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10 Symptoms of Hyperthyroidism in Cats

Developed in collaboration with Andrea Johnson, DVM | Co-Founder | PetVet365

Last reviewed: December 14, 2023

What is Feline Hyperthyroidism?

Hyperthyroidism in cats, or feline hyperthyroidism, is a condition where your cat’s thyroid gland is overactive. It is most common in older cats and, if left untreated, can cause heart failure, organ damage, and possibly death.

The thyroid gland is made up of two lobes in your cat’s neck and is part of the endocrine system. It uses hormones to control your cat's? metabolism, energy level, reproduction, growth and development, response to injury, stress, and mood.

There is no way to prevent hyperthyroidism, nor can it be cured. But it can be successfully managed and controlled with daily, long-term medication, diet, and, when appropriate, surgery or radioactive iodine therapy.

Key Facts
  • Feline hyperthyroidism is the medical term for an over-active thyroid gland.
  • If left untreated, hyperthyroidism can cause organ damage or failure.
  • The causes of hyperthyroidism are unknown.
  • There is no cure for an over-active thyroid, but it can be managed and controlled.
  • The most common symptom is weight loss due to a high metabolism rate.
  • Can be diagnosed with a simple blood test and confirmed with several other diagnostic tests.
  • Prognosis is excellent once it is treated, but management is usually life-long.

10 Symptoms of Hyperthyroidism in Cats

  1. Weight Loss: The most common symptom of hyperthyroidism in cats is weight loss due to an increased metabolism rate. This may happen in spite of your cat eating more.
  2. Changes in Appetite: Your cat’s metabolism is speeding up to the point that its body is burning calories faster than they can take them in.
  3. Increased Water Consumption (polydipsia).
  4. Frequent Urination (polyuria): Often peeing outside of the litter box.
  5. Vomiting
  6. Diarrhea
  7. Mood Changes: Your cat may become more cranky or aggressive than usual or vocalize more, especially during the night.
  8. An Enlarged Thyroid Gland: A lump on the neck.
  9. Restlessness or Hyperactivity
  10. Unkempt Appearance: From lack of self-grooming.


Causes of Hyperthyroidism in Cats

There is no definitive known cause of feline hyperthyroidism. It is suspected that environmental factors and exposure to toxins may be at fault. Some of the hypothesized causes include:

  • Diet: Eating primarily canned cat foods, especially canned fish, which has high levels of dietary iodine, puts cats at a higher risk. Instances of hyperthyroidism increased about a decade after canned cat foods became commercially available in the 1960’s. Veterinary scientists suspect a correlation between the two.
  • Environmental Exposure: Studies show that cats with hyperthyroidism have higher levels of polybrominated diphenyl esters (PBDE), a chemical used in flame retardants, in their blood. Other studies suspect that chemicals in kitty litter, flea collars, carpet cleaners, and processed foods trigger hyperthyroidism.
  • Genetics: Siamese and Himalayan breeds are less susceptible to hyperthyroidism.

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Your veterinarian may prescribe a customized, compounded medication. These medications are mixed by trained, licensed compounding pharmacists and often come in dosage forms designed to make giving or applying the medication easier and more accurate.

Diagnosing Hyperthyroidism in Cats

If your cat is showing signs of hyperthyroidism, your veterinarian will perform a physical examination to look for enlarged thyroid nodules and run a thyroid serum blood test to check thyroxine (T4) levels. Typically, if the T4 levels are over a certain threshold, there is no question about the diagnosis.

Before treatment, they may perform one or more additional tests to evaluate your cat’s overall health and predict the likelihood of complications with treatment. These may include additional blood tests, a urinalysis, chest X-rays, an echocardiogram (ECG), a technetium (Tc-99m) scan or scintigraph (thyroid scan), an ultrasound, and/or blood pressure measurements. During treatment, your veterinarian will monitor for hypothyroidism (the slowing of thyroid function) to make sure it is not getting too much of a treatment.

Complications From Feline Hyperthyroidism

There are a number of conditions that can develop from complications of feline hyperthyroidism, some of which are serious and life threatening.

  • Hypertension (high blood pressure) 
    Hypertension affects about 25% of cats and develops due to the increased pumping pressure and elevated heart rate that accompanies hyperthyroidism. In some cases, a cat's blood pressure can become so high that retinal bleeding or retinal detachment occurs, resulting in sudden blindness. A heart murmur, rapid heart rate, or an abnormal heartbeat known as a “gallop rhythm” may also occur.
  • Thyrotoxic cardiomyopathy 
    Thyrotoxic cardiomyopathy is a disease of the heart muscle that makes it harder for the heart to pump blood to the rest of the body. Cardiomyopathy can lead to heart failure. The primary types being:
    • Dilated cardiomyopathy (DCM): A condition in which the heart muscle becomes weakened and enlarged
    • Enlarged hypertrophic cardiomyopathy (HCM): A condition affecting the left ventricle, the main pumping chamber of the heart.
    • Restrictive cardiomyopathy (RCM): A condition where the chambers of the heart become stiff over time. Though the heart is able to squeeze well, it's not able to relax between beats normally. This makes it harder for the heart to fill with blood, so the blood backs up in the circulatory system.

Both cardiomyopathy and hypertension can be manageable with medication and lifestyle changes, depending on the stage of disease.

  • Chronic Kidney disease 
    Technically, cats diagnosed with hyperthyroid disease already have kidney disease because the kidneys are part of the endocrine system along with the thyroid. But untreated hyperthyroidism can affect renal function and cause kidney disease because of the damage hypertension does to the kidneys.

Treatments for Feline Hyperthyroidism

There are several treatment options for hyperthyroidism in cats. Which one is best for your cat depends on the cause of its hyperthyroidism, its age, and its other underlying health conditions.

  • Medication: Methimazole is an antithyroid drug and the first line medication for hyperthyroidism. It is generally a safe and effective treatment. Your veterinarian may prescribe a specific dosage methimazole (possibly under the brand names Tapazole® or Felimazole®) based on your pet’s weight and condition. It can be given as an oral tablet, oral suspension (liquid), or applied topically as a transdermal gel, usually in the cat's ear.

Methimazole is given long-term to manage thyroid hormone levels and can also be used to normalize thyroid levels before thyroid surgery. Treatment with methimazole is a form of medical management; it does not cure hyperthyroidism. If your veterinarian determines your pet has special needs that are not satisfied by the commercially available methimazole medications, they may prescribe compounded methimazole that is both the appropriate dose and strength for your cat from a compounding pharmacy.

If your veterinarian determines your pet has special needs that are not satisfied by the commercially available methimazole medications, they may prescribe compounded methimazole that is both the appropriate dose and strength for your cat from a compounding pharmacy.

Wedgewood Pharmacy specializes in compounded products and provides medication options that help ensure accurate dosing, especially for uncooperative cats. Click here for a complete list of Wedgewood’s dosing forms and strengths for methimazole.


  • Radioiodine (I-131) treatment: Radioactive Iodine is absorbed into the overactive thyroid tissue to kill abnormal thyroid cells. It is usually performed by a feline endocrinologist and has a cure rate of over 95%. However, it can be expensive for many cat parents.
  • Thyroid surgery: Surgery to remove abnormal thyroid tissue. But surgery can be risky, especially in older cats or those with certain health conditions. It can also cause issues, like low calcium levels in the blood. In some cases, another surgery is needed to remove any remaining tissue. However, surgery for thyroid carcinoma (malignant thyroid gland tumors) is usually recommended.
  • Low iodine diet: The thyroid gland needs Iodine to make thyroid hormones. A prescription diet restricting the amount of iodine in your cat’s diet can lower thyroid hormone levels and improve symptoms. Like methimazole, this treatment option will need to be continued for the rest of your cat’s life. Due to cats frequently finding other things to eat, even indoors, low iodine diets have a low success rate in controlling the disease.
Frequently Asked Questions

Frequently Asked Questions

Will Feline Hyperthyroidism Be Cured with Treatment?

No. Hyperthyroidism in cats can be successfully managed, but it cannot be cured. It is likely that your cat will be on medication (methimazole) or another treatment for the rest of its life. Recurrence of hyperthyroidism is rare after I-131 therapy, but possible.

What Is the Prognosis for Cats with Hyperthyroidism?

The outcomes of most hyperthyroid treatments are usually excellent, and most cats have a very good chance of returning to a normal state of health. Cats managed with diet or medication generally do well as long as their feeding is consistent, their medication is administered routinely, and follow-up blood and diagnostic tests are performed as scheduled. Early diagnosis is key to a positive long term outcome, so screening older cats for thyroid disease annually is recommended.

How Can Feline Hyperthyroidism Be Prevented?

The exact cause of feline hyperthyroidism is unknown, so there are currently no preventative measures. Early diagnosis often leads to a better prognosis.

Is There Any Monitoring That Needs to Be Done with Methimazole?

Your cat should be monitored closely for adverse effects such as tiredness, vomiting, loss of appetite, yellowing of the skin or eyes, or itchiness. If these are seen, discontinue the medication and contact your veterinarian.

Your veterinarian will most likely check your cat’s blood before starting methimazole to establish a baseline of thyroid function, and then check again every 2-3 weeks for the first 3 months to track thyroid hormone levels. Once dosing is stabilized, thyroid levels should be checked every 3-6 months.

This article is meant to provide general and not medical advice. We strongly recommend that a veterinarian be consulted for the specific medical needs of your animal.


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