Geriatric endocrinology: Hyperthyroidism (Proceedings)
Hyperthyroidism is a common disease of cats over 6 years of age. Feline hyperthyroidism is caused by adenomatous hyperplasia or adenoma of the thyroid gland; carcinomas are rare. The disease is bilateral in 70% of cases. The cause remains unknown. Clinical signs can vary depending upon which stage the disease is identified. Weight loss is present in about 90% of cases with polyphagia in about 50% of cases. Cats can become hyperactivity, anxious, and are prone to this especially when being examined by a veterinarian. Most cases have a thyroid slip.
In rare cases apathetic hyperthyroidism can occur with decreased appetite, weight loss and severe lethargy.Routine laboratory tests
• Elevated liver enzyme (ALT, AST, alkaline phosphatase) activity is commonly present.
Indicated when dyspnea, tachypnea, or muffled heart and lung sounds are present.
• Electrocardiography is indicated when an arrhythmia is suspected.
Specific tests for diagnosis
Hyperthyroidism is usually readily diagnosed by documenting elevated serum total T4 concentration. Normal serum T4 can be seen in hyperthyroid cats with mild hyperthyroidism, hyperthyroidism with a concurrent nonthyroidal illness, or a disease other than hyperthyroidism. If the total T4 is normal and hyperthyroidism is still suspected, serum T4 should be measured again in 1-4 weeks. Alternatively free T4 with a dialysis procedure can be run. This is a more sensitive test (98% vs. 90% with just T4). Falsely elevated fT4 can be seen in cats with nonthyroidal illness.
Thyrotropin releasing hormone (TRH) stimulation test
As effective as the T3 suppression test in diagnosing hyperthyroidism in difficult cases.
T3 suppression test