Roughly 5-10% of cats over 10 years old are hyperthyroid, and the disease is easy to miss because the cat looks better than it should: bright, hungry, and thinner. A baseline panel done once, at the right moment, tells you where the cat sits before any drug touches it. The line that matters is T4 above 2.5 ug/dL with compatible signs, and the habits that make the number trustworthy are a fixed lab, a morning draw, and a recheck at 3 weeks rather than 3 months.
Where the 2.5 ug/dL line comes from
Most laboratories report total T4 with a reference interval of 1.0-4.0 ug/dL, which is 13-52 nmol/L. The intervals are set from healthy young and middle-aged cats, so a 13-year-old with a T4 of 3.8 ug/dL can sit inside the printed range and still be thyrotoxic. Clinical practice therefore uses 2.5 ug/dL as the working cut-off when weight loss, a ravenous appetite, or a heart rate over 200 bpm are present. Below 2.5 ug/dL, hyperthyroidism is unlikely; between 2.5 and 4.0 ug/dL, the result is a grey zone and needs a second test rather than a prescription.
Who should be tested, and how often
Every cat from 10 years up should have a T4 at least once a year, and twice a year from 13. The prevalence climbs with age, so a cat tested at 10 with a normal 1.8 ug/dL is not cleared for life. Weight loss of 10% over 6-12 months with a good appetite is the single most useful trigger for testing, and so is a new heart murmur in a cat that had none 12 months earlier. Palpating a thyroid nodule in the neck is possible in most hyperthyroid cats, but a small nodule in a cat with a short neck is easy to miss, which is why the blood test leads and the neck check follows.
Weight down 10%, appetite up, heart over 200
Thyroid hormone raises metabolic rate, so the cat burns through food faster than it can absorb it. A 4.5 kg cat that drops to 4.0 kg in 8 months has lost 11% of its body weight, and that is a finding, not a compliment. Resting heart rate above 200 bpm, measured at home while the cat sleeps, is the second signal; the same cat often has a systolic blood pressure over 160 mmHg on 2 of 3 readings, and 20-40% of hyperthyroid cats do. Vomiting 2-3 times a week, a greasy coat, and 3 am yowling complete the picture in many cases.
The baseline panel, by the numbers
Draw the panel before any treatment, on a fasted morning sample, and keep the same laboratory for every recheck so the numbers are comparable. A minimum baseline is T4, free T4 by equilibrium dialysis when the total is in the grey zone, TSH, creatinine, phosphorus, ALT, CBC, urine specific gravity, and blood pressure. Adding NT-proBNP is worth it when a murmur or arrhythmia is present.
| Test | Typical reference | Reading that suggests hyperthyroidism | Why it is on the list |
|---|---|---|---|
| Total T4 | 1.0-4.0 ug/dL | above 2.5 ug/dL with signs | first-line test |
| Free T4 (dialysis) | 0.7-2.5 ng/dL | above 2.5 ng/dL | splits the grey zone |
| TSH | 0.03-0.4 ng/mL | under 0.03 ng/mL | suppressed in most cases |
| Creatinine | 0.8-2.4 mg/dL | low-normal, under 1.2 mg/dL | may hide kidney disease |
| Urine specific gravity | 1.035-1.060 | under 1.030 | concentration ability |
| Resting heart rate | 140-200 bpm | above 200 bpm | cardiac load |
| Systolic pressure | 120-160 mmHg | above 160 mmHg | hypertensive damage |
Every laboratory and analyser sets slightly different limits, so compare against the interval printed on your own report rather than the numbers above.
Why the recheck lands at 3 weeks
T4 has a half-life of about 1-2 days, so a treated cat reaches a new steady state within 7-10 days, and 3 weeks is long enough for the dose to show its true effect without letting a too-high dose run for a quarter. Book the first recheck at 3 weeks, the second at 3 months, and every 6 months after that. The 3-week visit is also when creatinine gets rechecked, because once thyroid hormone falls, kidney filtration drops with it and a previously hidden creatinine of 2.0 mg/dL can surface at 2.8 mg/dL. A cat whose T4 was 6.5 ug/dL at diagnosis and 1.2 ug/dL at 3 weeks is over-treated and needs a dose cut, not congratulations.
When T4 sits in the grey zone
A T4 between 2.5 and 4.0 ug/dL with no signs is a 2-test problem. Repeat total T4 in 3 weeks, and if it stays in the same band, add free T4 by equilibrium dialysis plus TSH. A free T4 above 2.5 ng/dL with a TSH below 0.03 ng/mL supports the diagnosis; a normal free T4 with a normal TSH argues for waiting another 3 months. Do not start methimazole on a single grey-zone result, because the drug needs monitoring and a wrong diagnosis means a lifetime of blood draws for nothing.
Kidney disease hides behind a high T4
Hyperthyroidism increases kidney blood flow and inflates the glomerular filtration rate, which pushes creatinine down. That is why 20-40% of these cats are diagnosed with chronic kidney disease only after treatment brings the thyroid down, and why the 3-week creatinine matters more than the 3-week T4 in some cats. Urine specific gravity under 1.030 before treatment is another hint that the kidneys are already struggling. If creatinine rises above 2.4 mg/dL after the thyroid normalizes, the treatment plan and the diet both need revisiting on the same day.
What to bring to the first appointment
- A 3-week weight log, weighed at home on the same scale, with the starting weight and today's weight.
- A video of the cat breathing while asleep, 30 seconds, to count a sleeping respiratory rate against a normal of under 30 breaths a minute.
- A list of every food and supplement, since some treats carry enough iodine to shift a T4.
- The date of the last T4 and the laboratory that ran it, so the new result can be compared like for like.
- 3 questions written down, the first being what the plan is if T4 is normal but the cat keeps losing weight.
If the baseline comes back high, the next decision is the treatment route, covered in hyperthyroidism in cats; read the reference intervals on cat bloodwork ranges before you argue with a lab value, and price the monitoring schedule with the cost calculator so the 3-week and 3-month visits are budgeted rather than improvised.
Terms mentioned in this article
Frequently asked
My cat's T4 is 3.2 ug/dL and the lab says normal. Is it hyperthyroid?
Possibly, and the lab is not wrong. The printed interval of 1.0-4.0 ug/dL comes from healthy younger cats, so 3.2 ug/dL sits in the grey zone for a cat over 10. Repeat total T4 in 3 weeks and add free T4 by equilibrium dialysis with TSH; a free T4 above 2.5 ng/dL and a suppressed TSH support treatment, while 2 normal results justify waiting another 3 months.
How much weight loss should trigger a thyroid test?
Around 10% of body weight over 6-12 months, especially when appetite is good or increased. On a 4.5 kg cat that is about 450 g, which is easy to dismiss as old age. Pair it with a resting heart rate above 200 bpm, vomiting 2-3 times a week, or a new murmur, and the case for a T4 at the next visit is strong enough that waiting a year is the riskier choice.
Why recheck 3 weeks after starting treatment instead of 6 weeks?
T4 settles into a new steady state within 7-10 days of a dose change, so 3 weeks already shows the true effect and catches over-treatment before it costs the kidneys or the heart. The 3-week visit also rechecks creatinine, since filtration falls once thyroid hormone drops and hidden kidney disease often surfaces at that point rather than later.