Hyperthyroidism in Cats: Symptoms and Treatment

Hyperthyroidism in Cats: Symptoms and Treatment

by: Dr. Lorie Huston as seen in PetMD 

 Reviewed and updated on March 18, 2020, by Jennifer S. Fryer, DVM


Hyperthyroidism in cats is a disease that’s usually caused by a benign tumor within the thyroid gland. This tumor causes an overproduction of the thyroid hormone called thyroxine. One of the primary functions of this thyroid hormone is to regulate an animal’s metabolism

Cats with too much thyroid hormone have a greatly increased metabolic rate, which leads them to lose weight despite having a ravenous appetite. Other symptoms can include anxiety, vomiting, diarrhea, and increased thirst and urination.

These excessive hormone levels push a cat’s body into constant overdrive, which frequently leads to high blood pressure and a type of heart disease called hypertrophic cardiomyopathy

Here’s everything you should know about hyperthyroidism in cats so you can spot the signs and get your cat on a treatment plan as soon as possible.

How Common Is Hyperthyroidism in Cats? 

There is no known genetic predisposition for hyperthyroidism, but it is quite common in cats. 

In fact, hyperthyroidism is the most common hormonal (endocrine) disease in the cat population, often seen in late middle-aged and older cats. 

The average age of diagnosis is approximately 13 years. The possible age range is 4-20 years, although seeing young hyperthyroid cats is very rare.

What Does the Thyroid Gland Do?

In cats, the thyroid gland has two parts, with one on each side of the trachea (windpipe), just below the larynx (voice box).

The thyroid gland makes several different hormones (mostly thyroxine, or T4). These thyroid hormones affect many of your cat’s body processes:

·         Regulation of body temperature

·         Metabolism of fats and carbohydrates

·         Weight gain and loss

·         Heart rate and cardiac output

·         Nervous system function

·         Growth and brain development in young animals

·         Reproduction

·         Muscle tone

·         Skin condition

Symptoms of Hyperthyroidism in Cats

Here are the major symptoms of hyperthyroidism that you should look for in your cat:

·         Weight loss

·         Increased appetite (ravenous)

·         Unkempt appearance

·         Poor body condition

·         Vomiting

·         Diarrhea

·         Drinking more than usual (polydipsia)

·         Peeing more than usual (polyuria)

·         Rapid breathing (tachypnea)

·         Difficulty breathing (dyspnea)

·         Heart murmur; rapid heart rate; an abnormal heartbeat known as a “gallop rhythm”

·         Hyperactivity/restlessness

·         Aggression

·         Enlarged thyroid gland, which feels like a lump on the neck

·         Thickened nails

Less than 10% of cats suffering from hyperthyroidism exhibit atypical signs such as poor appetite, loss of appetite, depression, and weakness.

What Causes Cats to Be Hyperthyroid?

Hyperfunctioning thyroid nodules (where the thyroid nodules produce excess thyroid hormones outside of the control of the pituitary gland) cause hyperthyroidism. But what causes the thyroid to go haywire?


There are several theories about what causes cats to become hyperthyroid:

·         Rarely, thyroid cancer

·         Some reports have linked hyperthyroidism in cats to some fish-flavored canned food diets

·         Research has pointed to flame-retardant chemicals (PBDEs) that are used in some furniture and carpeting and circulated in house dust

·         Advancing age increases risk

How Do Vets Test for Feline Hyperthyroidism?

In most cases, diagnosing hyperthyroidism is straightforward: high levels of thyroid hormone in the bloodstream (total T4 or TT4) along with the typical signs. 

In some cases, however, your cat’s T4 levels may be in the normal range, making a diagnosis of hyperthyroidism more difficult. This is especially true in the early stages of this disease. 

If your cat is showing the symptoms of hyperthyroidism but the blood tests are not conclusive, you will need to return to your veterinarian for further blood tests or for a referral for a thyroid scan.

The signs of feline hyperthyroidism can also overlap with those of chronic renal failurediabetes mellitus, chronic hepatic disease, and cancer (especially intestinal lymphoma). 

These diseases can be excluded on the basis of routine laboratory findings and thyroid function tests. Your veterinarian will conduct a battery of tests to zero in on a reliable diagnosis.

Kidney disease is commonly diagnosed along with hyperthyroidism in cats. Cats suffering from both diseases may need treatment for both, and the diagnosis of kidney disease in a cat with hyperthyroidism can affect the cat’s prognosis

Treatment for Hyperthyroid Cats

The gold standard therapy is radioiodine (I131) treatment, which can cure the hyperthyroidism in most cases. Daily medication (methimazole) or feeding a low-iodine diet are good options when radioiodine therapy is not an option due to financial considerations or the cat’s overall health.

Radioiodine Therapy (Radioactive Iodine Treatment)

Radioiodine therapy, or I131 treatment, uses radioactive iodine to kill the diseased tissue in the thyroid gland. Most cats undergoing I131 treatment are cured of the disease with one treatment.  

The cat’s thyroid levels are monitored after treatment. Rare cases require a second treatment. Hypothyroidism is not common after treatment, but it can occur, and it can be managed with a daily thyroid medication.

The use of radioiodine is restricted to a confined medical facility, since the treatment itself is radioactive. Depending on the state you live in and the guidelines in place, your cat will need to be hospitalized from several days to a few weeks after being treated with radioactive medicine, to allow the radioactive material to leave your cat’s body before coming home.

Precautions will still need to be taken after bringing your cat home. Your veterinarian will give you specific instructions to reduce your risk of exposure to the radioactive material, which will probably include storing your cat’s used litter in a sealed container for a period of time before disposing of it in the garbage.


Surgically Removing the Thyroid Gland

Surgical removal of the diseased thyroid gland is another potential treatment. Like I131 treatment, surgical treatment is curative, but these cats also must be monitored afterward for hypothyroidism.

Surgical removal of the thyroid gland is best performed when only one thyroid gland is affected, as removal of both can possibly lead to hypothyroidism. Another complication that can occur after surgical removal of the affected thyroid gland is the successive hyperactivity of the remaining thyroid gland.

Methimazole Medication

Giving your cat a medication called methimazole is probably the most common treatment choice. It’s administered by mouth in pill form, or it can be formulated by a compounding pharmacy into a transdermal gel that can be applied to your cat’s ear. Methimazole is often given before radioiodine treatment or surgery to stabilize your cat’s clinical signs.  

Methimazole is effective in controlling the symptoms of hyperthyroidism. However, it does not cure the disease—your cat will need to receive the medication for the rest of his life. If a cat is younger at diagnosis (under 10 years old) and does not have underlying diseases, the cost of methimazole for a lifetime may exceed surgery or radioiodine. 

Methimazole has rare but significant side effects in some cats, so make sure to make and keep regular monitoring appointments with your veterinarian. 

Iodine-Restricted Diet


Feeding a diet that restricts iodine is a newer alternative for treatment of feline hyperthyroidism. Like methimazole treatment, this alternative is not curative, and your cat will require lifelong treatment. 

This diet must be given exclusively. The hyperthyroid cat on this diet must not have access to or be given any treats, other cat food, or human food. Other cats in the household may eat this food, but they must be supplemented with an appropriate cat food for their age and health in order to provide adequate iodine. 

Follow-Up Care for Hyperthyroid Cats

Once treatment has begun, your veterinarian will need to reexamine your cat every two to three weeks for the initial three months of treatment, with a complete blood count to check their T4. Treatment will be adjusted based on the results, such as changing methimazole dosage to maintain T4 concentration in the low-normal range.

If your cat has had surgery, particularly removal of the thyroid gland, your veterinarian will want to closely observe your cat’s physical recovery. Development of low blood-calcium levels and/or paralysis of the voice box during the initial postoperative period are complications that will need to be watched for and treated, should they occur. 

Your doctor will also measure your cat’s thyroid hormone levels in the first week after surgery and every three to six months thereafter, to check for recurrence of thyroid gland overactivity. 

By: Dr. Lorie Huston, DVM, Dr. Jennifer Coates, DVM, Dr. Jennifer S. Fryer, DVM


Milk Thistle for Pets


By Rodney Habib,  Dogs Naturally Magazine and Pet Wellbeing

Milk Thistle is a well-known ‘liver herb’ for both people and pets. It can be used to support general liver health and detoxification, and has an overall excellent safety* profile.  A company called Pet Wellbeing offers a full-spectrum Milk Thistle, extracted from the whole seeds.

Milk Thistle can provide natural support for pets experiencing:

  • Long or short term prescription drug administration
  • Fatty liver
  • Liver diseases
  • Cholestasis (bile obstruction from liver to small intestine)
  • Cancer (including liver cancer)
  • Diabetes (with careful monitoring of blood sugar levels)

Milk Thistle is commonly given to pets while concurrently administering prescription drugs. Examples include: treatment for Heartworm, pain medications, steroids, and a vast number of other medications that can affect the liver. The liver is the organ with a primary role in metabolizing drugs and helping the body excrete toxic substances. If your pet is being given medications, some liver support is highly recommended.

So how does it work?

Milk Thistle helps protect and strengthen liver cells, stimulates their repair and regeneration, and even promotes production of new liver cells. Milk Thistle is an excellent herb for supporting general detoxification as well as recovery after infections.

Milk Thistle is often prepared in an alcohol base for optimizing extraction of its beneficial compounds; however, alcohol is not ideal for our pets. Pet Wellbeing Milk Thistle is first alcohol-extracted to ensure potency, and then the alcohol is removed and replaced with glycerin, providing a safe, alcohol-free product for your pet. Milk Thistle is gentle enough for long-term use.

Additional Uses

Milk thistle not only helps treat and prevent liver disease. Below is a list of treatment claims linked to the plant:

  • Kidney disease: If there has been kidney damage to your pet because of an infection milk thistle has been show to greatly decrease the amount of time it takes your pet to heal. • Pancreatitis: Although rare in cats, pancreatitis is very common in dogs. Milk thistle can be given to your dog and cat to help alleviate the symptoms.
  • Cancer: Milk thistle has been shown to decrease the effects of cancer in a pet’s body. While there haven’t been many studies done, there have been enough that it has shown improvement and helps protect against the potential for cancer in your pet.
  • Diabetes: Pets suffering from diabetes that are given milk thistle at least once per week have been shown to not have to have nearly the amount of insulin that they would have had to take otherwise.

Clearly, milk thistle has an array of positive effects throughout the body.

However, milk thistle should not be used as a preventative, or a “just in case”; rather, it should be used as a means of cleansing the liver after exposure to toxins (i.e. drugs, vaccines, chemicals, etc.) or as a treatment to liver damage. Milk thistle stimulates the growth of new liver cells in order to replace those that are dead or dying, and helps protect against toxins which could cause further damage.

Remember, medicinal herbs shouldn’t be used for extended periods of time.  Gregory L. Tilford and Mary L. Wulff explain in their book entitled Herbs for Pets: The Natural Way to Enhance your Pet’s Life: “Despite much of the publicity that has been generated about this ‘wonder herb’, milk thistle should not be used as a daily food supplement. Milk thistle is a medicine that is best reserved for situations in which the liver is already under abnormal stress.” Most holistic doctors feel that milk thistle should be administered for 3-6 weeks with a 1-3 week break.

Also, pregnant and lactating pets should not be given milk thistle because the research is still inconclusive.

You can find milk thistle in most health food stores or health sections of your local grocery store. It can be found in capsule, tablet and tincture forms.

The usual recommended extract of milk thistle contains 70 to 80 percent silymarin. Each extract should be labeled with the silymarin percent. The tincture can be administered at a starting dose of 1/4 tsp. per 20lbs of animal’s body weight per day. The daily dosage should be taken in 2-4 equally divided doses. With the powder format, administer 2-5mg per 1lb of the animal’s body weight, 2 to 3 times per day. Read labels carefully, discuss dosages and, although there are no known drug interactions, it’s always important to consult with your veterinarian.

 *If your pet is on any medications, always inform their vet of any natural remedies or supplements that are being considered for use, to avoid possible drug-herb interference.

You can read more about Milk Thistle and order it with the URL below:




Milk Thistle for Cat Liver Disease

  • Helps maintain liver health in cats
  • Health support during use of drugs (like chemotherapy)
  • Helps support regeneration of damaged liver cells
  Milk Thistle for Dog Liver Disease
Essential for dogs with liver disease & dogs taking medications such as:

  • Steroids
  • Heartworm prevention
  • Pain or anti-inflammatory medications

Milk Thistle

Conventional vs Alternative Animal Healing Methods, part 2

Photo courtesy of Natural Healers (http://www.naturalhealers.com/alternative-medicine/animal-therapy/)
Photo courtesy of Natural Healers (http://www.naturalhealers.com/alternative-medicine/animal-therapy/)

In last week’s blog I discussed the differences between traditional veterinary, alternative, complementary and integrative care.  In today’s blog I am going to explain some treatment options that fall under alternative and complementary care.

A holistic vet can bring a wide variety of treatment options that may have a wide encompassing affect than just using traditional veterinary medicine.  After all, we are simply seeking the best for our pets, right?  A holistically trained vet or alternative therapy provider can bring the following treatment options to the table for consideration and use with your pet which can be used as a stand- alone healing modality or in conjunction with traditional veterinary care:

Aromatherapy using therapy grade essential oils to heal emotional and physical issues can be used  alone or with other healing techniques.

Animal communications can be used to heal emotional issues.  It uses telepathy to communicate a thought from one person/animal to another. Technically, telepathy is the communication between beings using thoughts, feelings, desires, or other means that cannot be understood in terms of known scientific laws.  Telepathy is considered a form of extra-sensory perception and is often connected to various paranormal phenomena such as precognition, clairvoyance and empathy.

Bach flower essences are all natural, very dilute solutions made from spring water, an alcohol preservative, and the parts of specific flowers. They are used to help balance the emotions and bring about a state of equilibrium in living organisms, and have been successfully used with people and animals to treat a specific emotion or state of mind such as fear, anger, apathy, anxiety, anger, grief, etc.  This healing technique does not negatively interact any other healing modalities.

A Certified Nutritionist can recommend changes including mineral supplements, enzymes, vitamins, fatty  and amino acids to make up nutritional shortfalls in the diet which will complement any other medical steps.

Massage uses the sensation of touch to engage your pet’s mind. A light touch brings awareness to the coat and upper layers of connective tissue and surrounding superficial muscles. Stronger pressure heightens awareness of deeper muscles, tendons, joints and ligaments. Joint mobilization promotes body-movement awareness and gives the animal permission for exploration of movement to rediscover his “place of comfort.”

Chiropractic care maintains each joint, especially the spine.  By freeing the spinal nervous system which is connected to the brain, they can establish pain free flow of energy from the brain through to the extremities.

Energy-based body work which includes: TTouch, Healing Touch for Animals, Reiki, and acupressure to elicit a state of well-being emotionally or physically and is great for relieving pain and stress.

Healing with crystals uses the energy of the crystal to invoke both physical and emotional healing.

Color therapy has been known to strengthen, cleanse, invigorate, balance and may regulate metabolic processes positively influencing bodily functions and moods.  It harnesses the nutritional aspects of color to provide emotional, physical, anti-aging, and spiritual benefits.

Certified Herbalist using Traditional Chinese Medicine (TCM) uses plant remedies to treat a variety of ailments. For example, alfalfa is used for arthritis and allergies.

Acupuncture uses very fine needles inserted into specific areas on your pet’s body to balance the flow of energy.

Homeopathic remedies are used to jumpstart the animal’s own healing response with very diluted substances that cause the same symptoms the dog is suffering from. For instance, a dog with diarrhea would be given tiny amounts of a substance that causes diarrhea.

Be aware that some veterinarians don’t care for alternative therapies since, unlike conventional veterinary medicine; most of them haven’t been scientifically proven to work.  However, that doesn’t mean they’re ineffective; it just means they haven’t been put to the test in well-conducted studies.

There are plenty of vets who are open to the alternative approach. Some veterinary schools now provide studies in holistic medicine, and some vets offer alternative therapies alongside conventional treatments as they see the benefit in treating the whole animal not just the disease.  Why not try everything possible to bring your beloved animal companion into wellness?

Conventional vs Alternative Animal Healing Methods

Photo courtesy of ct.gov.
Photo courtesy of ct.gov.

Conventional (Traditional) western veterinary medicine is exactly what you have known all your life. You take your pet to the vet when they are sick or for an animal check- up much like you would do for yourself. If your pet is ill, the vet may prescribe antibiotics, pills, salves etc. Your vet will use diagnostic tests like x-rays, MRIs, ultra sounds and even exploratory surgery could be an option to obtain information to enable your veterinary professional to make a diagnosis. If your pet is healthy, they will perform several techniques to verify you pet’s health, for instance, listening to their heart or looking in their mouth and ears. They will probably run their hands over your pet looking for “issues”. You may be required to bring in a stool or urine sample for analysis. You can obtain maintenance prescriptions like heartworm or flea and tick medicines from your neighborhood vet.

While there is nothing wrong with using traditional western medicine to help your pet, there are; however, options for expanding the care you provide to your beloved animal companion. When you use alternative medicine to help your pet feel better or remain healthy you employ many different tactics for wellness.

Complementary medicine refers to healing practices and products that work in conjunction with traditional medicine. For example, a cancer patient receiving chemotherapy may also undergo acupuncture to help manage chemo side effects.

Alternative medicine is not used as a complement to, but rather as a substitute for traditional therapy. An example would be a pet with cancer whose owner forgoes recommended chemotherapy or surgery and instead chooses to treat the disease with specific dietary changes.

Complementary and alternative medicines are categories of medicine that includes a variety of treatment approaches that fall outside the realm of conventional medicine. Employing holistic therapies and medicine is about viewing and treating the body as a whole. This is very different from the conventional medical approach.

Integrative medicine draws from both complementary medicine and alternative medicine and combines these with traditional Western therapies.

Many pet owners find that the common sense principles integrative medicine relies on really resonate with them. Often, common sense principles for creating wellness are overlooked in the search for hard-core science but not without consequences.

Lots of different therapies fall under the umbrella of alternative medicine (also called holistic medicine). Many of these therapies have this philosophy in common: consider and treat all aspects of the pet’s life, not just the symptoms. Check in with my blog next week to learn more about alternative and complementary medicine.