Total T4 (thyroxine) in cats and dogs
Total T4 is the main hormone of the thyroid gland, measured together with the part carried on blood proteins. On a lab form it is the usual first thyroid test: in older cats it is used to look for an overactive thyroid, in dogs to screen for an underactive one. Other illnesses and some medicines lower it, so one low value is never a diagnosis on its own.
What it shows
The thyroid gland releases thyroxine (T4), a hormone that sets the pace of metabolism. Almost all T4 in the blood rides on carrier proteins; only a tiny share is free. “Total” T4 counts both together.
The test reads differently in the two species. In cats a high total T4 is the hallmark of hyperthyroidism. In dogs total T4 is a good first screen for hypothyroidism: a value well inside the reference range makes it unlikely. A low value is not specific — a dog with low T4 may have a healthy thyroid and be ill with something else.
Higher than reference — what it may be a sign of
- Hyperthyroidism in cats — an overactive thyroid, most often from a benign growth (adenoma or adenomatous hyperplasia); in about 70% of cats both lobes are affected.
- A thyroid tumour in a dog — in dogs an overactive thyroid is almost always linked to thyroid carcinoma.
- Thyroid hormone tablets — in a dog receiving thyroid hormone, the value reflects the tablets.
- Antibodies against T4 — rare antibodies in some dogs make the measured value falsely high.
Lower than reference
In dogs: hypothyroidism — in more than 95% of clinical cases the thyroid gland itself is destroyed, mostly by lymphocytic thyroiditis or idiopathic atrophy. But low T4 has many other causes: other illnesses (“euthyroid sick”: T4 falls roughly in proportion to how sick the animal is), some medicines — among them glucocorticoids, sulfonamides and phenobarbital, with effects that reverse after they are stopped, age (T4 slowly declines in elderly dogs) and breed. In cats a low T4 is usually due to an illness other than thyroid disease. Naturally occurring low thyroid function is extremely rare in adult cats; when it happens, it usually follows radioiodine, thyroid surgery or antithyroid medication.
Look at together with
- Free T4 — the next step when total T4 is borderline.
- TSH — in a dog, low T4 together with high TSH supports hypothyroidism; TSH is never read on its own.
- Cholesterol — high in about 80% of hypothyroid dogs; a mild non-regenerative anaemia is seen in 40–50% of them.
- Creatinine, urea and urine specific gravity — hyperthyroidism in cats can mask chronic kidney disease, which may show up once thyroid function returns to normal.
- ALT and ALP — increases are common in hyperthyroid cats.
Units and conversion
Total T4 is reported in nmol/L or µg/dL; some forms use ng/mL. One µg/dL equals 12.87 nmol/L, so 2.0 µg/dL is about 26 nmol/L; one ng/mL equals 1.287 nmol/L. Do not confuse it with free T4: that row is printed in pmol/L or ng/dL, units that look similar but are a thousand times smaller. In the app each row keeps the unit printed on the form; the chart converts to one unit so forms from different labs line up.
Reference range
The reference range is on your lab’s form. It depends on the method and the laboratory. Some manufacturers also mark an ambiguous zone: IDEXX’s guide to its in-clinic total T4 test calls 2.3–4.7 µg/dL (30–60 nmol/L) in older or symptomatic cats a gray zone, and 1.0–2.0 µg/dL (13–26 nmol/L) in dogs low-normal and ambiguous. These are values for one test, not a reference range — a result there leads to further tests, not to a conclusion. T4 also fluctuates during the day.
Cats and dogs
Hyperthyroidism is a common disease of middle-aged and older cats, most common over the age of seven. About 5–10% of affected cats still have total T4 inside the reference range — in mild or early disease, or when another illness pulls T4 down — and then free T4 or a repeat test helps. T4 also declines naturally with age, so in a senior cat with typical signs, such as weight loss despite a good appetite, a result near the top of the range deserves attention.
In dogs total T4 is a screen for hypothyroidism, which is most common at 4–10 years in mid- to large-size breeds. Greyhounds have considerably lower total T4 than most breeds, and Basenjis, Salukis, Whippets and Scottish Deerhounds can also sit below the usual range with healthy glands. That is why a vet does not diagnose hypothyroidism from one low T4, especially in a dog that is ill for another reason.
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 Feline Health Center — Hyperthyroidism in Cats
- Cornell Animal Health Diagnostic Center — Thyroid Testing Interpretation
- Cornell Animal Health Diagnostic Center — Canine Thyroid Testing
- IDEXX — Total T4 Testing Guide: canine hypothyroidism, feline hyperthyroidism (PDF)
- VCA Animal Hospitals — Thyroid Hormone Testing in Cats
- 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.