Hypothyroidism in Cats

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Hypothyroidism in cats occurs when a cat’s body is either unable to effectively synthesize thyroid hormones at all, or produces thyroid hormones in such a small quantity that they are unable to do their job.

Thyroid glands produce two iodine-containing hormones: T3 and T4. These hormones affect many of the processes in your cat’s body and regulate metabolic rate. When a cat has hyperthyroidism, the body produces too much of the thyroid hormones, and bodily processes speed up. With hypothyroidism, which occurs when there is too little thyroid hormone, bodily processes slow down. While hypothyroidism is markedly less common than hyperthyroidism in cats, it can occur, especially as a secondary result of treatment for hyperthyroidism.

Understanding the symptoms and potential causes of hypothyroidism in cats can help you better monitor your cat’s thyroid function. This is especially important for owners of cats who have previously been treated for hyperthyroidism.

Signs & Symptoms of Hypothyroidism in Cats

Symptoms of hypothyroidism in cats may vary depending on the severity of the disease: cats with mild cases may show only mild symptoms, or none at all. Some of the symptoms will depend upon the cause of the hypothyroidism itself.

Here are some common signs of hypothyroidism in cats:

  • Lethargy
  • Dullness
  • Hair loss
  • Decreased appetite
  • Low body temperature
  • Decreased heart rate
  • Unexplained weight gain
  • Obesity
  • Dwarfing
  • Constipation

Because thyroid hormones affect how all the organ systems in your cat’s body function, a decrease in these hormones typically slows systems down. Slower metabolism can lead to lethargy, mental dullness, hair loss, decreased appetite, lower-than-normal body temperature, and sometimes even a decreased heart rate. Obesity is especially common in cats who develop the disease as a result of treatment for hyperthyroidism.

Cats born with hypothyroidism, or who develop it when they are very young, may exhibit signs of hypothyroidism like dwarfing, constipation, lethargy, mental dullness, and a decreased heart rate.

How Did My Cat Get Hypothyroidism?

Most cats with hypothyroidism develop the condition as a result of treatment for hyperthyroidism. However, some cats can be born with the disease or can develop it at a young age.

Most common causes of hypothyroidism in cats:

  • Treatment for hyperthyroidism
  • Congenital abnormalities

Hyperthyroidism, which is much more common in cats than hypothyroidism, occurs when the cat’s thyroid glands secrete an excess amount of thyroid hormones. Effective treatment for hyperthyroidism in cats often involves the removal or destruction of one or both of the thyroid glands. If both glands are removed, synthetic thyroid hormone supplements must be administered. However, this can inadvertently lead to hypothyroidism.

While rare, some cats are born with congenital abnormalities that cause hypothyroidism, or develop juvenile-onset hypothyroidism. These conditions can be caused by defects in how the thyroid hormones are synthesized by the body, an inability of the gland to respond to thyroid stimulating hormones, or abnormal organ development.

Diagnosing Hypothyroidism in Cats

Veterinarians diagnose hypothyroidism in cats through a combination of symptoms and diagnostic testing. If your cat’s symptoms suggest possible thyroid gland involvement, your veterinarian will most likely recommend laboratory tests that measure blood concentrations of thyroid hormones in conjunction with high levels of TSH (thyroid stimulating hormone).

Depending on your cat’s symptoms, the veterinarian may also suggest additional diagnostic testing to rule out other potential causes.

Treating Your Cat for Hypothyroidism

Luckily, there is a treatment for hypothyroidism in cats. Synthetic thyroid hormone replacement drugs like levothyroxine can provide regulation and are usually effective at alleviating or resolving symptoms.

Is There a Cure for Hypothyroidism?

There is no cure for hypothyroidism in cats, but it can be treated. In most cases, synthetic hormone replacement drugs improve or resolve symptoms. However, these drugs will be necessary for the remainder of your cat’s life.

Is Hypothyroidism Contagious for Humans and Other Pets?

Hypothyroidism is not contagious.

What is the Cost of Treating Hypothyroidism?

The cost for treating hypothyroidism will depend on the individual cost of your veterinary visits; how many tests are required for diagnosis; and how long it takes to determine the appropriate dosage of synthetic hormone replacement/medication. The cost of the medication itself may also vary depending on where you get the prescription filled, but it generally ranges around $25 to $40 per month.

Bear in mind that treating hypothyroidism is a life-long commitment, and your cat will require medications and monitoring for the rest of her life. All of this adds up to more than it would cost to maintain a typical cat’s health.

Recovery and Management of Hypothyroidism

Most cats recover from hypothyroidism with treatment from synthetic thyroid hormones like levothyroxine, although the extent and duration of recovery may vary. Veterinarians measure the success of treatment by how much a cat’s symptoms improve.

Finding the appropriate dosage of synthetic thyroid hormone can take some adjustment. During this time, cats will require more frequent monitoring. Once a dosage is shown to be effective, monitoring and laboratory work can be decreased to once or twice a year for the rest of your cat’s life.

Preventing Hypothyroidism in Cats

Hypothyroidism cannot always be prevented. However, cats who undergo treatment for hyperthyroidism should have their thyroid levels monitored to ensure hypothyroidism does not develop, and to correct dosages of synthetic thyroid hormones if it does.

Is There a Vaccine for Hypothyroidism in Cats?

No, there is no vaccine for hypothyroidism.


Hypothyroidism in cats is a disorder where the body does not produce enough thyroid hormone, often resulting in decreased appetite and unexplained weight gain. While there’s no cure, it’s easily treatable with medication.

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