Hyperthyroidism in Cats

Hyperthyroidism in cats is fairly common, especially in cats aged 12-13, but symptoms may present in cats as young as seven years old.

Hyperthyroidism is a condition characterized by enlarged thyroid glands that produce excess amount of thyroid hormone (T3 and T4). Typically, hyperthyroidism is caused by a benign tumor on the thyroid gland. The chances of this tumor being malignant are slim, 2%-5%.

There are a number of symptoms of hyperthyroidism that may present, but upon noticing one or two of them, you'll want to consult your vet. Some of the symptoms that may present are:

Increased appetite;
Excessive thirst;
Increased temperature;
Diarrhea;
Increased stools and urine output;
Weight loss and loss of muscle tone;
Irritability, nervousness;
Frequent vomiting (beyond the occasional hairball);
Lack of luster to the coat;
Weakness.

Your vet will diagnose your cat by two processes, physical exam and CBC test. The physical exam consists of the vet palpating or feeling the cat's neck area. Often, the vet will be able to detect the enlarged gland. The vet will also take pulse rate and blood pressure as elevated numbers here may indicate hyperthyroidism (or a kitty who's freaked out about going to the vet!)

Because hyperthyroidism can mask or mimic other diseases (more on this later), a CBC blood panel is conducted along with a thyroid specific (usually a T4 test) which will specifically show thyroid hormone levels. An elevated T4 number is a clear indication of hyperthyroidism.

Complications of Hyperthyroidism in Cats

Because thyroid hormones impact nearly all the organs of the body, elevated levels of thyroid hormones can cause complications beyond the disease symptoms. Thyroid hormones stimulate a faster heart rate and a stronger contraction of the cardiac muscle. Over time, if left untreated, cats with hyperthyroid can develop an enlarged heart muscle and a thickening of the left ventricle (hypertrpohic cardiomyopathy). When the underlying thyroid disease is effectively controlled, these cardiac changes will often improve or be resolved altogether.

Hypertension (high blood pressure) is another complication of untreated hyperthyroidism, and can cause damage to other vital organs,such as the eyes, kidneys, heart and brain. If hypertension is diagnosed along with hyperthyroidism, drugs may be required to treat this condition until the underlying hyperthyroidism is under control. Once the hyperthyroidism is managed, the hypertension will usually resolve, no longer requiring drug treatment.

Another complication of hyperthyroidism in cats is the high incidence of concurrent kidney disease, and often, hperthyroidism can mask symptoms of this condition. For this reason, many vets will first treat with a pharmaceutical called methimazole (Tapazole (TM)). Methimazole renders the thyroid gland less efficient in hormone production, but is reversible. In other words, the thyroid will return to the pre-drug level of hormone production. By administering a reversible treatment, and potentially revealing kidney disease, the vet may make a more informed decision as to the course of treatment for both conditions. Hyperthyroidism actually serves as a renal protectant, causing a higher than normal blood flow and increased filtration rate. It is however important to control thyroid production to safeguard the overall health of the cat, but in the presence of kidney disease, at a rate high enough to keep increased filtration and blood flow at an optimal level.

Once hyperthyroidism has been diagnosed in your cat, what are the available options for treatment? Click here to explore.