Total T4

A low T4 raises the possibility of an underactive thyroid (hypothyroidism) — overwhelmingly a dog disease, typically in middle age, causing weight gain, lethargy, a poor coat and skin problems. But there is a crucial trap: any unrelated illness can lower T4 without the thyroid being at fault, a phenomenon called "sick euthyroid" (or non-thyroidal illness). Because of this, a low T4 alone does not diagnose hypothyroidism. It is confirmed with additional tests — free T4, thyroid-stimulating hormone (TSH) — and always read against the clinical picture, so an unwell dog is not mislabelled as hypothyroid on a single low value.

A high T4 raises an overactive thyroid (hyperthyroidism) — overwhelmingly an older cat disease, and one of the most common conditions of senior cats. It speeds everything up: weight loss despite a big appetite, restlessness, a fast heart rate, sometimes vomiting or diarrhoea. A clearly high T4 in an older cat with these signs is often diagnostic on its own; a borderline value in a suspicious cat may need repeating or a free T4 to confirm, because early hyperthyroidism can flicker in and out of the range.

T4 is central to monitoring treatment of both conditions. In a hypothyroid dog on thyroid supplementation, T4 is checked to ensure the dose has restored a normal level. In a hyperthyroid cat on medication, T4 is tracked to confirm it has been brought back into range without overshooting into a low (over-treated) state. Watching T4 over time is how these lifelong conditions are kept in balance, and it is read alongside kidney values in cats, because treating hyperthyroidism can unmask underlying kidney disease.

This is the clearest example in the whole panel of species determining meaning. The same gland, the same hormone — but a low T4 sends you looking for canine hypothyroidism (with the sick-euthyroid caveat firmly in mind), while a high T4 sends you looking for feline hyperthyroidism. Reading a T4 without knowing the species and the direction of change is meaningless; reading it with both is powerful.

Total T4 is measured from a blood sample, in µg/dL (US) or nmol/L (UK/SI). It is the usual screening thyroid test; free T4 (the active, unbound fraction) and TSH are added to confirm and to sort out the sick-euthyroid trap. No special preparation is needed, though some medicines affect thyroid tests, so your vet will factor those in.

Because a single T4 can mislead in both directions — lowered by unrelated illness in dogs, borderline in early feline cases — the constructive step is to interpret it with the clinical signs and confirmatory tests (free T4, TSH) rather than in isolation. Once a diagnosis is made, tracking T4 over time is how the treatment of these common, very manageable conditions is guided.