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Hyperthyroidism in a Cat Cicero, a 13 year old male cat, presented to our hospital for his yearly exam and vaccines. On examination, Cicero appeared very healthy. He did have some mild tartar on his teeth and according to his owner, had lost some weight. She had switched all her cats over to timed feedings, rather than free feeding them, and felt his weight loss was because of that. We performed a feline leukemia and FIV test as this had never been done. His vaccines were administered and we drew some blood for a senior wellness blood panel. This included a chemistry panel, a urinalysis, a CBC (complete blood count) and a thyroid level.
Cicero was started on a medication, called methimazole, which blocks the production of thyroid hormone. The goal was to lower his thyroid level to the low end of the normal range. His thyroid level was rechecked one month after being on the methimazole twice daily. His thyroid level was significantly lowered to less than 0.2. There was also one other abnormality on his lab work. His red blood cell count was lowered to 20%. Previously it was 44%. The normal range is 29% to 48%. After several adjustments to his medication over the course of the following 2 months, we finally were able to get his thyroid level to 0.88 and his red blood cell count to 43%. Cicero did have his teeth cleaned once we were able to control his thyroid level and he is doing very well. Hyperthyroidism is a fairly common disease of senior cats. While advancing age is a risk factor for developing the disease, environmental and dietary risk factors have been investigated and may play a role in predisposing cats to hyperthyroidism. The thyroid gland is in the neck and plays a very important role in regulating the body’s rate of metabolism. Hyperthyroidism is a disorder characterized by the overproduction of thyroid hormone and a subsequent increase in the metabolic rate. On average, affected cats are about 12 years of age. The most consistent symptom is weight loss secondary to the increased rate of metabolism. The cat tries to compensate for this with an increased appetite. In fact, some of these cats have a ravenous appetite. Despite the increased intake of food, most cats lose weight. The weight loss may be gradual or rapid. Affected cats often drink a lot of water and urinate frequently. These cans may have periodic vomiting or diarrhea. Their hair coat may be unkempt. In some cases, the cats completely lose their appetite as the disease progresses. Two secondary complications of this disease can be significant. These include hypertension (high blood pressure) and a heart disease called hypertrophic cardiomyopathy. Hypertension develops as a consequence of the increased pumping pressure of the heart. In some cats, the blood pressure can become so high that the blood vessels in the retina of the eye can burst or cause the retina to detach all together. This can lead to temporary or permanent blindness. Heart problems develop because the heart must enlarge and thicken to meet the increased metabolic demands. Both of these problems may be reversible with early detection and treatment. Diagnosis of hyperthyroidism is relatively straightforward and generally diagnosed with a blood test. The first step is to determine the blood level of one of the thyroid hormones, called thyroxine (T4). Usually the T4 level is so high that there is no question as to the diagnosis. Occasionally, a cat suspected of having hyperthyroidism will have T4 levels within the upper range of normal cats. When this occurs, other tests, such as a Free T4 level or a T3 suppression test are performed. A thyroid scan can also be performed at a veterinary referral center or the T4 can be measured again at a later time. Once a cat has been diagnosed with hyperthyroidism, other tests will need to be performed to ensure that the overall health of the cat is optimum before any treatment is instituted. This includes a CBC (complete blood count), chemistry panel, urinalysis, chest radiographs (x-rays), blood pressure, ECG (electrocardiogram) and possible cardiac ultrasound. There are several treatments for hyperthyroidism and many factors must be taken into consideration before choosing the best therapy for a cat with this disease. All the treatment options have pros and cons and should be weighed before making a decision. Radioactive iodine is considered the gold standard for treatment. In a high percentage of cases, this process destroys all the abnormal thyroid tissue without causing harm to any of the other organs. However, this procedure is costly and can only be performed by veterinary referral centers. In a few cases, the radioactive iodine treatment may need to be repeated. Surgery is also an effective option for treatment of hyperthyroidism. This, however requires anesthesia and a skilled surgeon. Finally, oral medication can control the effects of an overactive thyroid gland by blocking the production of thyroid hormone. The drug must be given for the remainder of the pet’s life. The medication, called methimazole, can also have some undesirable side effects in some cats such as vomiting, diarrhea and inappetance. Many owners of cats with hyperthyroidism are hesitant to have radiation therapy or surgery because their cat is older. But remember, age is not a disease. The outcomes following both surgery and radiation are usually excellent and most cats have a good chance of returning to a normal state of health. Cicero’s case is important for several reasons. First, the only symptom of hyperthyroidism that was observed by his owner was weight loss. The owner was certain it was because she placed all her cats on a diet and controlled their portions. It is important to look for trends of weight loss. If your pet has always had a difficult time losing weight and then suddenly begins to lose weight, be aware that there could be something more occurring than just your new diet plan. Cicero appeared to be a very healthy cat on his physical examination, aside from some dental tartar. He did not appear to be an underweight cat and his body condition was actually good. Cicero’s case teaches us that pets do not necessarily follow the textbook. While there are many symptoms of hyperthyroidism, your pet may show very few. Sometimes, they appear to not have any symptoms. This prompts us to recommend performing bloodwork regularly, annually if possible. Second, Cicero’s case teaches us the importance of performing annual wellness blood panels in senior pets. Bloodwork panels can give information about the function of their internal organs and catch problems before they occur. Most problems can be detected on physical examination, however bloodwork catches those things that are not detected with a thorough physical examination. Had we not performed bloodwork on Cicero, we would have likely discovered his thyroid disease when he was very symptomatic and possibly very ill. We recommend annual bloodwork for all pets, primarily senior pets. Thirdly, we are reminded that methimazole can have some side effects. One of the side effects we may observe is bone marrow suppression. We can also see vomiting, diarrhea, decreased appetite and liver problems. Cicero did appear to have some bone marrow suppression as seen by his low red blood cell count. Thankfully, his bone marrow seemed to rebound when his dose of methimazole was lowered. Finally, Cicero’s case teaches us the importance of follow-up. It is very important to have regular follow-up examinations and bloodwork when a pet has been diagnosed with a chronic disorder and placed on a long-term medication. Cicero will need to be on medication for the rest of his life. As he ages, his medication requirements and doses will change. The only way for us to monitor him carefully is to perform regular bloodwork panels and examinations. We will monitor his weight closely as well his heart and other organ function. Hyperthyroidism is a common disease diagnosed in older cats. There is no way to prevent your cat from developing hyperthyroidism. By giving your pet special attention and monitoring their daily behaviors and attitudes, you may be able to help your pet before he or she begins to have problems. Cicero continues to be a healthy and happy senior pet. |
His bloodwork results came back with one primary abnormality, his thyroid level was very high. Normally the thyroid level falls between 0.8 and 4.0. Cicero’s thyroid level was 9.47. There appeared to be some evidence of mild dehydration, but overall the rest of his bloodwork was normal.