Hyperthyroidism in Cats

Hyperthyroidism is a common disease of the thyroid gland in cats caused by increased production of thyroid hormones by enlarged glands. The excess in circulating thyroid hormone results in a high metabolic state, which causes a variety of symptoms and changes. Most cats afflicted by hyperthyroidism are middle-aged or older with the average age at onset being 12 years of age.

Causes

Cats have two thyroid glands, one on either side of the larynx (voice box). Hyperthyroid patients have enlargement and increased hormone production of the thyroid glands. In most cases, enlargement of thyroid glands is caused by a non-cancerous benign tumor called an adenoma. Some rare cases of hyperthyroid disease are caused by malignant tumors known as thyroid adenocarcinomas.

Symptoms

Cats afflicted with hyperthyroidism gradually develop a variety of symptoms that may be subtle at onset but will become more severe as the disease progresses.

• Weight loss and muscle wasting
• Increased ravenous appetite
• Vomiting and diarrhea
• Increased thirst and urination
• Nervousness and hyperactivity
• Increased vocalization
• Poorly kept or matted haircoat

On physical exam a veterinarian may find a palpably enlarged thyroid glands.  A thin body condition with muscle weakness is also a common finding. Some patients will have an elevated heart rate, a heart murmur, and arrythmia or irregular heartbeat. In rare cases some cats may ocular changes including have loss of sight or sudden blindness.

Diagnostic Tests

A veterinarian who suspects a cat has a thyroid problem will likely order a variety of tests including bloodwork, urinalysis, blood pressure, and imaging. 

• Blood tests to measure circulating thyroid hormones including T4, fT4, and TSH. Repeat measurements of T4 may be needed over time to identify trends and response to therapy.
• Tests to evaluate kidney health including blood tests for BUN, Creatinine, and SDMA will be recommended. Urine may also be collected for a urinalysis.
• Blood pressure monitoring to assess for hypertension.
• A cardiac evaluation may be needed in some patients including chest x-rays, an electrocardiogram (ECG), and an echocardiogram (heart ultrasound) to assess the heart function.

Treatment

Several therapy options for hyperthyroidism exist including radiation, medication, nutrition, and surgery.

• Radioactive Iodine (I-131) Therapy – Radioactive iodine therapy is the treatment of choice for cats with hyperthyroidism. The I-131 procedure is performed at a referral specialty hospital. The radioactive iodine is given to the cat by injection. Radioactive iodine irradiates and destroys the hyper-functioning thyroid tissue. The treated cat then remains hospitalized until its radiation level has fallen to acceptable limits. The radioactivity carries no significant risk for the cat, but precautionary measures are required for people who come into close contact. The advantage of this therapy radioactive iodine therapy is curative in approximately 95% of cases with most cats needing no additional therapy. Disadvantages of this therapy include an elongated stay in the hospital and upfront costs.
• Medication Therapy – The most common medication used to treat hyperthyroidism in cats is methimazole. The FDA approved methimazole for cats comes in a pill form called Felimazole. The medication is given once to twice daily. Periodic monitoring is needed to adjust the dosage and to detect side effects. Side effects arise in 15-20% of treated cats and include loss of appetite, vomiting, lethargy, and blood work changes. Administration of this medication is required for the life of the cat. If a cat is difficult to medicate the methimazole medication may be compounded into a flavored liquid to ease oral administration or topical gel to apply to the ear.
• Nutritional Therapy – A prescription food called Hill’s Prescription Diet Y/D Feline Thyroid Health is an available treatment option. The prescription diet is very iodine restricted. Iodine is necessary for the synthesis of thyroid hormone. A diet with restricted iodine will inhibit the thyroid gland from producing excessive amounts of thyroid hormone. A caveat is that a cat on this diet can only eat Hill’s y/d. No other food or treats can be eaten.
• Surgical Therapy – Removal of the thyroid glands, called surgical thyroidectomy is a treatment option hyperthyroidism. Surgery removes the adenoma or benign enlargement of the thyroid gland. Surgical therapy requires general anesthesia which can be risky for older felines with heart and kidney complications. Medication and radioactive iodine therapy are equally effective therapies, but are less invasive so surgical treatment has become a less popular treatment option.

Follow- up Care & Secondary Complications

All feline patients with hyperthyroidism will need regular progress exams and diagnostic tests. Routine blood tests to measure thyroid hormone levels are done after starting treatments and are repeated periodically for cats to monitor response to therapy.

For many felines hypertension, or high blood pressure, is a complication of hyperthyroidism. Long term high blood pressure causes additional damage to the eyes that results in sight loss and to the kidneys that may result in renal failure. Hypertension may need to be controlled with medication and monitoring to reduce the risk of damaging these organs. After the hyperthyroidism has been successfully treated, high blood pressure will often resolve, and long-term therapy may not be required.

Some cats with hyperthyroidism may require additional diagnostic tests and treatment to for secondary heart disease. Elevated thyroid hormones stimulate an increased heart rate and a stronger contraction of the heart muscle which can cause thickening of the left heart muscle over time. These changes eventually compromise the normal function of the heart and can even lead to heart failure. However, once the underlying hyperthyroidism has been controlled, the cardiac changes may improve.

Prognosis

The prognosis for cats that receive Radioactive I131 treatment early in the course of the disease is excellent. Most feline patients will be cured of the disease following one treatment and will have a normal life expectancy free of symptoms. The prognosis for cats that receive methimazole medication or dietary therapy with Y/D food is good. Medication and food therapy can help to alleviate symptoms and can prevent some of the secondary kidney, heart, and ocular complications. Most cats receiving consistent therapy will have a good quality of life with their symptoms controlled.

Cats that have developed secondary complications including heart disease, loss of sight, and renal failure have a guarded prognosis. Some heart, kidney, and eye changes can be permanent. Many of these patients will have a poor quality of life and needed extensive therapy and monitoring. Cats that do not receive treatment or are diagnosed very late in the disease process have a poor prognosis. The disease will progress over time and the symptoms will become more severe.