LISTEN - Hyperthyroidism is a disease caused by excess amounts of circulating thyroid hormone in the bloodstream. This is a common medical condition in middle-aged to geriatric cats, and it usually results from one or more benign tumors (ie, adenoma or adenomatous hyperplasia) of the thyroid gland. Carcinoma (cancer) is a very rare cause of hyperthyroidism in cats, at least in the initial stages of the disease.

Excess thyroid hormone accelerates most bodily processes and gradually causes the clinical signs that are apparent to most owners. The most consistent finding is weight loss secondary to the hyperthyroid cat’s increased rate of metabolism. Most cats try to compensate for their weight loss and increased rate of metabolism by eating more. In fact, some of these cats have a ravenous appetite and will eat literally anything in sight! However, despite the increased food intake, most cats lose weight. While the weight loss may happen quickly, it may also be so gradual that some owners will not even realize it has occurred. Other signs of hyperthyroidism may include anxiety or nervousness, a rapid, pounding heart rate, muscle weakness, vomiting, diarrhea, increased thirst or urination, heat intolerance, panting, and an unkempt hair coat. Over time, untreated or poorly controlled hyperthyroidism leads to deleterious effects on many of the cat's internal organs. Heart failure, kidney and liver disease, muscle wasting, chronic emaciation, and/or severe metabolic dysfunction can develop, ultimately leading to death.

Fortunately, hyperthyroidism is both familiar to veterinarians and relatively easy to diagnose. The veterinarian can diagnose hyperthyroidism based on the cat's history, physical examination abnormalities, and results of routine laboratory tests and thyroid hormone tests. Most cats have one or two palpable, enlarged thyroid tumors nodules in their ventral neck area, an important finding for making the diagnosis. Almost all cats will also have clearly high serum thyroid hormone levels, diagnostic for hyperthyroidism. In those cats in which the thyroid tumor cannot be palpated or that have only borderline high thyroid concentrations, thyroid imaging (or scanning) can be extremely useful in confirming hyperthyroidism.

See “Why does my cat need a thyroid scan?" question in the Thyroid Scanning FAQ section.

Cat Thyroid Storm: A Hyperthyroid Cat, Before Radioiodine Treatment Click here to watch the video

Formerly Hyperthyroid Cat, Now Cured of Hyperthyroidism With Radioiodine Click here to watch the video

Yes, all hyperthyroid cats have thyroid tumors – every last one. However, if your cat is hyperthyroid, there is no need to panic! Most of the thyroid tumors that cause hyperthyroidism in cats are benign or non-cancerous, so it is very unlikely that your cat has cancer, especially if the hyperthyroid condition was only recently diagnosed.

All thyroid tumors in hyperthyroid cats are made up of thyroid cells that grow and function at an uncontrolled rate. This makes your cat’s thyroid gland grow larger and larger with time. In almost all hyperthyroid cats, the veterinarian can palpate the thyroid tumor.

Click here to read the rest of Dr. Peterson's informative blog post on this topic.

Hyperthyroidism can be treated medically, surgically, by life-long feeding of an iodine deficient diet, or with radioiodine. Medical treatment consists of administering methimazole (Felimazole™) one to three times per day. Methimazole treatment usually controls the signs of hyperthyroidism, but it is not a cure (i.e., the drug will block thyroid hormone secretion but will not remove or destroy the thyroid tumor). The drug must be given for the rest of the cat's life, and it is very difficult to maintain serum thyroid hormone values within the normal range over many months. Finally, because the thyroid tumors continue to grow while on methimazole, many cats develop very large thyroid tumors after a few years, and some of these benign thyroid tumors will undergo malignant transformation into thyroid carcinomas (cancer).

Feeding a low iodine diet can work to lower thyroid hormone values, but does not destroy the thyroid tumor. In addition, the commercially available iodine deficient diet (y/d) is not very palatable and is relatively low in protein content, especially for the cat —a carnivore with high protein needs.

Surgery cures the hyperthyroid condition by removing part or all of the thyroid gland. Obviously, this requires anesthesia, and postoperative complications are not uncommon.

Radioiodine (radioactive iodine; I-131) also cures hyperthyroidism. The procedure for this treatment is simple; it is a single subcutaneous injection of a radioiodine, given much like a routine vaccine.

Click here to see for yourself how radioiodine compares to medical or surgical treatments

Medical therapy works in most cats, but there are several reasons it may not be the best choice. First, some cats are difficult or impossible to pill. Second, mild reactions (e.g., loss of appetite and vomiting) are common, and a few cats develop serious adverse reactions to the medication (blood or liver problems; facial itching; drug allergy). Because of these side effects, it is necessary to periodically monitor the cat’s condition with blood tests. Finally, some owners may not want to have to medicate their cat daily for the rest of his or her life, especially if the cat is young or only middle-aged.

It is important to remember that while medical therapy "blocks" production of thyroid hormone secretion, the thyroid tumor(s) will still grow and enlarge over time. Some of these thyroid tumors will also transform from a benign to a cancerous tumor over time (months to years). In many cats treated with long-term medical therapy, the signs of hyperthyroidism recur when the tumor becomes so large that the medication can no longer effectively block thyroid hormone secretion.

Surgery is generally an effective treatment for hyperthyroidism, but it has disadvantages in many cats. Many cats with hyperthyroidism have heart problems and thus have more risks with anesthesia. There is also a considerable risk that there will be damage to the parathyroid glands (4 little glands located within or adjacent to the thyroid) during thyroid surgery, resulting in hypocalcemia (low blood calcium). This complication can be life-threatening and results in substantial additional hospitalization costs. After surgery, some cats develop hypothyroidism and require treatment with thyroid hormone pills for a period. Finally, there is a considerable risk that the surgery will not completely cure the hyperthyroidism, or that the condition will recur (i.e., radioiodine has a much smaller chance for recurrence).

Radioiodine therapy has some distinct advantages over medical or surgical treatment, and virtually all authorities consider it to be the treatment of choice for hyperthyroidism. With radioiodine, there is no need for anesthesia and the risk of postoperative hypocalcemia is eliminated. Methimazole (Felimazole™) treatment is not needed. The major draw back is that after receiving radioiodine, the cat must be kept in the hospital for a period (usually 4 to 5 days). Overall, radioiodine provides a simple, effective, and safe cure for cats with hyperthyroidism, regardless of age.

Normal thyroid function requires oral intake of the element iodine (iodine is a normal part of both human and cat diets and is commonly added to salt). Once ingested, iodine is taken up by the thyroid gland, where the iodine becomes incorporated into thyroid hormone. Radioiodine (radioactive iodine; I-131) is a radioactive form of stable iodine. In its radioactive state, it naturally decays, giving off two types of radiation: gamma rays (similar to X-rays) and beta particles (electrons). The half-life of I-131 is 8 days. In other words, one-half of the radioiodine decays every 8 days.

When a cat receives radioactive iodine, the thyroid tumor tissue takes up the radioiodine just as readily as normal, nonradioactive iodine (the kind in iodized salt). All thyroid tumor tissue will take up the radioiodine in this fashion, no matter where in the body it is located. This is an important feature of radioiodine therapy because 10% of hyperthyroid cats have thyroid tumors in the chest cavity, where surgical removal is not feasible. Once the thyroid tumor tissue has taken up the radioiodine, the radioiodine emits radiation, destroying the overactive thyroid tumor cells.

Because the hyperactive thyroid tumor suppresses the function of any normal thyroid tissue, the normal cells do not concentrate any of the radioiodine. Because almost all hyperthyroid cats have some residual, normal thyroid tissue, this means that cats rarely develop an underactive thyroid gland (hypothyroidism) after radioiodine therapy. These spared normal thyroid cells then can “turn back on,” and make normal amounts of thyroid hormone after the radioiodine has destroyed all of the tumorous tissue.

After treatment, the residual (normal) thyroid tissue resumes full function within 3 to 6 months. On average, over 95% of all hyperthyroid cats treated with radioiodine are permanently and safely cured with a single injection! As an added benefit, radioiodine poses no risk to the parathyroid glands, the four small glands located adjacent to the thyroid gland, which are extremely important for maintaining normal calcium levels. Therefore, hypocalcemia (low blood calcium) secondary to parathyroid damage never occurs after treatment with radioiodine. This is markedly different from surgical treatment, which very commonly damages the parathyroid lands, causing life-threatening hypocalcemia.

Dr. Mark E. Peterson Interviewed About Hyperthyroidism in Cats Click here to watch the video

How Does Radioactive Iodine Work in a Cat with Hyperthyroidism? Click here to watch the video

Hyperthyroid cats of any age can be successfully treated with radioiodine, as long as they have no other life-threatening medical conditions. In order to rule out any additional medical conditions, your veterinarian needs to perform a number of screening tests before you schedule an appointment at Hypurrcat for the radioiodine treatment.

Dr. Peterson was the first veterinarian to develop this procedure for use in hyperthyroid cats. He has over 45 years of experience successfully administering radioiodine therapy to cats. It is not a new or experimental treatment; Dr. Peterson has treated over 10,000 hyperthyroid cats in his long career.