Hyperthyroidism is a clinical condition resulting from the excessive production and secretion of thyroxine (T4) and triiodothyronine
(T3) by the thyroid gland which was first reported around 1979. These hormones regulate the body's metabolic rate and affect
every system in the body. The production of the thyroid hormones is controlled by the hormone called thyroid stimulating hormone
(TSH). TSH is produced by the pituitary gland, which is found at the base of the brain. The thyroid gland is small and consists
of two lobes, one on each side of the trachea (windpipe) in the neck. If the thyroid gland produces excess amounts of the
thyroid hormones, the condition called hyperthyroidism results. The most common cause is a benign (non-cancerous) increase
in the number of cells in the thyroid gland. Groups of these abnormal cells form small nodules (multinodular adenomatous hyperplasia
or goiter)on the thyroid gland or adenomas. Multiple adenomas may form in the same lobe, and in approximately 70% of the cases,
both lobes are involved. Only 1-2% of hyperthyroid conditions in cats are caused by malignancy (cancer). The incidence of
hyperthyroidism in cats has increased remarkably in the last 25 years. The etiology is unknown, but probably due to multiple
factors. The ingredients and types of foods fed, immunological factors, and environmental influences may be involved. Because
of the rising incidence of this condition the need for veterinary nurses to be educated and able to provide good care to these
patients is indeed important.
It is common in middle-age to older cats with >95% of the cases occur in cats over 8 years of age. The actual age range is
4y-22y with the mean age being 13years old); so these are typically older cats. Only 5% of hyperthyroid cats develop the disease
before 8 years of age. There does not appear to be a breed or sex predilection.
Signs commonly observed by owners of cats
Thyroid hormone affects every organ system, so signs can be variable. The most common clinical signs include weight loss,
increased appetite, vomiting, diarrhea, drinking and urinating more, and nervousness or hyperactivity. Thyroid hormone affects
the heart, causing fast heart rates, heart murmurs, abnormal heart beats and high blood pressure. Many times owners will
report that their cat appears "healthy", that they have a great appetite and are very active.
Breakdown of Documented Clinical Signs
Rapid heart rates are common in cats with hyperthyroidism, and heart murmurs and high blood pressure can also occur. Cats
with hyperthyroidism that are not treated often develop hypertrophic cardiomyopathy, in which the muscle of the heart becomes
excessively thick. This can lead to heart failure and death. It is common for these patients to be co-management by a cardiologist.