VetVaultCat T4 blood test: thyroid levels and hyperthyroidism explained
A cat's T4 blood test measures thyroxine, the main thyroid hormone. A total T4 above your lab's reference range in a middle-aged or older cat with weight loss, a big appetite or extra thirst strongly supports hyperthyroidism. A high-normal result in a cat with signs is a grey zone, so vets often add free T4 or repeat the test.
What a T4 test measures
The thyroid is a pair of small glands in the neck. They make T4, the main thyroid hormone, and smaller amounts of the more active T3. These hormones set the pace of metabolism, including heart rate and how quickly calories are burned.
A routine thyroid test reports total T4 (sometimes written TT4): all the T4 in the sample. Most of it is bound to carrier proteins. The tiny unbound fraction is free T4 (fT4), the part that can enter cells, and it needs a separate, more specialized test.
Total T4 is often part of senior wellness panels and is usually the first test when a vet suspects an overactive thyroid. It is reported in micrograms per deciliter (µg/dL) or nanomoles per liter (nmol/L), depending on the lab.
Hyperthyroidism in cats: who gets it and the signs
Hyperthyroidism means the thyroid is producing too much hormone. It is one of the most common hormone disorders in cats and mainly affects middle-aged and older cats. The usual cause is a benign (non-cancerous) growth of thyroid tissue; thyroid cancer is uncommon. Signs tend to creep in slowly:
- Weight loss despite a good or even increased appetite
- Increased thirst and urination
- Vomiting, and sometimes diarrhea
- Restlessness, irritability or night-time vocalizing
- An unkempt, matted or greasy coat
- A fast heart rate, sometimes with a heart murmur
Some cats instead become lethargic and eat less, which makes the diagnosis less obvious. Hyperthyroidism also strains the heart and is often linked with high blood pressure, so vets usually measure blood pressure too.
How to read your cat's T4 result
A total T4 above the lab's reference interval in a cat with compatible signs strongly supports hyperthyroidism. Your vet will also feel the neck for an enlarged thyroid lobe and look at the rest of the bloodwork and a urine test.
Reference ranges differ between labs. Many laboratories use a total T4 reference interval of roughly 0.8 to 4.7 µg/dL (about 10 to 60 nmol/L), but the exact range depends on the lab and analyzer. Always use the range printed on your cat's own report. The patterns below are illustrative and approximate.
| T4 result pattern | What it usually means | What vets often do next |
|---|---|---|
| Above the range (for example > 4.7 µg/dL or > 60 nmol/L on a lab using that interval) in a cat with signs | Supports hyperthyroidism | Check the neck, kidney values, urine and blood pressure, then discuss treatment |
| Upper half of the range in a cat with signs | Grey zone: possible early hyperthyroidism, or another illness holding T4 down | Add free T4 by equilibrium dialysis, repeat total T4 in a few weeks, or refer |
| Low to middle of the range, no signs | Usually normal thyroid function | Keep as a baseline for future senior checks |
| Below the range in an untreated, unwell cat | Most often non-thyroidal illness; true hypothyroidism is rare in untreated cats | Investigate the other illness; retest later if needed |
| Above the range on treatment | Not yet controlled, or a dosing, timing or absorption issue | Review how and when the medicine is given |
| Below the range on treatment | Possible over-treatment (treatment-induced hypothyroidism) | Review kidney values and the treatment plan; sometimes add TSH |
Converting between µg/dL and nmol/L
To convert total T4 from µg/dL to nmol/L, multiply by 12.87; to go back, divide by 12.87.
| µg/dL | nmol/L |
|---|---|
| 0.8 | 10 |
| 2.0 | 26 |
| 3.0 | 39 |
| 4.7 | 60 |
| 6.0 | 77 |
| 10.0 | 129 |
Free T4 uses smaller units (often pmol/L or ng/dL), and its range depends heavily on the method, so compare it only with the range printed beside it.
The grey zone: high-normal T4 and sick cats
Two situations cause most of the uncertainty:
- High-normal T4 in a cat with signs. Early or mild hyperthyroidism can produce a total T4 in the upper part of the range rather than above it, and T4 fluctuates from day to day.
- Another illness lowering T4. Kidney disease, diabetes, gut disease, cancer and other conditions tend to push total T4 down. This is called non-thyroidal illness, or “sick euthyroid” syndrome (euthyroid means a normally working thyroid). In a hyperthyroid cat that is also ill, it can pull a high T4 back into the normal range.
When the picture is unclear, vets have several tools:
- Free T4 by equilibrium dialysis. This is the most reliable free T4 method. A high free T4 with a high-normal total T4 supports hyperthyroidism. Free T4 is always read together with total T4, because on its own it can be falsely high in sick cats whose thyroid is normal; a high free T4 with a low or low-normal total T4 points more toward non-thyroidal illness.
- Repeat testing. Rechecking total T4 after a few weeks, while watching weight and signs, often clarifies a borderline case.
- TSH. Some labs offer TSH using a dog-based assay. A low TSH fits hyperthyroidism, but many older cats with normal thyroids also have low values, so it is not used alone.
- T3 suppression test or scintigraphy. In referral settings, a T3 suppression test (giving T3 and checking whether T4 falls, as it should in a normal cat) or thyroid scintigraphy (a nuclear-medicine scan showing overactive tissue) can settle difficult cases.
Other bloodwork changes with hyperthyroidism
Excess thyroid hormone affects many organs, so other values often shift too. Our guide to cat bloodwork explains each value in more depth.
| Value | Common pattern | Likely reason |
|---|---|---|
| ALT | Mildly to moderately increased | Thyroid hormone effect on the liver; often improves once T4 is controlled |
| ALP | Often increased | Liver and bone sources |
| Red cell values (PCV, hematocrit) | Sometimes mildly higher | Thyroid hormone stimulates red cell production |
| Creatinine | Often low or low-normal | Higher kidney filtration and muscle loss |
| Urine specific gravity | May be lower (more dilute) | Increased thirst and urination |
Treatment options at a glance
There are four main treatments. Each has trade-offs involving cost, availability, the cat's temperament and other illnesses, especially kidney disease, so the choice is one to make with your vet.
| Option | What it involves | Points owners often ask about |
|---|---|---|
| Methimazole or carbimazole | Daily tablets, or methimazole gel rubbed onto the inner ear | Controls rather than cures, so usually lifelong; regular blood tests; side effects such as vomiting or facial itching are possible |
| Radioactive iodine (I-131) | Usually one injection at a licensed center, then a hospital stay | Often curative; limited availability; some cats later develop an underactive thyroid |
| Iodine-restricted diet | A prescription food fed exclusively | Works only if the cat eats nothing else, including treats or prey |
| Surgery (thyroidectomy) | Removal of the affected thyroid tissue under anesthesia | Can be curative; anesthetic risk; calcium is watched afterwards because nearby parathyroid glands can be affected |
Some cats are stabilized on tablets first, partly so the vet can see how the kidneys respond once T4 is normal.
Monitoring blood tests after treatment starts
Vets track two things: whether T4 is coming into the target range, and how the kidneys cope.
- On methimazole or carbimazole: T4 and kidney values, and often a complete blood count (CBC) and liver values, are commonly rechecked a few weeks after starting and after each dose change, then every few months once stable. Early blood counts look for uncommon side effects on white cells or platelets. Many vets aim for a T4 in the lower-to-middle part of the range.
- After radioactive iodine or surgery: T4 and kidney values are rechecked over the following months, as T4 can fall below the range.
- On an iodine-restricted diet: T4 is rechecked after several weeks of strict feeding.
Results are interpreted alongside weight, appetite, heart rate and blood pressure. Any change to the dose or plan is your vet's decision, not something to base on a single number.
The kidney link
Hyperthyroidism and chronic kidney disease (CKD) often occur together in older cats, and an overactive thyroid can hide kidney problems. Excess thyroid hormone raises the glomerular filtration rate (GFR), the speed at which the kidneys filter blood, and wastes muscle, which lowers creatinine production. Both push creatinine down, so a hyperthyroid cat can have kidney disease with normal-looking creatinine.
Once T4 is controlled, filtration returns toward the cat's true baseline and creatinine often rises. A modest rise is expected; in some cats it unmasks kidney disease that was there all along. SDMA is less affected by muscle mass but still reflects filtration, so it can rise too. That is why kidney values, urine specific gravity and blood pressure are rechecked after treatment starts, and why kidney staging is most meaningful once the thyroid is stable. Over-treatment that pushes T4 below the range can worsen kidney values.
Untreated hyperthyroidism can also harm the kidneys and heart, so the usual aim is careful control with close monitoring. See our guide to cat kidney values for creatinine, SDMA and urine specific gravity.
When to call the vet urgently
Contact your vet or an emergency clinic straight away if your cat has:
- Very fast, open-mouth or labored breathing
- Collapse or fainting
- Sudden hind-leg weakness or paralysis, often with pain or cold back feet (a possible blood clot)
- Not eaten for 24 hours or more
- Sudden blindness or widely dilated pupils (very high blood pressure can damage the eyes)
If your cat is on thyroid medication, also call promptly about yellow gums or eyes, unexplained bruising or bleeding, severe facial scratching or marked lethargy.
Questions to ask your vet
- Is this T4 clearly high, or in the grey zone for this lab?
- Would free T4 by equilibrium dialysis or a repeat T4 help?
- What are the kidney values now, and what change do you expect after treatment?
- What T4 range are we aiming for, and when is the next recheck?
Keeping each report with its date, units and reference interval makes trends easier to see, such as T4 coming down while creatinine creeps up.
Frequently asked questions
What is a normal T4 level for a cat?
Many labs use a total T4 reference interval of roughly 0.8 to 4.7 µg/dL (about 10 to 60 nmol/L), but intervals differ between laboratories and analyzers. Always compare your cat's result with the range printed on its own report.
What does a high T4 mean in a cat?
A total T4 above the lab's range in a cat with signs such as weight loss despite a good appetite, increased thirst or vomiting strongly supports hyperthyroidism. Your vet will usually also check kidney values, urine and blood pressure before discussing treatment.
Why would my vet run a free T4 test?
Free T4, ideally measured by equilibrium dialysis, helps when total T4 sits in the upper part of the range in a cat with signs, or when another illness may be lowering total T4. It is read together with total T4, because free T4 on its own can be falsely high in sick cats whose thyroid is normal.
Can a cat have hyperthyroidism with a normal T4?
Yes, especially early in the disease or when another illness is pulling T4 down. Vets may repeat total T4 after a few weeks, add free T4, or in referral settings use a T3 suppression test or thyroid scintigraphy.
How often is T4 checked when a cat is on methimazole?
Vets commonly recheck T4, kidney values and often a blood count a few weeks after starting treatment or changing the dose, then periodically once the cat is stable. Your vet sets the exact schedule, and any dose change should come from them.
Why did my cat's creatinine go up after thyroid treatment?
Hyperthyroidism increases kidney filtration and reduces muscle mass, both of which keep creatinine low. Once T4 is controlled, creatinine often rises and can reveal kidney disease that was already present, which is why vets recheck kidney values after treatment starts.
Sources
- Hyperthyroidism in Animals, Merck Veterinary Manual.
- Hyperthyroidism in Cats, Cornell Feline Health Center.
- Hyperthyroidism in cats, International Cat Care.
- 2016 AAFP Guidelines for the Management of Feline Hyperthyroidism, American Association of Feline Practitioners.
- IRIS guidelines and staging of chronic kidney disease, International Renal Interest Society.