Total T4 is the screening test, free T4 (fT4) is the tiebreaker, and the reference interval belongs to the laboratory that printed it. A cat with a T4 of 3.6 µg/dL, a heart rate of 220, and 12% weight loss over 6 months is not a normal cat even though 3.6 sits inside many printed intervals. Three interference categories — non-thyroidal illness, drugs, and the assay itself — explain most of the confusing results, and they are reasons to repeat a test, not to conclude anything from 1 number.
What total T4 actually measures
More than 99% of circulating thyroxine rides on binding proteins; under 1% is free and biologically active. Total T4 therefore reflects 2 things at once: how much hormone the thyroid is making, and how much binding protein is available to carry it. Free T4 by equilibrium dialysis measures the small unbound fraction directly and is far less sensitive to protein shifts, which is why it costs 2 to 4 times as much and takes 1 to 3 days to return. Typical feline intervals sit at 0.8 to 4.0 µg/dL (10 to 51 nmol/L) for total T4 and 0.7 to 2.6 ng/dL (9 to 34 pmol/L) for fT4 by dialysis. Textbook prevalence of hyperthyroidism in cats over 10 is usually given as 2% to 11%, and 98% of those cases are benign adenomas rather than carcinoma.
Reference intervals move with the method
Two laboratories can print different upper limits for the same cat and both be right, because immunoassay platforms, calibration, and antibody pairs differ. Some feline total T4 intervals end at 4.0 µg/dL, others at 4.5, and a few reference populations push the top to 5.0. That is a 1.0 µg/dL spread, which is wider than the difference between a clearly hyperthyroid cat and a borderline one. Practical rule: keep every sample with the same laboratory and the same method for the life of the cat, and read the interval printed on that report. A result between 2.5 and 4.0 µg/dL is the gray zone where clinical signs, a palpable thyroid nodule (present in roughly 90% of cases), and a repeat test decide more than the number does.
Interference 1: non-thyroidal illness lowers T4
Sickness suppresses thyroxine. Chronic kidney disease, diabetes mellitus, neoplasia, feline infectious peritonitis, and any febrile or anorexic state can push total T4 to 0.5 to 1.2 µg/dL, well below the reference interval, in a cat whose thyroid is perfectly normal. The mechanism is reduced peripheral conversion and altered protein binding, not thyroid failure; true hypothyroidism is rare enough in cats that a low T4 is treated as an interference factor first. This is why a T4 drawn during a crisis is close to uninterpretable, and why the value is usually repeated 4 to 8 weeks after the cat is stable and eating again.
Interference 2: drugs shift the number
Glucocorticoids lower total T4, sometimes within 1 to 2 weeks of starting, and the effect scales with dose and duration. Phenobarbital and other anticonvulsants accelerate hormone clearance and can drop T4 by 30% to 50%. Sulfonamide antibiotics suppress thyroid hormone synthesis during the weeks they are given. If a cat is on any of these, the T4 result describes the drug as much as the gland, and most clinicians note the drug on the lab form and interpret against a wider band rather than a strict cut point. Timing matters too: T4 peaks about 4 to 6 hours after a dose in a cat receiving thyroid hormone, so samples taken at random hours are not comparable with each other.
Interference 3: the assay itself
Analog free T4 assays are the commonest source of a number that does not fit the cat. Analog methods are sensitive to albumin concentration and to binding-protein changes, so a hypoalbuminemic cat can read falsely low, and a cat with anti-T4 antibodies can read falsely high or low depending on the platform. Hemolysis above roughly 1 g/dL of free hemoglobin, visible lipemia, and a sample left unseparated for more than 2 to 4 hours all shift results. Equilibrium dialysis removes most of these effects but not all: heterophile antibodies still interfere. When the clinical picture and the number disagree, the useful next step is a different method, not a bigger dose of belief in the first result.
When to repeat, and how often to screen
A borderline value with no clinical signs gets a repeat in 2 to 4 weeks, at the same laboratory, at the same time of day. A cat with weight loss, a heart rate above 200 at rest, and a palpable nodule but a normal T4 gets fT4 by dialysis plus a repeat total T4. Cats over 10 with no signs are screened every 6 to 12 months as part of the senior panel. After any change in thyroid status, the meaningful recheck window is 3 to 6 months, because kidney values and blood pressure often shift once the thyroid hormone level settles. Blood pressure should be measured at each of those visits; 160 mmHg or higher is a separate problem worth its own plan.
The panel at a glance
| Test | Conventional units | SI units | Turnaround | Main interference |
|---|---|---|---|---|
| Total T4 | 0.8 to 4.0 µg/dL | 10 to 51 nmol/L | same day | non-thyroidal illness, glucocorticoids, phenobarbital |
| fT4 by dialysis | 0.7 to 2.6 ng/dL | 9 to 34 pmol/L | 1 to 3 days | heterophile antibodies, sample age |
| fT4 analog | 0.6 to 3.1 ng/dL | 8 to 40 pmol/L | same day | albumin level, lipemia, hemolysis |
| TSH | 0.03 to 0.5 ng/mL | 0.03 to 0.5 µg/L | 1 to 2 days | low sensitivity in cats, rarely diagnostic alone |
| T3 | 0.5 to 1.5 ng/mL | 0.8 to 2.3 nmol/L | same day | poor screening test, normal in 30% of hyperthyroid cats |
Five steps for the next appointment
- Ask which laboratory and which method produced the T4, and file the printed interval with the result.
- Request fT4 by equilibrium dialysis, not the analog version, when total T4 is 2.5 to 4.0 µg/dL with clinical signs.
- List every drug given in the past 8 weeks, including steroids, anticonvulsants, and sulfa antibiotics.
- Never draw the sample during vomiting, anorexia, or dehydration if the question is thyroid function; wait until the cat has eaten normally for 7 days.
- Repeat a borderline result in 2 to 4 weeks at the same laboratory before acting on it.
Go to the clinic the same day for a resting heart rate above 240, labored breathing, a systolic blood pressure of 180 mmHg or higher, or a cat that has stopped eating for 24 hours. Those findings outrank any single hormone value. Background on how the gland works sits in the notes on thyroid physiology and the hormone axis; the breed quiz covers which lines are screened earlier.
Terms mentioned in this article
Frequently asked
Why can a hyperthyroid cat have a normal total T4?
Total T4 fluctuates, and early or mild disease often sits inside the printed interval, especially when the upper limit is 4.5 or 5.0 µg/dL. Non-thyroidal illness can also drag the value down into the normal band. That is why a cat with weight loss, a heart rate above 200 and a palpable nodule gets free T4 by equilibrium dialysis plus a repeat total T4 in 2 to 4 weeks.
Does a low T4 mean my cat has hypothyroidism?
Almost never. True hypothyroidism is rare in cats, and a low total T4 is usually an interference factor from another illness, from glucocorticoids, or from phenobarbital. Chronic kidney disease, diabetes, and any febrile or anorexic state can push T4 to 0.5 to 1.2 µg/dL in a cat with a healthy thyroid, so the value is repeated 4 to 8 weeks after the cat is stable.
Which free T4 test should I ask for?
Equilibrium dialysis. The analog method is cheaper and faster but shifts with albumin concentration, lipemia, and hemolysis, which is exactly the situation in a sick cat. Dialysis costs more and takes 1 to 3 days, yet it removes most protein-binding effects, and it is the version most laboratories recommend when total T4 and the clinical picture disagree.