The signs
Signs of hyperthyroidism in cats
An overactive thyroid is the most common hormone disorder of older cats, and it is most often found in cats over 10.
What owners notice
- Weight loss despite a big, sometimes ravenous appetite
- Restlessness, and yowling at night
- Vomiting or diarrhea
- Drinking and peeing more
- A fast heart rate at rest: above the normal 220 beats a minute, often 240 or more
- An unkempt, greasy or matted coat
What it does inside
Thyroid hormone sets the body’s pace. Too much of it burns calories faster than a cat can eat them, and drives the heart harder. Left untreated, it can lead to heart disease and high blood pressure, which in turn damages the eyes and kidneys.
The cause is almost always a benign growth on the thyroid gland. And it is very treatable.
Source: AAFP Guidelines for the Management of Feline Hyperthyroidism 2016 · Merck Veterinary Manual · Cornell Feline Health Center
Diagnosis
How hyperthyroidism is diagnosed in cats
The T4 test
Most cats are diagnosed with one blood test: total T4 (thyroxine) above the laboratory’s reference range. Ranges differ between labs; about 0.8–4.7 µg/dL is typical.
In early or mild cases, or when another illness is pulling it down, T4 can sit in the upper half of normal, and the vet may repeat it or add a free T4 test. Your vet will usually check kidney values, blood pressure and the heart at the same time, for a reason explained further down.
Source: AAFP Guidelines for the Management of Feline Hyperthyroidism 2016
Treatment
Treatment for hyperthyroidism in cats
There are four options. The right one depends on your cat’s age, kidneys, other illnesses and temperament, and on your circumstances.
Methimazole or carbimazole
Tablets, or a gel rubbed on the inside of the ear, usually for life. It controls the thyroid rather than curing it, and the dose is adjusted from T4 rechecks. Often the first step, because it shows how the kidneys cope once the thyroid is normal.
Radioiodine (I-131)
A single injection that destroys the overactive tissue and cures most cats. It needs a specialist center and a short hospital stay while the radiation level falls.
Iodine-restricted diet
A prescription food so low in iodine that the thyroid cannot make too much hormone. It works only if it is the only thing your cat eats: no treats, no other food, no hunting.
Surgery (thyroidectomy)
Removing the affected gland can cure it. It needs an anaesthetic in a cat that may be older or have a heart already affected, and the tiny parathyroid glands beside it must be protected to keep calcium normal.
Source: AAFP Guidelines for the Management of Feline Hyperthyroidism 2016 · Cornell Feline Health Center
In FelineSense
The treatment, the T4 results and the home signs
- The treatment your vet chose: methimazole or carbimazole, radioiodine, an iodine-restricted diet, surgery, or not decided yet
- Your lab’s own T4 range; 0.8–4.7 µg/dL is filled in until you change it
- Each T4 placed above the range, in the upper half, in the lower half (the AAFP goal for a treated cat) or below it
- Creatinine and the vet’s heart-rate reading logged beside each T4
- A 90-day watch for early methimazole side effects, with weight and heart rate as home signs
- Methimazole doses ticked under Medications, exactly as prescribed. See all three chronic conditions
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On treatment
What the T4 result means on treatment
The AAFP guidelines aim for a treated cat’s T4 in the lower half of the lab’s reference range.
| Where the T4 sits | What it usually means |
|---|---|
| Above the range | Still overactive. The dose, diet compliance or timing may need a change; your vet decides. |
| Upper half of the range | Acceptable, but if signs persist your vet may aim lower. |
| Lower half of the range | The goal for a treated cat. Keep the recheck schedule. |
| Below the range | Over-treated, which can harm the kidneys. Tell your vet; the dose may come down. |
Rechecks: 2–4 weeks after starting or changing a dose, then every 3–6 months, with kidney values each time.
Source: AAFP Guidelines for the Management of Feline Hyperthyroidism 2016
Methimazole side effects to watch for
Most side effects appear in the first three months. Vomiting, loss of appetite and lethargy are the common ones, and intense itching and scratching of the face and neck often starts at around 4–8 weeks. Rarer but serious reactions affect the liver (yellow gums or eyes) and the blood cells, which is why your vet checks bloods in the early months. Any of these means a call to your vet that day, and a cat that stops eating needs the vet today.
Source: Cornell Feline Health Center (Feline Hyperthyroidism) · AAFP 2016
Treating the thyroid can unmask kidney disease
An overactive thyroid pushes extra blood through the kidneys. That lowers creatinine and can hide kidney disease. Once the thyroid is brought back to normal, creatinine often rises, and some cats turn out to have had CKD all along. Correct treatment does not cause kidney disease, but it can reveal it, and over-treatment (a T4 below the range) can make kidney values worse.
That is why kidney values are checked with every T4, why many vets start with methimazole before a permanent treatment, and why a T4 below the range is avoided. More in the kidney disease guide.
Source: AAFP Guidelines for the Management of Feline Hyperthyroidism 2016
At home: weight and heart rate
Weigh your cat weekly. Losing 5 % or more over two months is the main sign the thyroid is not controlled, or that something else is going on. A resting heart rate over 220 beats a minute points the same way. Both are worth a call; a T4 recheck is likely. Here is how to count your cat’s heart rate.
In the app
Doses, weight and the record
Questions
Frequently asked
What are the signs of hyperthyroidism in cats?
How is hyperthyroidism diagnosed in cats?
What is a normal T4 level for a cat?
Can hyperthyroidism in cats be cured?
What are the side effects of methimazole in cats?
Can treating hyperthyroidism cause kidney problems?
FelineSense keeps the treatment your vet chose, each T4 against your lab’s range and the AAFP goal, kidney values beside it, and the home signs, in one record for the vet.
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