There are many clinical signs associated with feline hyperthyroidism:
Weight loss despite excellent appetite.
Enlarged Thyroid Glands (found at physical exam)
Hyperactivity or restlessness, especially at night
Excessive vocalization and irritability
Excessive shedding and an poor haircoat
Hyperthyroidism is generally a disease of older cats. The average age at diagnosis is about thirteen.
The diagnosis is based on several factors including clinical symptoms, the finding of a thyroid nodule on physical exam, and expected lab work abnormalities associated with this disease. Once hyperthyroidism is suspected, a comprehensive blood panel is ordered to assess, not only the thyroid, but the general health of the patient including hepatic, renal, hematological, and other body systems. If thyroid hormone (T4) is markedly elevated the diagnosis is clear. And since the thyroid is important regulator of metabolic drive and numerous other important roles, disorder can result in other abnormalities including elevated liver enzymes and, if unrecognized or left untreated, can result in kidney failure. Untreated hyperthyroidism can severely affect the heart as well so additional diagnostics may be recommended based on the results of lab work and physical exam.
Sometimes diagnosis can be unclear:
As previously mentioned, a high T4 in a patient with symptoms of hyperthyroidism and a palpable thyroid nodule make diagnosis easy. The thyroid is an endocrine organ however (an organ that secretes hormones) and therefore is easily influenced by the condition of the body: variables in T4 include normal day-to-day fluctuations, discrepancies related to the age and/or physique, and the existence of other coexisting illnesses (not uncommon in older felines unfortunately.) Often a T4-by Equilibrium dialysis (T4ED) will be ordered which most of the time can confirm the diagnosis. Even an elevated T4ED is irrefutable evidence of hyperthyroidism however; whereas T4 can be suppressed by coexisting illness, T4ED can be falsely elevated in this same situation. So, on rare occasions, a Technetium scan (a nuclear imaging technique) will necessary for definitive diagnosis (for more information click here).
Scan of cat with normal thyroid glands
Scan of hyperthyroid cat, showing enlarged thyroid glands
What Causes Hyperthyroidism?
Hyperthyroidism is caused by an adenomatous growth in the thyroid gland that produces a toxic amount of T4 (thyroid hormone.) Approximately 98% of the time these tumors are benign but occasionally can be cancerous. Many people want to know what caused their cat’s thyroid gland to grow a tumor, benign or otherwise. Although no cause has been identified it has been speculated that diet may be a factor. Feline hyperthyroidism correlates most with multi nodular toxic goiter in humans. Iodine seems to play an important though undefined role in the development of this disease.
Why is it so Important to Treat Hyperthyroid Cats?
Patients with hyperthyroidism are frequently brought to the veterinarian for obvious problems such as weight loss, muscle deterioration, and gastrointestinal disturbance. These visible symptoms however may be less important to longevity and quality of life than the damage that untreated hyperthyroidism does that is unseen: kidney damage and cardiovascular disease. These problems can result in heart failure, sudden blindness, or sudden death and all can be prevented with timely treatment for thyroid disease. The longer treatment is delayed the greater the risk for physical illness not just symptoms.
Treatment versus Cure:
There are numerous reasons why curative treatments are recommended over control-treatments but the most compelling reason is proposed by Dr. Mark E. Peterson who discovered feline hyperthyroidism 1979 who recently speculates that cancer is suspected to be a result from unrecognized and/or untreated hyperthyroidism. This theory is based on the fact that thyroid tumors continue to grow in size despite treatments that control symptoms rather than cure the disease.