In the Cat

Hyperthyroidism in the cat

Hyperthyroidism is an excessive production of the thyroid hormone by the thyroid glands, which are situated in the neck area overlying the trachea (windpipe). The cause of this overproduction is unknown

The disease was first documented some 25 years ago and has become the most common endocrine (hormonal) disorder of cats, surpassing even diabetes. It is primarily a disease of older cats (mean age 13 years). Because thyroid hormone controls the rate of body metabolism, many of the body systems are affected

The most commonly observed symptom is a voracious appetite. I have known a cat to eat four 400gram cans of food a day, most of which was voided as soft or sloppy faeces. Despite the massive appetite, hyperthyroid cats lose weight. Other common signs include vomiting, increased thirst, hyperactivity and increased vocalisation, especially at night

Upon veterinary examination, 70% of these cats have enlarged thyroids, which can be palpated. Heart rate is often increased and a murmur may be audible on auscultation with a stethoscope. Diagnosis is made by a blood test of the circulating T4 hormone. Because these cats are often elderly, treatment can be complicated by the concurrent presence of kidney or heart disease

There are three treatment options

Surgical removal of the thyroid glands

This is complicated by the fact that embedded in the thyroid glands are the parathyroid glands which control calcium metabolism. Seaforth Veterinary Hospital does not recommend this procedure

radioactive iodine

This which destroys part of the thyroid tissue. This is a specialist procedure, which is quite expensive. Cats are hospitalised for some time after treatment, as they are radioactive

Both the above treatments have the disadvantage of being irreversible


There are a variety of drugs that can be used

Carbimazole - tablets are given 2-3 times daily

Methimazole - orally. A once daily tablet

Methimazole - topically. This cream is rubbed into the cat's inner aspect of the ear. It is well tolerated an da good second option for clients who just cannot pill their cat

The advantage here is that the T4 levels can be monitored and adjusted accordingly. Also if any kidney disease is exacerbated, the drug can be withdrawn. The disadvantage is that the treatment is ongoing for the rest of the cat's life
With regular monitoring, cats can live many more years, gain weight and live happy lives