What is hyperthyroidism?

Hyperthyroidism, a condition in which the cat’s thyroid gland produces too much thyroid hormone (abbreviated T4), is a very common condition in middle aged and elderly cats.  Current studies suggest roughly one in 10 cats will develop this disease over the course of their lifetime.

Thyroid hormone is responsible for regulating the entire body’s overall metabolic rate; as a result, the hormone has far-reaching effects on many organs, including the heart, kidneys, liver, gastrointestinal tract, brain, etc.  An excess of thyroid hormone (T4) develops slowly, but over time, the patient tends to lose weight and muscle mass despite a good appetite, while the liver, kidneys, and other organs are slowly damaged. 

Hyperthyroid cats often develop heart disease, vomiting, diarrhea, liver problems, high blood pressure, and declining kidney function if the disease is not treated.

The cause of hyperthyroidism is not known, but most likely involves an interplay of the cat’s genetic makeup, certain compounds in their diet/water, and exposure to other chemicals in their environment.

Most cats with hyperthyroidism, especially the ones diagnosed early in the course of the disease, have small, benign tumors in the thyroid gland that are producing far more T4 than the body requires.  Hyperthyroidism is easily treated during this stage of the disease.  Roughly 1-2% of cats will have a malignant thyroid tumor that is more difficult to control; these tend to be the cats that have been hyperthyroid for extended periods of time, even if they were managed with medication or special diet to lower their T4 levels.