Hyperthyroidism in Cats
Hyperthyroidism is a disease caused by overproduction of thyroxine, a thyroid hormone that increases metabolism in the body. The thyroid gland normally produces thyroid hormones in response to stimulation by the pituitary gland, the "master gland" of the body. Thyroid hormones normally increase chemical processes occurring within the cells of the body, especially those related to metabolism; however, in hyperthyroidism, the excessive hormone levels push the cells and body into overdrive, resulting in increased metabolism with concurrent weight loss, anxiety, and diarrhea, among other symptoms.
There is no known genetic predisposition for hyperthyroidism, but it is quite common in cats. In fact, hyperthyroidism is the most common hormonal (endocrine) disease in the cat population, often seen in late middle-aged and older cats. (The mean age of discovery is approximately 13 years, with a range of 4-22 years.)
Symptoms and Types
- Involves many organ systems due to the overall increase in metabolism
- Weight loss
- Increased appetite
- Unkempt appearance
- Poor body condition
- Increased thirst (polydipsia)
- Increased urine (polyuria)
- Rapid breathing (tachypnea)
- Difficulty breathing (dyspnea)
- Heart murmur; rapid heart rate; particularly an abnormal heart beat known as a “gallop rhythm”
- Enlarged thyroid gland, which can be felt as a lump on the neck
- Thickened nails
Less than 10 percent of cats suffering from hyperthyroidism are referred to as apathetic. These patients exhibit atypical signs such as poor appetite, loss of appetite, depression, and weakness.
- Overfunctioning thyroid nodules (where the thyroid nodules produce excess thyroid hormones outside of the control of the pituitary gland)
- Rarely, thyroid cancer
- Some reports have linked hyperthyroidism in cats to some canned food diets
- Advancing age increases risk
The signs of feline hyperthyroidism can overlap with those of chronic renal failure, chronic hepatic disease, and cancer (especially intestinal lymphoma). These diseases can be excluded on the basis of routine laboratory findings and thyroid function tests. Your veterinarian will conduct a battery of tests to zero in on a reliable diagnosis.
Thoracic radiography and echocardiography may be useful in assessing the severity of myocardial disease. Abdominal ultrasound may be useful for exploring underlying renal disease.
Thyroid gland scintigraphy (a diagnostic test in which a two-dimensional picture of a body radiation source is obtained through the use of radioisotopes) can be used to diagnose hyperthyroidism and to determine the location of abnormal thyroid tissue. A high concentration of T4 (tetraiodothyronine) in the blood serum is the most common finding, confirming the diagnosis of hyperthyroidism. In some cases, however, the T4 levels may be in the normal range, making a diagnosis of hyperthyroidism more difficult. This is especially true in the early stages of this disease. If your cat is showing the symptoms of hyperthyroidism but the blood tests are not conclusive, you will need to return to your veterinarian for further blood tests.