11December

Feline Hyperthyroidism

Case of the Month, Sep 2005

Feline Hyperthyroidism

Byron has always been a very relaxed cat, content to nap the day away. Now, he acts nervous and restless, pacing the house and meowing more frequently. His thirst, urination, and stools appear normal, but he is losing weight despite a marked increase in his appetite. There has been no couching or sneezing but he has vomited a few times.

SIGNALMENT:

Ten year old, neutered male, domestic shorthair cat named "Byron".

HISTORY:

For the past few months a change in his usual behavior has become apparent. Byron has always been a very relaxed cat, content to nap the day away. Now, he acts nervous and restless, pacing the house and meowing more frequently. His thirst, urination, and stools appear normal, but he is losing weight despite a marked increase in his appetite. There has been no couching or sneezing but he has vomited a few times.

PHYSICAL EXAM:

  • Weight = 9.5 (he was 12 lbs one year ago)
  • Temperature = 100.0 F
  • Heart rate = 200 beats/minute
  • Respiratory rate = 16 breaths/minute

Byron's physical exam reveals some notable problems. His weight loss is apparent, he has also lost fat and muscle over his back making his spine appear more prominent. There is a smooth nodular growth felt on the left side of the thyroid cartilage in his neck. Byron also now has a sustained increase in his resting heart rate and his pulse would be described as bounding, and his haircoat, which used to be smooth and slick, now appears dull and unkempt.

PROBLEMS:

  • Abnormal change in behavior
  • Weight loss
  • Elevated heart rate
  • Thyroid nodule
  • Vomiting

DIFFERENTIAL DIAGNOSIS:

  • Gastroinestinal disease
  • Cardiac disease
  • Hyperthyroidism

DIAGNOSTIC PLAN:

The first step in the diagnostic work up was to run a senior profile. The senior profile includes a complete blood count, chemistry profile, and a urinalysis.

RESULTS OF SENIOR PROFILE:

The complete blood count revealed no abnormalities, but the chemistry profile showed mild increases in the liver enzymes AST and ALT. Byron's kidney enzymes and electrolytes are normal and his urinalysis reveals well-concentrated urine without evidence of inflammation. Thyroid hormone concentrations called T3 and T4 were markedly elevated out of the normal range.

DIAGNOSIS:

A diagnosis of feline hyperthyroidism was made based on history, physical exam findings, and blood results showing elevated thyroid hormones.]]>

DISCUSSION:

Hyperthyroidism is a pathologic, sustained, increased metabolism caused by high circulating concentrations of thyroid hormones. It is caused by the growth of abnormal thyroid nodules that secrete excessive amounts of thyroid hormones. Hyperthyroidism is the most commonly diagnosed endocrine disorder in middle aged and older cats. The average age at diagnosis is 13 years and there is no gender predilection. Clinical signs are a result of metabolic changes caused by excessive thyroid hormones. Classical symptoms include weight loss despite increased hunger, restlessness, and hyperactivity. Vomiting is frequently observed. Concurrent problems frequently associated with hyperthyroidism include thyrotoxic changes in the heart muscle and insufficient kidneys masked by enhanced kidney perfusion.

There are two treatment options available for cats diagnosed with hyperthyroidism. Anti thyroid drugs are available and they inhibit the synthesis of thyroid hormones. They are administered once to twice daily for the life of the cat. They are generally well tolerated but adverse reactions such as lethargy, anorexia, and vomiting may occur. They can also cause blood disorders such as decreasing platelet numbers. Tapazole is the most commonly prescribed anti thyroid drug and is available in tablet and liquid forms or it can be compounded and administered transdermally. The other treatment option is radioactive iodine therapy. This is a safe and effective treatment for hyperthyroidism. There are treatment centers that specialize in administering a radionucletide Iodine 131. Administered by injection, this radionucletide concentrates in the abnormal thyroid tissue and destroys it while sparing the normal thyroid and surrounding structures. Cats are kept at the center for 3-5 days allowing for excretion of the "hot" bodily fluids. The success rate is high with greater than 90% of the cats having normal thyroid levels within three months.

OUTCOME:

We ruled out concurrent heart disease with thoracic radiographs and an ultrasound of his heart (echocardiogram). We discussed the treatment options and radioactive iodine therapy was chosen. We chose this treatment because he was free of concurrent heart disease and kidney disease. It was also the treatment of choice due to the high success rate, lack of adverse side effects, and Byron's parents did not want to have to medicate him daily for the rest of his life.

He was referred to Radiocat and the treatment was performed. Three months later we rechecked his blood work and both his liver enzymes and thyroid levels had returned to normal. He gained back all the weight he lost and his coat has regained its luster. His behavior as well returned to normal, his parents are very happy with the results.

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