Diagnosing Hyperthyroidism in Cats

Hyperthyroidism in cats is a very common disease. It involves the endocrine system which regulates hormone levels. It affects many different systems in the cat's body. In hyperthyroidism, the cat's thyroid gland produces too much of the thyroid hormone. The disease can be very serious, especially for the older cat.

The Thyroid Gland

The thyroid gland serves several purposes for the body. Two small lobes make up the gland and are located one on each side of the neck of the cat on each side of the windpipe. The thyroid is responsible for producing two hormones referred to as T4 and T3. The thyroid stimulating hormone or TSH is produced by the pituitary gland which is located at the base of the cat's brain. TSH controls the production of T4 and T3. These hormones regulate metabolism and affect the heart rate and many systems in the body.

Symptoms of Hyperthyroidism

There are many different symptoms of hyperthyroidism that vary from cat to cat. Most cats exhibit unexplained weight loss along with increased food consumption. In around half of the cases, vomiting and increased water consumption develop. The heart rate often speeds up which can result in increased activity, nervousness or other behavior changes. Approximately one third of cats with the disease will stop caring for their fur and may lose hair and develop diarrhea. In a few cases, the cat manifests tremors or weakness.

Physical Effects of Hyperthyroidism

In cats with this disease, the thyroid gland is producing too much of one or both of the thyroid hormones. This is often caused by an increase in the number of abnormal cells in the thyroid gland which form small nodules on one or both lobes of the gland.

Confirming the Diagnosis

There are several ways that a vet can confirm the diagnosis. Most owners report the above symptoms and then the vet does a physical exam. A cat with hyperthyroidism will have a thyroid gland that is large enough to feel with the hand. A blood or urine test will also confirm abnormally high levels of T4. This is more of an indicator than higher levels of T3.

Often other diseases have similar symptoms to hyperthyroidism. If the T4 levels test as normal but signs and symptoms are present, the vet might opt to treat another disease first. Then the thyroid test can be re-administered with more accurate results. Other tests can confirm hyperthyroidism such as a thyrotropin-releasing hormone stimulation test or thyroid radionuclide uptake and imaging.

Which Cats Develop Hyperthyroidism

This disease does not discriminate against breed or gender. Usually cats develop the disease around 13 years of age or older. The incidence of hyperthyroidism has increased in recent years but no one knows why. It could be due to nutritional or environmental factors. Some, but very few cats, develop the disease under 8 years of age.