Greenbriar Animal Hospital
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Feline Hyperthyroidism

Feline Hyperthyroidism

Hyperthyroidism (overactive thyroid gland) is a hormone disorder that occurs most commonly in older cats. This is usually because of a benign tumor in the thyroid gland(s). The thyroid gland manages our metabolism- when it is high, the metabolism is increased. 


The signs of hyperthyroidism can vary tremendously. You may notice weight loss in the face of an excellent appetite, hyperactivity or irritability.  Cats may have episodes of decreased appetite followed by bouts of greatly increased food consumption.  Heart irregularities are commonly seen as the condition worsens. It also will cause kidney disease and high blood pressure.

The "ravenous" appetite may contribute to your cat eating too quickly, vomiting, developing diarrhea, or having an increased number of bowel movements.  Untreated, the disorder can weaken your cat and shorten his/her life.

The physical examination and your history may imply hyperthyroidism. Typically these cats eat well but still lose weight. They will also have an increased heart rate.  A blood panel is needed to diagnose the disease. Most cats will have an elevated T4 level. Some may need an additional thyroid test called "free-T4" to confirm the diagnosis.  Because the thyroid influences the blood pressure, heart muscle and kidney blood flow, all of those organs must also be checked.

The recommended treatment is radioactive iodine which selectively attacks the thyroid gland. This is an oral medication given at a specialist clinic.  It is generally curative, and the need for post procedure monitoring is limited.  The cat stays at the facility for 3-5 days (has he/she is temporarily radioactive), but then does not usually require any further treatment.  It is the most expensive option up front, costing $2000-$3000.

Alternative treatment is with daily medicine called methimazole. It must be monitored regularly with bloodwork.  This is a lifelong treatment and may need adjustment as the tumor grows.  The medication comes in a pill form, but can be readily compounded into a liquid or transdermal (absorbed through the skin of the inner ear).

Some cats will eat a speciality diet by Science Diet called y/d.  If cats like the iodine restricted diet, and that is ALL the cat eats, and he/she drinks bottle water, the diet alone can control the disease.

Often when we treat hyperthyroidism hidden kidney disease is exposed which then also needs to be addressed. If there is high blood pressure it must be treated.

With the proper care, hyperthyriodism can be managed and many cats live for years with this disease, although requiring life long monitoring.