Radioiodine (I-131) Therapy

Effective Treatment Options for Hyperthyroidism in Cats

Hyperthyroidism in cats is a common issue that primarily impacts older cats. It results from too much thyroid hormone, leading to significant health problems. This article will cover its causes, symptoms, diagnoses, and treatment options.

Key Takeaways 

● Feline hyperthyroidism is primarily caused by a very small benign tumor(s) of the thyroid gland and is characterized by symptoms like weight loss, increased appetite, and behavioral changes. 

● Diagnosis involves a thorough veterinary examination and specific blood tests to assess thyroid hormone levels, ensuring an accurate diagnosis and effective treatment planning. 

● Treatment options include anti-thyroid medication, surgical removal of the thyroid gland, dietary management, and radioiodine therapy, with radioiodine therapy being the most effective, easiest, and having the highest success rates. It is considered the gold standard and is how this disease is treated in humans. 

● Radioiodine therapy is only available in specially equipped facilities, with specially trained veterinarians and support staff. We are so fortunate to be able to offer this therapy in Central Oregon. 

Understanding Feline Hyperthyroidism

Feline hyperthyroidism is a condition resulting from the excessive production of thyroid hormone, and it predominantly affects middle-aged to older cats. This disorder arises due to the overactivity of one or both thyroid glands, often caused by a benign tumor or, in rare cases, by thyroid carcinoma. Hyperthyroidism is the most common endocrine disorder in cats, emphasizing the need for regular health checks, especially in older cats. It is our goal to have this disease diagnosed before it causes outward clinical changes, for the best care of your furry family member. 

This condition can lead to a variety of health problems if left undiagnosed and untreated. The thyroid hormone regulates many aspects of a cat’s metabolism, and excess thyroid hormone can cause significant disruptions in the cat’s body.

Awareness and early detection are crucial, as most cats treated for hyperthyroidism can return to a normal, healthy life. 

Common Symptoms of Hyperthyroidism

Recognizing hyperthyroidism symptoms early significantly improves treatment outcomes. Screening lab tests can reveal the disease before it causes outward changes. Common signs are subtle, and include weight loss despite a good appetite, a poor hair coat, increased thirst and urination, increased energy, anxiety or nervousness, increased vocalizing, and digestive issues like diarrhea or vomiting. Observing any of these changes warrants an appointment with the veterinarian, and when detected early on can lead to prompt diagnosis and treatment, preventing more severe complications. 

If hyperthyroidism is not noticed and allowed to progress, over time this increased thyroid hormone can cause significant heart disease, kidney damage, high blood pressure, muscle loss, and increase risk of blood clots. These problems can appear as a thin, unkempt appearance, difficulty breathing, sudden onset of blindness, an acute painful loss of function in the rear legs, and death. Of course, we never want to let it get to this point, which is why screening is recommended to try to catch these problems early. 

Causes of Hyperthyroidism in Cats

The primary cause of hyperthyroidism in cats is a functional benign enlargement of the thyroid gland, which accounts for 98% of cases. This benign growth, known as an adenomatous hyperplasia, is causing the overproduction of thyroid hormone. 

In rare instances, about 2% of cases, hyperthyroidism may be caused by thyroid carcinoma, a malignant tumor. Understanding these causes helps in tailoring the most effective treatment plans for our feline friends.

Diagnosis of Hyperthyroidism in Cats

Diagnosing hyperthyroidism in cats involves a combination of clinical evaluation and specific diagnostic tests. During a physical examination, a veterinarian will often check for weight loss, changes in heart rate, muscle and coat condition, and subtle changes in thyroid gland size and shape which are telltale signs of hyperthyroidism. Depending on how advanced the disease is, many cats early in the disease process will not have these outward signs. Screening blood tests to detect elevated thyroid hormone levels are routinely run on all cats over 7-10 years of age. 

More specific blood tests, including total T4, free T4, and TSH tests, can be used to confirm the diagnosis. Elevated T3 levels can also be indicative, especially when T4

results are inconclusive. Comprehensive diagnostic testing ensures accurate diagnosis and effective treatment planning.

Initial Veterinary Examination

A thorough veterinary examination is critical in diagnosing hyperthyroidism. It involves taking a detailed history, performing a physical examination, and reviewing any previous test results. The vet will likely order a blood sample to check the total T4 concentration, as this is a key indicator of thyroid function. In cases where symptoms include increased thirst and urination, these signs are carefully evaluated. 

If new concerns arise before the appointment or if it has been more than four weeks since the last lab work, repeated tests may be necessary. Should any abnormalities be detected during this initial evaluation, the vet will discuss additional tests to ensure a comprehensive understanding of the cat’s health. 

This comprehensive method ensures the most effective treatment plan is chosen.

Diagnostic Tests

Several diagnostic tests are essential for confirming hyperthyroidism in cats. These tests typically include a complete blood count, total T4 levels, urinalysis, and thoracic radiographs. Assessing kidney function is particularly important, as many older cats with hyperthyroidism may have concurrent kidney issues that need to be addressed before treatment. And, if hyperthyroidism has been going on for a while, the kidneys are particularly sensitive to the effects of hyperthyroidism, so can be damaged from it. But, with the increased blood flow to the kidneys, the hyperthyroidism can actually mask that damage. The heart is also particularly sensitive to the effects of overproduction of thyroid hormone. 

These diagnostic tests are crucial for both confirming the diagnosis and customizing the treatment plan to the cat’s overall health. A complete health assessment allows veterinarians to minimize risks and tailor the treatment strategy for optimal outcomes.

Treatment Options for Hyperthyroidism

Treating feline hyperthyroidism involves several options, each with its benefits and considerations. The four primary treatment methods are anti-thyroid medication, surgical removal of the thyroid gland, dietary management, and radioiodine therapy. 

Most cats respond well to these treatments, and the prognosis is generally positive, especially when the disease is not severe and does not affect multiple organ systems. Each treatment option has unique aspects that can guide cat owners in making informed decisions for their pets’ care.

Anti-Thyroid Medication

Anti-thyroid drugs, such as Methimazole, are commonly used to manage hyperthyroidism in cats. These medications work by inhibiting the production of thyroid hormones and must be administered consistently at the precise dosage for your cats’ condition twice daily for the cat’s lifetime to maintain normal thyroid function. Most of the tiny tumors that cause hyperthyroidism progress over time, so the dosage needs to be gradually increased over time. This is why blood tests are required at least every 6 months while on the Methimazole. Each dose adjustment needs to be rechecked with blood tests a month later to see if the adjustment keeps the thyroid production within the normal range. Side effects from this medication are not uncommon, with about 20% of cats developing side effects that are not tolerable. Humans can also have side effects when handling the medication. Side effects include severe skin itching, liver damage,  vomiting and diarrhea, and bone marrow damage. 

Though lifelong, this treatment offers a non-invasive way to manage the condition.

Surgical Removal of Thyroid Gland

Surgical removal of the thyroid glands, known as thyroidectomy, is another effective treatment option. This procedure involves removing the affected thyroid tissue, which can cure hyperthyroidism. However, the surgery requires anesthesia and poses risks, particularly for older cats with pre-existing health issues such as heart or kidney disease. 

Despite these risks, surgery can be a viable option, providing a permanent solution without ongoing medication. There is some risk of recurrence in any remaining thyroid tissue, and some cats have thyroid tissue in their chests, which is not addressed with regular thyroid surgery. There is also risk of damage to the neighboring parathyroid tissue, which would require medication for the rest of the cat’s life. Consulting with a veterinarian helps determine if surgery is suitable, based on the cat’s health and specific circumstances.

Dietary Management

Dietary management involves providing an iodine-restricted diet, which can effectively control hyperthyroidism by limiting the production of thyroid hormones. Strict adherence to the diet is necessary throughout the cat’s life for this approach to be effective, so no treats or other foods, no hunting outdoors. Some cats can’t get enough protein to maintain their muscle mass on this diet, others seem to do okay. Multi-cat households pose another challenge with dietary management. 

While non-invasive, it requires commitment from the owner to maintain the specific dietary regimen.

Radioiodine Therapy

Radioiodine therapy is considered the most effective treatment for feline hyperthyroidism. Administering radioactive iodine (I-131) selectively targets and destroys overactive thyroid tissue, sparing healthy tissue. 

The therapy is easy and effective, with most cats returning to normal thyroid hormone levels within weeks of treatment. Its high success rates and minimal side effects make it particularly favored over other treatments. It’s a “one and done” treatment. 

Radioiodine Therapy in Detail

Radioiodine therapy is considered the Gold Standard for hyperthyroidism in cats. This section explores how the therapy works, its administration, and the required hospitalization and isolation period post-treatment.

How Radioiodine Therapy Works

Radioiodine therapy works by using radioactive iodine, specifically I-131, which is absorbed selectively by the overactive abnormal thyroid tissue, destroying that abnormal tissue at a cellular level. The radioactive iodine emits both gamma rays and beta particles, effectively targeting the thyroid gland while sparing other tissues. This treatment is known as radioactive iodine therapy, radioiodine therapy, or I-131 therapy. 

Targeted destruction results in very high cure rates, with many cats permanently achieving normal thyroid hormone levels within a few weeks post-treatment. Others take a bit more time to reach the normal range for thyroid hormone production. Less than 5% of cats remain somewhat hyperthyroid six months after the therapy, and will need a second dose. For 3-10% of treated cats, their normal thyroid tissue takes a while to re-awaken, so we monitor at 1, 3, and 6 months post treatment to support them during this transition to their permanent normal thyroid status.

Administration of I-131

Administering I-131 involves a single subcutaneous injection, the same way a vaccine is given. We do this with provided comforts like an irresistible food treat and loving pets to distract and keep them thinking about other things. It takes just a few seconds to administer. 

Because I-131 is a radioactive substance, it takes very specialized handling, and special equipment to handle and monitor its use. It is highly regulated, so there is specialized training and documentation of its use. After it is administered to the patient,

your cat must remain isolated from others as the I-131 is metabolized and the waste (urine and feces) loses its radioactivity, known as radioactive decay.

Hospitalization and Isolation Period

After receiving the radioiodine injection, cats are typically hospitalized for 4 days to monitor their recovery and ensure their safety. Some cats may require more than 4 days to reach safe radiation levels. This takes place in a lead shielded ward used only for this purpose, so no dogs or other stressors are around, and no unauthorized personnel can enter. Daily monitoring for radioactive output is conducted during this period as a precaution and to determine when it is safe for them to be discharged from the hospital to their home and family. Basic care during the isolation period includes feeding and cleaning the cat’s accommodations. We also provide lots of comforts like cushy bedding, videos to watch, hiding places, whatever gives them the most at-home feel. If any of their regular medications are needed, that can be provided. If they need any extra support from the veterinary staff, that will be provided as well.

Post-Treatment Care and Monitoring

Post-treatment involves two weeks of keeping the cat indoors, special handling of the cat’s waste, and minimizing contact with people. Homes with vulnerable individuals like children, minors, or pregnant women may be best to have an extended isolation period of at least 5-8 days.

At-Home Care Guidelines

Post-radioiodine therapy at-home care involves several precautions to minimize radiation exposure. Limit interaction with the treated cat to 5-10 minutes twice daily. All other times, try to keep your cat at a distance no closer than 6 feet for as much of the 2 weeks as possible. After handling the cat or its items, washing hands is essential to reduce radiation exposure. 

Pregnant women and children under 18 should not handle the cat post-treatment for a period of 2 weeks. 

Collect all waste and litter from the treated cat for two weeks and store it securely for 80 days to allow radiation decay.

Follow-Up Testing

Follow-up testing is a critical component of post-treatment care. Blood tests to monitor thyroid hormone levels and kidney function should be performed at Bend Animal

Emergency and Specialty Center or your primary veterinarian’s office using either Idexx or Antech labs. We also want to monitor weight, body condition, coat condition, heart rate and their home habits such as appetite, thirst, activity and general well being. 

Regular follow-ups detect issues early and help us adjust treatment plans as needed, ensuring the cat’s continued improvement in health and well-being.

Potential Side Effects and Risks

While all treatment options for hyperthyroidism are generally effective, they come with potential side effects and risks. Knowing these risks helps cat owners make informed decisions and prepare for possible complications.

Hypothyroidism

One potential risk of treatments, including radioiodine therapy, is the development of hypothyroidism. Around 5% of cats treated with I-131 may develop hypothyroidism, some requiring daily hormone supplementation to maintain normal thyroid function. Hypothyroidism can happen with radioiodine, surgery, and methimazole treatment. 

Monitoring thyroid hormone levels post-treatment is essential to manage this risk and ensure the cat’s health.

Kidney Function Concerns

Kidney function is another critical concern, particularly in older cats. Pre-diagnostic tests ensure the best outcome before radioactive iodine treatment. But, even with those tests, pre-existing kidney disease can be masked by hyperthyroidism, and therefore can be unmasked after treatment. Successful treatment for hyperthyroidism can improve overall health, and stop ongoing kidney damage by alleviating hyperthyroidism complications like blood clots, heart failure and hypertension. 

Ongoing monitoring of kidney values is essential to manage any potential issues post-treatment related to chronic kidney disease.

Prognosis and Long-Term Outlook

The prognosis for cats with hyperthyroidism is generally good, particularly with early detection and appropriate treatment. 

This section discusses the success rates of various treatments and the importance of managing chronic conditions.

Success Rates of Treatments

Radioiodine therapy boasts the highest cure rate of approximately 95-98% after a single treatment. Most cats return to normal thyroid function within weeks, making it a highly desirable treatment option. 

Studies comparing survival times of cats treated with I-131 to cats treated long term with oral medication (Methimazole) show that cats live twice as long if treated with I-131. In our experience, we also have seen how much better the cats seem to feel after treatment with I-131, even after some time being on Methimazole. 

Another study comparing surgical treatment and Methimazole treatment also saw a 2-fold difference in survival time with surgery-treated cats living twice as long as Methimazole treated cats .This study also noted a recurrence rate of 13% in the surgically treated cats, 27% recurrence rate in the Methimazole treated cats. The effects of anesthesia on the cats were not considered. Yearly compared costs were considered the same.

Managing Chronic Conditions

Managing chronic conditions is essential for the long-term health of cats treated for hyperthyroidism. Anti-thyroid medications (methimazole) need careful management and regular veterinary monitoring to be effective. Dietary management and surgical options also contribute to long-term care, depending on the cat’s specific needs. Other chronic conditions that older cats can have include kidney disease, heart disease, high blood pressure, all directly affected by hyperthyroidism, especially if not perfectly controlled. These conditions can happen independently from hyperthyroidism as well. Other unrelated chronic conditions such as diabetes, asthma, obesity, GI disease including IBD, pancreatitis, and lymphoma, and many cancers also can affect older cats. (This is why we recommend a thorough screening workup prior to treating with radioiodine) 

Regular follow-up testing and ongoing monitoring help manage chronic conditions and ensure the cat’s well-being. 

Summary

Hyperthyroidism in cats, while common, can be effectively managed with the right treatment approach. From medication and diet to surgery and radioiodine therapy, the options available offer hope for a healthy future. Regular monitoring, early detection, and informed decision-making are key to ensuring your cat’s well-being. By understanding the symptoms, causes, and treatments, you can provide the best care for your feline friend, ensuring they lead a long and happy life.