Liza Cahn, DVM
As a veterinarian and pet parent, I understand the special place that cats fill in our hearts. In addition to staying up to date on routine veterinary care, diligent and knowledgeable pet parents can help keep their furry family members happy and healthy by being on the lookout for any changes in health or behaviors. It’s important for owners to be aware of the common medical conditions that affect cats. Let’s start with hyperthyroidism, a condition that frequently affects our senior feline friends, and the most common hormone imbalance in cats.
What is Hyperthyroidism?
Hyperthyroidism is an endocrine disorder, meaning it involves a hormonal imbalance. Cats have two thyroid glands located in their neck, on either side of the windpipe. These small glands play a critical role in regulating metabolism – the speed at which the body uses energy.
The thyroid glands produce thyroid hormones, primarily thyroxine (T4), and a smaller amount of triiodothyronine (T3). T3 is considered the more active form of thyroid hormone, directly influencing the function of the cells within your cat’s body to help control essential functions such as heart rate, body temperature, digestive and nervous system function, and even brain development in young animals. However, most of the thyroid hormone circulating in the blood is T4 (which is what we test for when diagnosing cats with hyperthyroidism). The body converts T4 into T3 as needed at a cellular level.
In hyperthyroidism, one or both thyroid glands become overactive and produce an excessive amount of thyroid hormones. This acts like an accelerator on your cat’s internal systems – it forces their heart and other organs to work in overdrive. Over time, this constant strain can have detrimental effects on your cat’s health and well-being.
Causes of Hyperthyroidism in Cats
Hyperthyroidism is most commonly seen in middle-aged and senior cats. It is the most common endocrine disorder in cats, affecting about 10% of those over the age of 10 years.
The condition is usually due to a benign (non-cancerous) growth within the thyroid gland, which may be associated with deficiencies or excesses of certain compounds in the diet and chronic exposure to thyroid-disrupting chemicals. In rare cases, hyperthyroidism can be caused by thyroid cancer.
Recognizing the Signs of Hyperthyroidism in Cats
The clinical signs of hyperthyroidism can vary and often overlap with other common feline medical conditions, such as diabetes and chronic kidney disease. Common symptoms include:
- Weight loss despite an increased appetite: A classic sign of hyperthyroidism is a cat with a voracious appetite who continues to lose weight.
- Increased thirst and urination: Hyperthyroid cats often drink and urinate more frequently.
- Hyperactivity and restlessness: Some cats become unusually restless or may exhibit increased activity levels and vocalization, especially at night.
- Changes in behavior: Irritability or behavioral changes might be observed due to the hormonal imbalance.
- Unkempt coat: A cat’s fur may become matted, greasy, and lackluster as grooming routines are neglected.
- Vomiting and diarrhea: Chronic vomiting and diarrhea can be due to many issues, including hyperthyroidism.
- Enlarged thyroid: Your vet may be able to palate an enlarged thyroid while feeling your cat’s neck.
- Less than 10% of affected cats may display atypical signs of hyperthyroidism, consisting of poor appetite and lethargy.
Despite the fact that some clinical signs of cat hyperthyroidism may be viewed as positive (for example, an older cat having the energy of a kitten or an overweight cat losing weight), untreated hyperthyroidism places a significant strain on your cat’s body. Excessive thyroid hormone can lead to serious health consequences, including:
- Heart disease: Hyperthyroidism can overwork the heart muscle, potentially leading to heart failure.
- Hypertension (high blood pressure): Elevated blood pressure is another common complication, seen in 20% of hyperthyroid cats. Untreated it can lead to other issues, such as retinal detachment and blindness.
- Kidney problems: Long-term, the kidneys can be damaged by the effects of hyperthyroidism. It is also common for senior cats to have preexisting kidney disease, which can sometimes be masked by hyperthyroidism.
How is Hyperthyroidism Diagnosed?
If you suspect your cat may be hyperthyroid, a veterinary visit is essential. Even if you are not noticing symptoms of hyperthyroidism, it is a good idea to check a thyroid level as part of your senior cat’s annual vet exam.
Diagnosis of hyperthyroidism in cats is generally straightforward and includes:
- Physical Examination: Your veterinarian will perform a nose-to-tail exam to assess your cat’s overall health. This will also include a thorough palpation of the neck where an enlarged thyroid may be felt in some cases, known as a thyroid slip.
- Bloodwork and urine test: A complete blood count, chemistry panel, and urinalysis will help your vet evaluate your cat’s overall systemic health, rule out other possible causes of their symptoms, and evaluate other organ systems that may be affected by hyperthyroidism.
- Thyroid testing: The primary test for diagnosing hyperthyroidism in cats is a blood test measuring total T4. Cats with hyperthyroidism will have elevated levels of T4. But it’s not always that simple. Some cats, especially those in the early stages of the disease or with other concurrent illnesses, may have T4 in the borderline range. In these cases, your veterinarian might recommend additional thyroid blood tests, such as Free T4 by equilibrium dialysis or a T3 suppression test.
- Blood pressure: Your vet may check for this common complication of hyperthyroidism.
Treatment Options for Hyperthyroidism
If your cat is diagnosed with hyperthyroidism, it can be overwhelming. Fortunately, hyperthyroidism is generally easy to manage, and there are several treatment options available.
Oral Medication
The most common treatment for hyperthyroidism in cats is methimazole, a medication that reduces the production and release of thyroid hormones. This small pill is generally given by mouth twice a day. If your cat is difficult to medicate, you may have methimazole made into a flavored liquid. It can also be made into a transdermal gel, which can be applied to the skin.
This treatment is effective and affordable, and many cats can do well on methimazole long-term. However, it requires a commitment to medication and monitoring blood tests for the rest of your cat’s life. There is also a potential for side effects such as vomiting, lethargy, appetite changes, facial itching, fever, and anemia.
Radioactive Iodine Therapy (I131)
This treatment involves a single injection of radioactive iodine, which targets and destroys the overactive thyroid tissue while leaving surrounding tissues intact. It offers a very high cure rate, eliminating the need for daily medications. Most cats who undergo this treatment have normal thyroid hormone levels within two weeks.
This procedure does not require anesthesia, but due to the use of radioactive materials, cats will require a brief hospitalization (normally 3-7 days) at a specialized facility. It may not be suitable for cats with severe kidney disease or other significant health concerns, and is typically more expensive than medication.
A potential risk of radioactive iodine therapy is the development of hypothyroidism (low thyroid hormone levels) in a small percentage of cats. Clinical signs of hypothyroidism include weight gain, lethargy, and poor hair coat. Hypothyroidism is generally easy to manage with daily thyroid hormone supplements.
Dietary Management
In some cases, pet owners may opt to try a prescription diet to manage hyperthyroidism. These diets drastically restrict iodine, which is necessary for thyroid hormone production. To be effective, this must be the cat’s only food – no treats, table scraps, or extra snacks allowed. This non-invasive option avoids medication and hospitalization, but might not be sufficient for cats with severe hyperthyroidism, other dietary restrictions, or those in multi-cat homes with access to other food.
Surgery (Thyroidectomy)
This procedure involves the surgical removal of the overactive thyroid gland(s). It may be considered when other treatments aren’t suitable or if thyroid cancer is suspected, however is less common than the other treatments. While surgery can be curative, it requires anesthesia and carries a small risk of hypothyroidism or damage to the nearby parathyroid glands, which are essential for calcium regulation.
Secondary Problems
In addition to treating hyperthyroidism itself, it’s important to address any secondary health problems that may have developed. This may involve medications for heart disease or high blood pressure. Once hyperthyroidism is under control, these medications may no longer be needed.
Lastly, many cats with hyperthyroidism also have kidney disease, but it may not always be evident. The abnormally high blood flow to the kidneys associated with hyperthyroidism enhances their ability to filter out waste products, making kidney function markers appear better than they truly are. Once treatment for hyperthyroidism begins and the metabolism normalizes, the “masking” effect disappears. Chronic kidney disease can be managed with dietary changes, medications, fluid therapy, and additional medications, depending on the severity.
Choosing the Right Treatment
The best treatment approach for your cat depends on several factors. Your vet can help guide you, so have a frank discussion with them about:
- Your cat’s health: Age, severity of hyperthyroidism, presence of other health conditions.
- Practical considerations: Your ability to administer medication, potential challenges with dietary restrictions, cost, access to radioactive iodine treatment centers.
- Your preferences: Your comfort level with lifelong medication vs a single curative treatment.
Living with a Hyperthyroid Cat
The prognosis for feline hyperthyroidism is generally excellent with appropriate treatment. Early diagnosis and intervention significantly improve the outcome and help prevent severe complications associated with uncontrolled hyperthyroidism. Prognosis can also depend on the severity of the condition, your cat’s response to treatment, and any other health conditions that your cat may have.
Luckily, with the right care, most hyperthyroid cats go on to live happy, healthy lives. Follow these tips to ensure you’re doing the best for your feline friend.
- Follow the plan: Stick to the medication schedule or dietary changes recommended by your vet.
- Vet visits are vital: Regular checkups are essential for monitoring your cat’s condition and adjusting treatment as needed.
- Stay alert: Be observant for any changes in your cat’s health or behaviors and let your vet know immediately.