In most cats (~99%), a small benign tumor (adenoma) or several
small benign tumors (adenomas) are responsible for the
overproduction of thyroid hormone that leads to the symptoms of
hyperthyroidism. In rare cats (~1.0 %) a functional thyroid
carcinoma (cancer) is responsible for the over-production of
thyroid hormone that leads to the symptoms of hyperthyroidism.
The most current data suggest that these tumors develop as the
result of a mutation in the gene for the production of an
inhibitory protein (G inhib ) that turns off the thyroid cell.
In other words the thyroid cell loses its ability to regulate
its growth and its production of thyroid hormone. What causes
this mutation is still under investigation. A.) Medical - Cats
with hyperthyroidism can be given pills called methimazole
(brand name = Tapazole®). Tapazole® works by suppressing the
thyroid gland's production of thyroid hormone. It does not
eliminate the thyroid tumor that causes hyperthyroidism nor does
to slow the continued growth of the tumor.