Does My Cat Have Hyperthyroidism?

What is hyperthyroidism?

Hyperthyroidism is an extremely common disease of the middle-aged and older cat. It is a multi-system disorder caused by an increase in the amount of thyroid hormones (called T3 and T4) produced by an enlarged thyroid gland. It was first documented in cats 31 years ago but the cause of the disease has been elusive. Although the enlargement in the thyroid gland is caused by one or more tumors, these thyroid tumors are usually non-cancerous or benign tumors (adenoma or adenomatous hyperplasia) of the thyroid gland. Carcinoma (cancer) is a rare cause of hyperthyroidism in cats, and tends to develop most common in hyperthyroid cats treated for months to years with medical treatments, such as methimazole.

What are the symptoms that a hyperthyroid cat could exhibit?

Excessive thyroid hormone secretion accelerates most bodily processes and gradually causes one or more clinical signs that are apparent to the cat owner. The most consistent finding is weight loss, secondary to the increased rate of metabolism. The cat tries to compensate for this by eating more (increased appetite - see Figure 1).

Figure 1

In fact, some of these cats have a ravenous appetite and will literally eat anything in sight! (Figure 2) 

Figure 2

Despite the increased intake of food, however, most cats lose weight. The weight loss may be so gradual that some owners will not even realize it has occurred, or it may happen more quickly.

Other signs that the hyperthyroid cat may show include anxiety or nervousness, a rapid, pounding heart rate, muscle weakness, vomiting, diarrhea, increased thirst or urination, panting (Figure 3), and an unkempt hair coat (Figure 4).

Figure 3

Figure 4

Over time, untreated hyperthyroidism leads to deleterious effects on many of the cat's internal organs. Heart disease, muscle wasting, chronic emaciation, and/or severe metabolic dysfunction can develop, ultimately leading to death.

What should you do if you think your cat could be hyperthyroid?

If you think that your cat may have hyperthyroidism, you should make an appointment for a check-up with your veterinarian. On physical examination, your vet may feel an enlarged thyroid nodule in the neck (Figure 5), a finding that is consistent with hyperthyroidism.  

Figure 5

If hyperthyroidism is still suspected based on your cat’s symptoms and exam findings, your veterinarian will order a complete blood count and blood chemistry panel to evaluate overall body health, as well as a thyroid hormone (T4) value.  The vet may also order a chest x-ray and an ultrasound to be thorough.

Most hyperthyroid cats will have high levels of the thyroid hormone T4 in their bloodstream, confirming hyperthyroidism   In a few cats, especially those with early disease or concurrent problems, more specialized thyroid testing or a thyroid scan may be needed to confirm the diagnosis.

Once hyperthyroidism has been confirmed, there are several treatment options. They include treatment with radioactive iodine (I-131), surgical removal of the gland (thyroidectomy), and treatment with anti-thyroid medications.  Overall, after treatment, the prognosis is good to excellent depending on your cat’s overall condition and treatment success.