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
- 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
- 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”
- 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.
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.
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.
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
Featured Image: iStock.com/borchee
Ettinger S, Feldman E, Coté E. Textbook of Veterinary Internal Medicine, Feline Hyperthyroidism. 8th edition. Philadelphia, PA: Saunder; 2016.
Nelson RW, Couto CG. Small Animal Internal Medicine, Feline Hyperthyroidism. 6th edition. St. Louis, MO: Elsevier; 2020.