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.
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.
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.
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.
A veterinarian who suspects a cat has a thyroid problem will likely order a variety of tests including bloodwork, urinalysis, blood pressure, and imaging.
Several therapy options for hyperthyroidism exist including radiation, medication, nutrition, and surgery.
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.
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.