Your cats thyroid gland, a vital part of his endocrine system, consists of two flat lobes located at the base of his neck, alongside the upper part of his trachea (windpipe). This gland secretes the hormones, thyroxine (T4) and tri-iodothyronine (T3), that it synthesizes from dietary iodine and releases directly into the bloodstream. Closely adhered to the thyroid gland are the four parathyroid glands. These produce parathyroid hormone (PTH) that regulates calcium and phosphorus metabolism. The thyroid is the thermostat for the metabolic rate of your cats body.
Hyperthyroidism is the most common endocrine disease in cats. About one in 200 older cats suffer from it, says Deborah Greco, DVM, endocrinologist at the Animal Medical Center in New York. Its a disease of cats older than 5 or 6 years of age. It hasnt been reported in young cats, but anything is possible. Only five percent of hyperthyroid cats are younger than 10 at diagnosis.
The Thyroid Connection
Hyperthyroidism occurs when a tumor (benign or malignant – most are benign) causes the thyroid to crank out excessive quantities of thyroxine and tri-iodothyronine. Untreated, hyperthyroid cats go into what may be described as uncontrolled metabolic overdrive.
Its not known whether tumors that cause hyperthyroidism in cats are due to a primary thyroid disorder or whether an external factor or combination of factors is involved. Theres lots of research ongoing about the pathogenesis of the disease, says Greco, who is board certified by the American College of Veterinary Internal Medicine. A lot of folks suspect an environmental or dietary cause. Feeding of canned, fish-flavored food was associated with increased incidence of hyperthyroidism in one recent study. Indoor cats seem to be more at risk. Proposed risk factors also include use of cat litter (goes with being an inside cat).
First reported as a clinical disease in cats in 1979, the prevalence of hyperthyroidism has been recognized only as more cats enjoy better health and care – and longer lives. Most veterinarians recommend routine thyroid screening for older cats. Bring your cat in for annual physical exams and laboratory screening for total T4 levels after age 5, advises Greco. Signs suggestive of hyperthyroidism call for immediate testing. Because of the potential for numerous and serious secondary complications, early diagnosis and prompt treatment are vital.
Because it causes disruption of regulatory mechanisms, hyperthyroidism can adversely affect virtually any organ or body system. About 25 percent of hyperthyroid cats have high blood pressure. Increased levels of thyroid hormones in the bloodstream can cause the heart to speed up perilously (often greater than 240 beats per minute), precipitate heart murmurs, and lead to a form of
cardiomyopathy, a dangerous heart condition that thickens the heart muscle. Without prompt treatment, up to 10 percent of hyperthyroid cats will experience heart failure. Fortunately, heart damage that occurs is usually reversed when successful treatment returns thyroid hormones to normal levels.
If your cat already suffers from kidney disease or reduced kidney function, hyperthyroidism can worsen his condition. In some cats, hyperthyroidism precipitates kidney disease. Because hyperthyroidism can increase blood flow through the kidneys, temporarily augmenting renal function in cats and obscuring underlying kidney problems, successful treatment sometimes unmasks pre-existing renal disease.
Signs to Watch For
Early signs can go unnoticed. Gradual weight loss despite normal appetite is often the first sign. As the disease progresses, the cat eats ravenously – but still loses weight. His rate of water consumption and urination may greatly increase, and he may vomit, have diarrhea, or show other signs of digestive distress. Behavioral changes can be dramatic and can include extreme agitation, pacing, crying and calling, and aggression in a normally placid animal; behavior symptomatic of urine marking can develop. If the condition has gone undetected for a long time, a hyperthyroid cat may become depressed, lethargic, or stop eating entirely. While any of these signs call for concern, their appearance in combination, especially in an older cat, calls for prompt thyroid testing.
Diagnosis and Testing
Since hyperthyroidism is a disease of older cats, other conditions, including diabetes and liver and kidney diseases, must be considered before making a definitive diagnosis. Any of these conditions can occur concurrently with hyperthyroidism and can produce similar signs. About 10 percent of hyperthyroid cats are also diabetic, notes Greco. Chronic renal failure and diabetes mellitus are two diseases that can mimic hyperthyroidism.
Presented with an older cat showing signs consistent with hyperthyroidism, your veterinarian will measure heart rate. Hell also palpate the thyroid, checking for enlargement or nodules (goiter). Enlargement of one or both nodes is detected in 80 to 90 percent of hyperthyroid cats. The presence of goiter, coupled with elevated heart rate, is a strong indicator, though not a definitive diagnosis, of hyperthyroidism. A serum thyroid hormone concentration test confirms the diagnosis.
The most common test is a TT4, total thyroxine level, coupled with the presence of a thyroid nodule, says Greco. Any older cat that shows signs consistent with hyperthyroidism should have the level of T4 in his bloodstream measured. Although most hyperthyroid cats will show elevated T4, a few – particularly very old cats – will have T4 values in the normal range, despite the presence of disease. For these individuals, re-testing after several days can confirm the diagnosis.
If not, and if signs consistent with hyperthyroidism persist and other conditions have been ruled out, more specialized testing is called for. Further testing such as free T4 or T3 suppression tests are indicated in 10-20 percent of hyperthyroid cats, says Greco. Free T4 (level of T4 unbound to protein and available to enter cells) tests are more likely to return consistent results and less likely to be influenced by factors like concurrent disease or ongoing drug therapy. T3 suppression tests are more complicated (involving re-testing thyroid hormone levels after the cat is given extra T3) but can confirm diagnosis in unusual cases when simpler tests cannot.
A range of treatments offers excellent results. Surgical removal of the diseased thyroid tissue and radioiodine therapy promise permanent cures. Long-term medical management with thioureylene drugs such as methimazole (Tapazole) or carbimazole or iodinated contrast agents (calcium ipodate) offers reversible treatment. All therapies carry risks and benefits. Relationship and convenience issues may influence the choice of treatment as much as medical considerations.
The treatment of choice is iodine-131 therapy, says Greco. Its effective in about 90 percent of cats, requires no surgery or anesthesia, and doesnt place the vital parathyroid glands at risk. The major disadvantage? It requires isolation at a treatment facility for at least seven to 14 days following the administration of radioactive iodine. During that time, safety concerns – and federal regulations – prohibit you from visiting. Even when your cat returns home, youll be instructed to minimize contact for an additional few weeks. But radioiodine therapy offers most hyperthyroid cats the best and safest chance of a permanent cure.