High T4 in cats: hyperthyroidism and what it hides

What this marker measures

Total T4 is the circulating thyroid hormone. The thyroid, two small glands sitting either side of the windpipe, controls its output. This hormone sets the speed at which the body burns its energy.

In a cat over ten, a T4 above the laboratory's range means something very different from what it would mean in a dog. Feline hyperthyroidism is the commonest hormonal disease of the older cat. It nearly always comes from a benign nodule that has started producing hormone without listening to orders from the brain.

The picture is quickly recognised. The cat loses weight while eating more than before. It becomes vocal at night, restless, sometimes aggressive. Its coat goes dull. It vomits or passes loose stools. Many owners arrive saying their cat has grown younger, before becoming worried about a weight that keeps falling.

What vets look into when the value sits outside the range

When T4 is high in an older cat, the diagnosis is rarely in doubt. The work then goes elsewhere: to what the hyperthyroidism is hiding.

This is the point almost no article for owners handles properly, and it is the one that decides what follows. Excess thyroid hormone speeds up blood flow through the kidneys. A kidney that is already damaged therefore carries on filtering properly for as long as the thyroid races. The day treatment restores a normal thyroid, that push disappears, and the kidney disease comes into plain view. It had been there all along.

That is why the vet looks at creatinine, SDMA, urine specific gravity and blood pressure at the same time. And it is why treatment often starts with tablets rather than with a permanent solution: tablets can be dosed, reduced, stopped. You test the kidney's reaction before burning any bridges.

High blood pressure often comes along with it. It damages the retina silently and can cause a detachment, so sudden blindness. Measuring blood pressure and examining the back of the eye have their place from the point of diagnosis.

In dogs, the reading is reversed. A high T4 is rare there and tends to prompt a search for a laboratory error or for supplementation being given. It is a low T4 that raises questions in dogs, and it is a trap: any serious illness lowers T4 without the thyroid having anything to do with it.

What this value does not tell you on its own

A T4 in the upper part of the range does not rule out hyperthyroidism. In a cat with something else wrong, T4 can be pulled down and mask early thyroid disease. When the clinical picture speaks loudly and the figure stays unremarkable, the vet rechecks a few weeks later or asks for a free T4.

The reverse also exists. The stress of a consultation does not push T4 up the way it pushes blood glucose up, so a high value in an older cat that is losing weight is taken seriously.

One last thing about monitoring. Once treatment has started, the aim is not the lowest possible value. A T4 pushed too low damages the kidney as much as an excess does. The target is the lower half of the range, not the floor.

Questions to ask your vet

1. Has kidney function been assessed before starting treatment, and with which markers?
2. Has blood pressure been measured, and the back of the eye examined?
3. Why this treatment rather than another in his case, and what stays reversible?
4. How often should T4 and creatinine be rechecked over the first three months?
5. Which T4 value are we aiming for, and at what point would the dose need reducing?

Does a normal T4 rule out hyperthyroidism in a cat?

No. In a cat with another illness, T4 can be pulled down and stay inside the range while the thyroid is already affected. When the clinical signs speak clearly, the vet rechecks a few weeks later or asks for a free T4.

Why start with tablets rather than with a permanent treatment?

Because a reversible treatment can be dosed, reduced and stopped. It allows you to see how the kidney reacts to a thyroid back to normal before taking an irreversible decision.

Should we aim for the lowest possible T4?

No. A T4 pushed too low damages the kidney as much as an excess does. The usual target sits in the lower half of the laboratory's range, not on the floor.