Fresno Veterinary Specialty and Emergency Center is proud to announce the addition of I-131 therapy for Hyperthyroid cats to our specialty services. Below you can find information about the referral process and what you can expect when you take your cat home. Please contact our office with any questions at 559-451-0800.


For the safety of all hospital patients, all dogs and cats must be leashed, crated or carried at all times. Retractable leashes must be locked at all times.

What is Hyperthyroidism?PDFWhat can I expect when I take my cat home?PDFI-131 Referral GuidelinesPDF

What is Hyperthyroidism?

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.

Hyperthyroidism is an overproduction of thyroid hormone by the thyroid gland. It is typically caused by a benign hyperplasia (overgrowth) of the thyroid gland. When hyperactive thyroid tissue develops, normal thyroid tissue atrophies because thyroid hormone production by the abnormal tissue is excessive. Excessive amounts of thyroid hormone cause a hyper metabolic state.

Malignant thyroid tumors also cause hyperthyroidism, but are very rare (an estimated 2-5% of cases). Radioactive iodine treatment is the best hyperthyroid treatment for cats with malignant thyroid cancer. In cats with a thyroid carcinoma, extension or metastasis to the thoracic cavity is common, so physical palpation alone will grossly underestimate the full extent of these malignant tumors.

Elevated thyroid hormone levels cause increased metabolism and the other common clinical signs of hyperthyroidism: weight loss, appetite changes, increased water consumption, vomiting, diarrhea, rapid heart rate, heart murmur, elevated blood pressure, increased vocalization, muscle weakness, and poor hair coat. Hyperthyroidism tends to be a gradual onset disorder with many cats losing weight over several months or a few years.

Hyperthyroidism is a common disease condition of older cats (average age is 12 to 14 years old). The majority of these cats will have abnormalities on routine chemistry panels reflecting damage to the liver, kidneys and muscle. Most hyperthyroid cats will have some degree of hypertrophic cardiomyopathy (which is a condition where the heart muscle becomes thickened). Since cats with the most common signs of hyperthyroidism (weight loss, appetite changes and vomiting) can also have other problems such as kidney disease, diabetes and inflammatory bowel disease, a thorough physical examination, blood testing and urinalysis are usually indicated to allow diagnosis of hyperthyroidism. Radiographs, cardiac ultrasonography and blood pressure determination are also sometimes performed.

Cats with both hyperthyroidism and chronic renal failure may have an increase in kidney waste products (azotemia) after treatment for hyperthyroidism. For the recently diagnosed hyperthyroid cat, the option of trial therapy with Tapazole should be considered so that the effect of treatment on renal functions can be evaluated.

Some thyroid nodules or tumors can be found by physical palpation of the thyroid gland in most cats, but about a third of hyperthyroid cats have thyroid tumors located within the thoracic cavity and cannot be palpated. Most of the hyperthyroid cats without palpable cervical nodules have thyroid tumors (adenomas or carcinomas) that have become extremely large. Because of the chronic effects of gravity on the progressively enlarging thyroid mass, these tumors migrate into the thoracic cavity. Some cats develop hyperthyroidism secondary to adenomas of ectopic, intrathoracic, thyroid tissue.

If this disease goes untreated multiple organ systems may be affected and damaged. Untreated, the increased metabolism will eventually cause organ failure (heart, kidney, and blindness due to retinal detachment) and premature death.