Do All Hyperthyroid Cats Have a Thyroid Tumor? Is It Thyroid Cancer?

Yes, all hyperthyroid cats have thyroid tumors – every last one.  However, if your cat is hyperthyroid, there is no need to panic!  Most of the thyroid tumors that cause hyperthyroidism in cats are benign (non-cancerous), so it is very unlikely that your cat has cancer, especially if the hyperthyroid condition was only recently diagnosed.

All thyroid tumors in hyperthyroid cats are made up of thyroid cells that grow and function at an uncontrolled rate. This makes your cat’s thyroid gland grow larger and larger with time.  In almost all hyperthyroid cats, the veterinarian can palpate the thyroid tumor (Figure 1).  In contrast, the veterinarian should not be able to feel a normal cat’s thyroid gland, much less see it! 

Figure 1: A hyperthyroid cat with an extremely large thyroid tumor.

In addition to making the thyroid larger, the tumor cells function at an increased rate and produce too much of the thyroid hormone thyroxine (T4).  This excess thyroid hormone in your cat's circulation is what causes the signs of hyperthyroidism, such as increased appetite, hyperactivity, and weight loss.

Thyroid tumors come in two varieties, adenoma and carcinoma.   Adenomas are by far the majority (97-98%) of tumors that cause hyperthyroidism in cats.  Adenomas are benign tumors; they do not spread (metastasize) to other parts of the body.  The normal feline thyroid gland is made up of 2 small, separate thyroid lobes, both lying adjacent to the trachea (windpipe).  In 70% of hyperthyroid cats, adenoma infiltrates both thyroid lobes (Figure 2), whereas 30% of cats have only one thyroid lobe affected.

Figure 2: Hyperthyroid cat with two thyroid adenomas at time of surgery. In this cat, the left thyroid lobe (top of image) is twice normal size, whereas the right lobe (bottom of image) is 4-6 times normal size. Both thyroid lobes were palpable on physical examination.

In 2-3% of cats, the hyperthyroid condition is caused by thyroid carcinoma, rather than thyroid adenoma.  Unlike adenomas, thyroid carcinomas are cancerous and generally grow to a massive size (Figure 3).  They are malignant tumors – carcinomas invade and metastasize to surrounding tissues, such as the chest cavity, and endanger the cat’s life.

Figure 3: Hyperthyroid cat a large thyroid carcinoma at time of surgery. In this cat, the left thyroid lobe is approximately 50 times normal size. These tumors are invasive and generally inoperable.

Although cats with thyroid carcinomas are harder to treat, radioiodine therapy is the best option.  By administering an ultra-high dose of radioiodine (e.g. up to 10 times the I-131 dose appropriate for an adenoma), it is easily possible to destroy all of the cat’s malignant thyroid tissue.  With this treatment, we can cure the cat of both their cancer and hyperthyroidism!

One question remains: what causes the 2 to 3% of hyperthyroid cats to develop carcinomas?  Again, most cats that have carcinomas have been hyperthyroid for a long time (on the scale of many months to years).  Recent evidence indicates that the tumors change over time in these cats, transitioning from normal tissue to fast growing tissue, soon turning into an adenoma.  When left untreated, or treated only with an anti-thyroid drug (see next week’s article), tumor growth continues and eventually the adenoma tissue transforms into a malignant carcinoma.