Important Concurrent Diseases

The presence of other diseases or abnormalities in hyperthyroid cats is an important factor when deciding on a treatment plan and giving a prognosis.

Chronic kidney disease (CKD)

Kidney disease is common in older cats and is an important complicating factor in hyperthyroid cats. It is often masked due to the increased glomerular filtration associated with the increased renal blood flow found in hyperthyroidism. Once the circulating thyroid hormone levels have been corrected, the underlying CKD may be unmasked. Initial medical treatment in cats with concurrent renal disease allows the veterinarian to monitor the renal function during treatment.

Permanent treatment options such as thyroidectomy or radioiodine have a poor prognosis when there is concurrent CKD and long-term medical treatment is recommended.The International Renal Interest Society (IRIS) staging system is used to assess, monitor and guide treatment of CKD. Read more about IRIS staging of CKD.

Cardiovascular disease in hyperthyroidism

Cardiac abnormalities are also common with tachyarrhythmia, premature beats and gallop rhythms commonly noted. Pleural effusion and overt heart failure at presentation are rare but could be precipitated by stress during the work-up of the case. Significant cardiac disease should also be addressed. In most cats the secondary cardiomyopathy appears to resolve after circulating thyroid hormone levels are controlled. In cases with severe tachycardia, medical treatment may be necessary to protect against stress tachycardia but must be used with care if there is evidence of heart failure. Treatment of tachycardia can be continued until the effects of excess circulating thyroid hormones have been resolved.

Hypertension has been documented in as many as 87% of cats with hyperthyroidism. Cats with hypertension should be stabilized as appropriate and be monitored carefully.

The most important complicating concurrent disease in hyperthyroid cats is renal dysfunction which is relatively common in older animals.