Hyperthyroidism is the most common endocrine (hormone) disorder affecting cats. As it commonly affects senior cats, it is an important reason for regular
wellness exams and blood testing.
The thyroid gland consists of two lobes that are situated on either side of the trachea, or windpipe. The gland measures mere millmetres in size, but helps to regulate many important processes in the body.
The hypothalamus and the pituitary gland in the brain are involved in regulating the levels of thyroid hormones by secreting hormones of their own to stimulate the thyroid gland. Hyperthyroidism affects cats of both sexes equally, with cases appearing in those as young as four and as old as 22, with an average age of 13 years. The most common cause of the disease is benign hyperplasia of the thyroid gland; only 1% to 2% of cases are caused by malignant tumours. Several studies implicate dietary and environmental factors in the development of the disease. Feeding cats predominantly canned food (especially fish) and the use of cat litter increase risk. The Siamese and Himalayan breeds were found to be at a decreased risk of developing the disease. Clearly, more research needs to be done.
How can you tell if your cat may have hyperthyroidism? Owners should be monitoring the following: weight loss, increased appetite, increased thirst, diarrhea, hyperactivity, vomiting and bulky, foul-smelling feces. In 10% of cases, cats will exhibit extreme lethargy and weakness, lose weight and have a poor appetite. If your pet is exhibiting any of these signs, they should be seen by a veterinarian. On physical examination, hyperthyroid cats have a rapid heart rate (over 220 beats per minute), a heart murmur and/or a gallop rhythm and poor body condition with an unkempt appearance and thickened nails. Fortunately, diagnosing hyperthyroidism is relatively easy. In most cases, a simple blood test can detect whether hormone levels are elevated.
If left untreated, many hyperthyroid cats will waste away using up first their fat reserves then their muscle tissues. The disease will also cause thickening of the heart, due to the excessive workload placed upon it, and increased blood pressure. This in turn can lead to retinal bleeding or detachment, resulting in sudden blindness. Many untreated cats become very restless, change their sleep patterns and can become quite vocal.
There are several treatment options available for treating hyperthyroidism. The oldest involves surgical removal of the affected thyroid tissue. However, surgery alone may not be curative, and given the risks of the surgery (inadvertently removing the neighbouring parathyroid glands and damaging nerves) it has fallen out of favour among many veterinarians.
Treatment with radioactive iodine will effectively control the disease in 95% of cases, with a single treatment. An oral dose is given in a controlled treatment facility; the radioactive iodine concentrates in the thyroid gland, destroying some of the gland and thereby restoring it back to a normal state. There is no surgery, anesthesia or daily medications involved with this procedure. The biggest drawback is the initial cost, due largely to the fact that patients are required to remain in the treatment facility for a minimum of one week so their urine and feces can be collected and handled as radioactive waste.
The mainstay of treatment among most veterinarians has been the use of antithyroid medications, specifically methimazole. This drug effectively blocks the effect of circulating hormones on the body. Patients start out on an initial dose and then their thyroid levels are retested until the levels are in the normal range. Some patients will vomit or show a diminished appetite on this medication. A complete blood cell count and kidney parameters should be tested, as well. Sometimes by regulating the thyroid hormone levels we uncover an underlying kidney problem. When a cat is in a hyperthyroid
state, their heart rate and blood pressure are usually elevated, which presents more blood to the kidneys for processing. When we reverse this with treatment, we sometimes find that the kidneys were also unhealthy. Now, we have two problems to manage.
The latest treatment for hyperthyroidism is a diet formulated with ultra-low levels of iodine. It has been shown to regulate the thyroid hormone thyroxine (T4) levels in 75% of cats in four weeks; 90% have normal T4 levels by eight weeks and by 12 weeks almost all cats will have normal T4 levels. In one longer-term study, the majority of cats on this diet had normal thyroid levels after one year on the diet. Cats with mild to moderate hyperthyroidism may be more effectively controlled on this diet than those with more severe disease. To work effectively, cats cannot eat any other food including treats or, for those that spend time outdoors, mice or birds.
Hyperthyroidism, as the most commonly diagnosed hormone condition in cats, warrants vigilance on the part of every cat owner and veterinarian. Fortunately, it is treatable, with many effective options available to us, enabling our furry friends to live well into their senior years.
Dr. Dieter Kohlmaier runs Westoak Animal
Hospital in Oakville, ON.