Hyperthyroidism in Cats

March will be dedicated to Hyperthyroid awareness!!

Hyperthyroidism is the most common endocrine (hormone related) disease in cats.  As many as 10-15% of cats will develop this disease sometime in their lifetime. Luckily, this disease is generally very easy to control in feline patients.

The thyroid gland is an organ that lies just below the larynx (voice box).  Cats have a single, bi-lobed thyroid gland that is roughly shaped like a butterfly that rests on the trachea (windpipe).  The thyroid gland is responsible for producing thyroid hormone which is important for many processes that affect metabolism.

In cats, hyperthyroidism is caused by a tumor of the thyroid gland.  Generally (greater than 98% of the time), these tumors are classified as functional adenomas of the thyroid gland.  An adenoma is a growth of abnormal cells that is NOT cancer, and therefore benign.  In the normal thyroid gland, thyroid hormone is produced only when thyroid stimulating hormone is produced in the brain in response to low thyroid hormone concentrations.  In cats with hyperthyroidism, the tumor is producing thyroid hormone without any direction from the brain.  This excess hormone production leads to many problems with many of the body’s organ systems.

Cats affected by hyperthyroidism almost always lose weight despite eating well.  Increased thyroid hormone concentrations will increase the metabolic rate of almost all cells in the body.  The heart will beat more quickly and with greater force.  Blood flow to the organs and muscles will increase and energy utilization will quickly rise.  This greater demand for energy makes affected cats lose large amounts of weight and they eat more in the attempts to maintain their original weight.  Many cats will also increase their water intake and will use the litter box more.  Increased anxiety, vomiting, diarrhea, more or less attention seeking, urinating or defecating outside of the litter box, increased vocalizations (especially at night), and poorly cared for hair and skin are other clinical signs that owners often notice.

Feline Hyperthyroidism is usually easily diagnosed.  Most affected cats will have signs noticed by their owners, obvious abnormalities with their blood work and high levels of thyroid hormone in their blood.  Among other things a total T4 test will be ordered by your veterinarian to measure the thyroid hormone in your cat’s blood.  Sometimes further testing is needed to confirm the presence of the disease.  Your doctor will discuss these options if necessary.

There are many treatment options for hyperthyroidism.

  • Medication:  The most common medication used to treat hyperthyroidism is methimazole.  This drug interferes with the enzyme pathway in the thyroid gland and prevents the production of thyroid hormone.  This drug does not destroy nor slow the growth of the thyroid tumor, but it generally manages the clinical signs associated with the disease very well.
  • Radioiodine therapy (I-131 therapy):  This is the Gold Standard of hyperthyroid treatments in cats.  Radioactive iodine is usually injected under your cat’s skin and the drug gets stored in the thyroid tumor.  The radiation produced by this material kills the thyroid tumor and in more than 90% of cases the disease is cured without complication.  Your doctor will generally recommend a trial with methimazole prior to treatment with I-131.
  • Surgery: The only other treatment that is potentially curative.  Unfortunately this procedure is invasive, has a high failure rate (almost 1 in 3 cats will fail surgical treatment), is often more expensive than I-131 treatment and has more complications than radioactive iodine.  We do not recommend surgery for the treatment of hyperthyroidism at Elm Creek Animal Hospital.
  • Diet:  There is a prescription diet that exists that is deficient in iodine.  Without adequate iodine, the thyroid gland cannot produce thyroid hormone.  This can be successful in the treatment of hyperthyroidism, but it does not cure the disease.  There is also a high treatment failure rate due to the difficulty of strictly restricting the diets of cats, especially those in multiple-cat households.  Except in certain, specific situations, we do not recommend diet to control hyperthyroidism for our feline patients at Elm Creek Animal Hospital.

Untreated hyperthyroidism can lead to high blood pressure, heart disease, strokes, kidney damage and even blindness.  Luckily, at Elm Creek Animal Hospital, screening for hyperthyroidism is part of every senior health screen for aging cats.  This disease is rarely left untreated in cats who maintain their regular check-ups with their doctor at Elm Creek.

If you have any questions or concerns about hyperthyroidism or your cat’s health, please contact Elm Creek Animal Hospital today!

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