney filtration and can hide an existing kidney problem, so vets usually assess kidney function at the same time.

When to follow up

If your vet suspects hyperthyroidism but T4 is in the upper half of the reference range, AAHA guidelines recommend repeating T4 together with free T4 in 2 to 4 weeks; if both are normal, other illnesses should be looked for. Cats already on treatment should have T4 and kidney values rechecked as your vet schedules.

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