Internal Medicine

Radioactive Iodine Therapy For Feline Hyperthyroidism

Systemic 131 Iodine therapy is a widely accepted treatment modality for cats with hyperthyroidism. Numerous reports in the veterinary literature advocate the use of radioiodine treatment for cats with thyroid adenoma and carcinoma. Advantages of systemic iodine therapy over medical or surgical treatment for cats with hyperthyroidism are that this procedure is noninvasive (compared to surgery) and usually only 1 dose is enough to return the animal to a euthyroid state (compared to daily administration of conventional anti-thyroid drugs.) It is expected that your cat will return to a euthryoid state (normal) between 2 and 4 months after being treated with 131 Iodine.

Hyperthyroidism is caused by hyperactive thyroid glands typically caused by a benign change to one or both thyroid glands where they stop responding to normal feedback mechanisms. This leads to the uncontrolled increase in thyroid hormone levels in the body. Rarely, this increase can be caused by a malignant tumor (thyroid carcinoma) of the thyroid gland (2-4%). Unfortunately, the excess of thyroid hormones leads to several changes in the body, which include heart disease (hypertrophic cardiomyopathy), systemic hypertension, cardiac arrhythmias, masking of chronic kidney disease, weight loss despite a ravenous appetite (rarely decreased appetite), increases in liver enzymes, unkempt hair coat, as well as gastrointestinal signs (e.g. vomiting and diarrhea).

I-131 is considered a permanent and effective treatment in more than 95% of cases. A small percentage of cats will require a second treatment or develop hypothyroidism (require thyroid hormone supplementation). Therefore, certain precautions should be taken before considering this permanent treatment. Excess thyroid hormone increases heart rate and effort leading to increased blood flow to the kidneys. Thus, it is possible that cats with underlying chronic kidney disease (loss of over 66-75% of kidney function) may have adequate urine concentration and normal kidney values due to the increased blood flow to the kidneys. Several attempts have been made in order to diagnose cats with underlying (masked) kidney disease before permanent I-131 treatment. However, the most reliable and predictable form of diagnosing underlying chronic kidney disease is treating hyperthyroid cats medically (temporary treatment) prior to I-131 therapy. It may take 1 to 3 months of medically controlled hyperthyroidism before chronic kidney disease is unmasked (normalization of blood flow to the kidneys). Even though hyperthyroidism is thought to cause further damage to the kidneys due to hyperfiltration because of increased blood flow to the kidneys, cats with the combination of hyperthyroidism and chronic kidney disease tend to eat well. The concern of permanent treatment (I-131) is that a cat that was initially eating well may develop decreased appetite and lethargy once chronic kidney disease is unmasked and kidney values may become elevated. The decreased appetite and lethargy is due to accumulation of toxins that were previously being excreted by the kidneys.

In summary, we recommend medically (methimazole) treating cats with hyperthyroidism for 3 months after becoming euthyroid (normal thyroid hormone level) to maximize the chances of unmasking kidney disease before I-131 treatment.

Unfortunately, methimazole can have deleterious side effects that need to be considered. Some cats can develop gastrointestinal signs (vomiting, diarrhea and decreased appetite), facial scratching, bone marrow toxicity [e.g. neutropenia (low white cell count), thrombocytopenia (low platelet count)], liver toxicity (increased liver enzymes), besides unmasking kidney disease. In this case, it is very important to monitor blood work (CBC and chemistry panel), urine specific gravity, total T4 (thyroid hormone) every 3 weeks until thyroid hormone is controlled. Gastrointestinal signs are more common when methimazole is given orally and can be circumvented by the use of topical methimazole. However, the other side effects occur regardless of the route of administration, thus the importance of discontinuing methimazole if these side effects occur. It is important to remember that topical methimazole takes on average 2 weeks longer to achieve thyroid hormone control.

Other important considerations in a cat with hyperthyroidism include addressing possible cardiac abnormalities (systemic hypertension, hypertrophic cardiomyopathy, tachycardia (fast heart rate), and cardiac arrhythmias. In this case, we recommend getting a blood pressure measurement, ECG, and chest radiographs to assess cardiac function. We may recommend an evaluation by a cardiologist depending on these findings. This is especially important before considering I-131 treatment because it requires isolation. This is aimed at decreasing hospitalization risks. Even though the hypertrophic cardiomyopathy can be secondary to hyperthyroidism, it is important to know the severity of the cardiac disease and assess the risk of cardiac failure during hospitalization.

Furthermore, we recommend stopping methimazole for 1 to 2 weeks (ideally) prior to I-131 treatment. Hyperthyroidism leads to atrophy of the normal thyroid glands because these glands respond to normal feedback mechanisms. Once hyperthyroidism is controlled via medical therapy, these glands go back to secreting thyroid hormone. Thus, they will also be affected (destroyed) if I-131 is given to cats that are normothyroid. Otherwise, atrophied glands are protected from I-131 therapy since I-131 has preferential uptake for thyroid secreting glands. Therefore, theoretically these cats are more predisposed to developing hypothyroidism after I-131. Recently, it has been shown that hypothyroidism is particularly detrimental if cats have concurrent chronic kidney disease. Cats with chronic kidney disease and post-treatment hypothyroidism live on average significantly less than cats with chronic kidney disease that are euthyroid (normal thyroid function). The normal thyroid glands in hyperthyroid (uncontrolled hyperthyroidism) cats are atrophied, therefore will be less affected by I-131 treatment, thus it will be less likely for these cats to become hypothyroid after I-131 treatment.

Thyroid scintigraphy has been recommended as the gold standard test for the diagnosis of hyperthyroidism. There is some controversy, but it is thought that normal cats undergoing methimazole therapy may appear hyperthyroid on thyroid scintigraphy. Besides the benefit of hyperthyroidism diagnosis, thyroid scintigraphy also provides further information regarding the number of thyroid glands affected, ectopic thyroid tissue, the potential for metastatic thyroid carcinoma (malignant cancer), and evidence of metastasis (spread) carcinoma if present. Even though thyroid scintigraphy is not a sensitive method in diagnosing thyroid carcinoma, it provides some support regarding this possibility. This is especially important because conventional I-131 treatment is not effective for thyroid carcinomas. Thyroid carcinomas can be treated via I-131, but much higher doses and hospitalization are required. Furthermore, there is some evidence that thyroid scintigraphy (using thyroid to background ratio) may predict I-131 treatment failure. Anecdotally, it has been suggested that cats with high thyroid to background ratios should receive a higher dose of I-131.

Even though I-131 treatment is very safe and effective, several considerations should be made prior to seeking this therapy. These recommendations are aimed to maximize treatment efficacy and minimize risks.