19203 Aurora Ave N
Shoreline, WA 98133
(206) 546-1243


5506 Pacific Avenue
Tacoma, WA 98408
(253) 471-9200

Restoring Health & Happiness to Senior Cats Since 1992

Restoring Health & Happiness to Senior Cats Since 1992

What is Feline Hyperthyroidism?

Hyperthyroidism is the most common hormonal disease of cats resulting from secretion of excess thyroid hormone (T4). Thyroid hormone (T4) is responsible for regulating the metabolic rate of all cells in the body, which results in an overstimulation of nearly all organ systems. This overstimulation puts significant and potentially fatal stresses on the body.

How does hyperthyroidism affect my cat?

95% of hyperthyroid cats are classical in their presentation and exhibit some or all of the following: excessive appetite, weight loss, attitude changes, hyperactivity, aggression, tremors, increased vocalization, hair loss/unkempt coat, excessive drinking, excessive urinating, vomiting and/or diarrhea.

5% of hyperthyroid cats are apathetic in their presentation. These cats may be lethargic and actually have a decrease in appetite, with or without the above clinical signs. Some cats alternate between the classical and apathetic forms, thus having increases and decreases in appetite over time.  

The most concerning feline patient is the one that manifests serious heart disease secondary to hyperthyroidism but without any other signs of hyperthyroidism to alert owners or veterinarians to the presence of the disease.

What can happen if my cat is left untreated?

In general, cats waste away, manifesting any of the signs listed above. Hyperthyroidism affects nearly every cell and organ in the body. The most significant problems are typically related to the cardiovascular system.  Severe heart disease secondary to hyperthyroidism can occur even prior to weight loss. Hyperthyroid cats’ heart rates are often between 220 – 350 beats per minute. This prolonged stimulation of the heart by the thyroid hormone will cause the heart muscle to thicken. This leads to smaller chambers within the heart and a subsequent loss of the heart’s ability to pump blood efficiently (hypertrophic cardiomyopathy). This is a life-threatening condition. If treated early, this problem can reverse itself after 131I treatment or surgery.  Methimazole (Tapazole/Felimazole) often does not control the progression of cardiac disease even though it may appear to resolve the other signs of hyperthyroidism. Another potentially serious complication to hyperthyroidism is hypertension. The elevation in blood pressure can cause debilitating strokes and /or blindness secondary to detached retinas. People with hyperthyroidism often experience severe anxiety and severe psychological disturbances.  In cats we certainly see behavioral changes that most likely represent this.

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Monty is 12 years old. Dr. Vaughan had been monitoring Monty’s thyroid over the years and identified that he was borderline for hyperthyroidism.  After discussing his overall appearance, behavior and appetite, we decided that it made sense to treat him.  We were so relieved to... read more »


Pox’s Story Does it make sense to treat hyperthyroid in a fourteen-year-old, mostly outdoor cat? That’s what we were asking ourselves in the summer of 2013. Pox let me know his opinion by following me around all summer, talking continually. He plainly said, “I feel lousy;... read more »

Seattle, WA

Will & Grace found me in 1999 when they were four weeks old.  They were born near in a parking lot off Highway 99.  They are twins and do everything together.  This year they celebrated their 15th birthdays and are thriving in excellent health due to the... read more »

Will,Grace,and Hunter
Edmonds, WA

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