Hyperthyroidism in Cats: A Guide for Cat Owners

What is hyperthyroidism?

Hyperthyroidism refers to an abnormal overproduction of thyroid hormones.
These hormones regulate metabolism—if there are too many, the entire body runs “at full speed.”
What does this mean? When too many thyroid hormones circulate in the blood, the cat’s metabolism is unnaturally accelerated. This means:

• the body uses much more energy than usual
• The heart, circulatory system, and organs must constantly work overtime
• Muscle, fat, and even the body's own reserves are broken down more quickly
• Processes such as digestion, blood circulation, and temperature regulation occur at an accelerated rate

The thyroid gland sends a constant "full-throttle signal" to the entire body—even when it isn't necessary.

Causes and risk factors

In most cases, hyperthyroidism is caused by benign changes in the thyroid gland. Benign adenomas or nodular hyperplasia account for approximately 98% of cases, and both lobes of the thyroid gland (right and left) are often affected.
Thyroid carcinomas (malignant changes) are rare and occur in about 2% of cats.
Hyperthyroidism is a disease of senior cats, typically those over 8 years of age.

Recognize signs

The following symptoms are common:

• Weight loss despite normal or even increased food intake
• Polyphagia (excessive appetite)
• Polyuria/polydipsia (excessive thirst and frequent urination)
• Vomiting/Diarrhea
• Hyperactivity, restlessness
• shaggy coat, increased meowing, behavioral changes
• Heart murmur, tachycardia

Less commonly, reduced food intake or aggression may occur.

diagnosis

The diagnosis of hyperthyroidism is based on a combination of clinical examination and laboratory test results. First, the cat is thoroughly examined. In some cases, the veterinarian may feel an enlarged thyroid gland, which is an initial indication of the condition.
A key component of the diagnostic process is a blood test, particularly the measurement of total T4 levels. An elevated T4 level often confirms the diagnosis.
In unclear cases, such as borderline values or the presence of other concurrent conditions, additional hormonal parameters such as free T4 (fT4) or TSH may be measured to confirm the diagnosis.
Since hyperthyroidism often coexists with other conditions or may initially mask them, a complete blood test is generally recommended. Depending on the individual findings, further diagnostic steps may be advisable, including blood pressure measurement, X-rays, a cardiac ultrasound, and an abdominal ultrasound.
These supplementary examinations help to obtain as complete a picture as possible of the cat’s general health and to plan the treatment optimally.

therapy

There are several established treatment methods. Which one is appropriate depends on the cat’s health, its living situation, and the owner’s preferences. And last but not least, the cat itself plays a small part in the decision—namely, whether it reliably takes pills or is willing to eat a specific type of special food.

• Drug therapy: This is the most commonly chosen form of treatment. The active ingredients used are methimazole or carbimazole, which inhibit the production of thyroid hormones and thus restore normal metabolism. A major advantage of this method is that it works relatively quickly—usually within 2 to 4 weeks. In addition, the treatment is reversible: if side effects such as loss of appetite, vomiting, apathy, or changes in blood counts occur, the medication can be reduced or discontinued, so that the symptoms usually subside quickly. The treatment is lifelong. The medication can be administered either in tablet form or as a transdermal patch applied to the inside of the ear.
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• Radioactive iodine therapy: In this form of treatment, the cat is injected with radioactive iodine (iodine-131), which specifically destroys the diseased cells in the thyroid gland without damaging healthy tissue.
  A major advantage of this method is its high success rate of about 85% and the low recurrence rate. The treatment usually results in a lifelong cure, and daily medication is no longer necessary.
  However, there are also some disadvantages. In Switzerland, radioiodine therapy is currently only offered at the Bern Animal Hospital. After the injection, inpatient isolation for up to two weeks is required, as the cat is “radioactive” during this time. In addition, this form of therapy is costly. Since radioiodine therapy is irreversible, it can in some cases lead to permanent hypothyroidism (underactive thyroid). Furthermore, the method is not suitable for all cats—certain medical criteria must be met for them to be considered candidates. See here for more information.
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• Special Low-Iodine Diet: Cats that live exclusively indoors can alternatively be treated with a special low-iodine diet. This food contains virtually no iodine—a trace element that is essential for the production of thyroid hormones.
  Without iodine, the thyroid cannot produce excess hormones, causing the T4 level to drop again after about 4–8 weeks.
  A major advantage of this form of therapy is that it is non-invasive and does not require the administration of tablets. However, it is crucial that the cat eats only this diet food—even small amounts of other food can negate the effect. This therapy is therefore not suitable for outdoor cats, as they could consume food outside.
  Important: Cats without hyperthyroidism should not be fed this food, as it can lead to iodine deficiency in the long term.
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• Surgical removal (thyroidectomy): Surgical removal of the thyroid gland is rarely performed today, but may be appropriate in certain cases—for example, if thyroid cancer is suspected or if the thyroid gland is affected on only one side. The surgery has the potential to be curative, but carries certain risks that should not be underestimated.
  The most significant risks include the stress of anesthesia in older cats, as well as potential damage to the parathyroid glands, blood vessels, or nerves in the neck area.
  Specific imaging tests (scintigraphy) must be performed before the procedure. These tests check for the presence of ectopic thyroid tissue—that is, thyroid tissue located in an anatomically unusual place in the body. If such tissue is overlooked, the disease could persist despite the surgery.

Whether it’s for diagnostic tests or treatment of hyperthyroidism, with Calingo, your cat and the associated veterinary costs are fully covered under your chosen plan and the applicable insurance terms and conditions.

Checkups – Why Are They So Important, and What Does This Have to Do with the Kidneys?

Once treatment has begun, regular and frequent checkups are essential for optimizing the treatment plan and detecting any potential comorbidities at an early stage. Typically, these checkups are conducted as follows:

• After about 2 weeks: T4 level, complete blood count, liver and kidney functiontests‍
• After 4–6 weeks: Follow-up and, if necessary, adjustment of themedication dose‍
• After 3 and 6 months:Follow-up visit‍
• At least once a year, often every 6 months, for life

Chronic kidney disease (CKD) is a common condition in older cats and affects up to 30% of the senior cat population. Therefore, it is not uncommon for a cat to suffer from both hyperthyroidism and CKD.
In untreated hyperthyroidism, the metabolism is significantly accelerated. As a result, the kidneys also work at full capacity, leading to an increased filtration rate. This condition can mask early-stage kidney disease.
Once the thyroid gland returns to normal function through treatment, kidney function normalizes—and as a result, a previously hidden kidney disease may become apparent.
For this reason, veterinarians monitor kidney values alongside thyroid values. This is an important part of treatment monitoring.

What happens if hyperthyroidism is left untreated?

Untreated hyperthyroidism can lead to serious complications, such as high blood pressure (hypertension), heart disease (particularly hypertrophic cardiomyopathy), or progressive kidney damage.

Conclusion – good prognosis with timely treatment

Although hyperthyroidism is a chronic condition, it is highly treatable. With the right treatment, most cats can lead completely normal lives. Early diagnosis and regular checkups are crucial for a normal life expectancy.

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If your senior cat has recently lost weight unintentionally, it’s a good idea to seek a professional evaluation as soon as possible. Calingo customers can take advantage of a free telemedicine service—available around the clock, 24/7, and from the comfort of your own home (see the button at the bottom right).

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