Feline Hyperthyroidism by Dr. Destinee Dummer

Feline hyperthyroidism is a commonly diagnosed condition in elderly kitties. About 10% of all cats older than 10 years develop hyperthyroidism. This disease is characterized by an overactive thyroid gland, causing an increased metabolism. We see significant weight loss despite a ferocious appetite, restlessness and often an increase in urination and thirst. Some cats will even begin vocalizing in the middle of the night, called night howling.

When the disease first begins, the signs are not as obvious, and may only be detectible with subtle changes on the blood work. Senior blood panels typically include a thyroid (T4) measurement. If this hormone is only mildly elevated, at Killarney Cat Hospital, we may want to run a more sensitive test called a free T4 test to support the diagnosis.

What exactly is happening?

Thyroid hormone (T4) is created and secreted by the thyroid gland, with iodine being an important ingredient in the process. This iodine is supplied through the diet. With hyperthyroidism, cells within the thyroid gland start to multiply and the thyroid gland enlarges. The increased number of cells cause an increased level of T4 produced and secreted. Normally the pituitary gland would warn the thyroid gland to stop over-producing based on a negative feedback loop, however, the diseased cells no longer respond to this signal.

Along with increased metabolism, blood pressure also becomes elevated, called hypertension which could lead to blindness and risk of seizure and stroke like behavior.

How is hyperthyroidism managed?

Most cases of hyperthyroidism in cats are benign tumors called adenomas – the cancer cells are localized to the thyroid gland only. Rarely do cats develop the more aggressive form called carcinoma. This is good news because it means there are several treatment options available and they are fairly effective allowing your cat to still live a long and comfortable life.

Radioactive Iodine Treatment (I131)

This treatment is almost always curative, with the best results seen when used early on in the disease process. It is a form of chemotherapy that is done at specialty centers with the facilities to handle radioactive iodine and house your cat until the radioactive isotopes have left their body. This usually takes less than a week. Not every cat is a good candidate for this procedure. The radioactive iodine obliterates the diseased cells within the thyroid and unless some of the tissue was missed (the dose of I131 was not enough), the tumor does not grow back.

Methimazole / Tapazole

This medication works by blocking thyroid hormone synthesis. It has a relatively short half-life in the body, meaning the drug wears off in less than 24 hours. This medication is generally given every 12 hours to sustain an adequate level within the blood stream to constantly block the overactive thyroid cells. When a dose is missed, the cells are free to overproduce T4 and your cat will begin to show symptoms again. At Killarney Cat Hospital, we understand that it is difficult to medicate your cat twice daily and it can affect the bond that you have. We offer this drug in a variety of preparations such as absorbable ointments, flavored liquids, coated and uncoated pills.

If your cat is receiving this medication, they will need regular blood testing of T4 levels to see if any dose adjustments are needed. Even if your cat is maintaining well for over a year on the same dose, it is still advisable to recheck the T4 levels as this medication does not prevent the diseased cells from multiplying – in essence, a higher dose will eventually be needed to block the increased number of diseased cells present.

Diet (Y/D)

The Hills pet food manufacturer has created a diet called Y/D that can be used to manage your hyperthyroid cat. To make this food, they shut down their production facilities and clean all the machines to remove even the most minute trace of iodine. The idea behind this treatment is to starve the overactive cells in the thyroid gland of an essential ingredient in creating thyroid hormone (T4) – Iodine! Without this ingredient, even though the diseased cells are present and still multiplying, they are unable to produce the T4, therefore your cat does not suffer from the side effects of too much T4 in their system.

Of course, this food ONLY WORKS if there is an absence of iodine. If your cat eats treats, other pet foods or people food, flavored medications, goes outside, etc, we see treatment failure.

Furthermore, there are insufficient quantities of protein in this diet to adequately maintain your geriatric cat’s muscle mass. This leads to muscle wasting as your cat steals protein from their own muscles to make up for the losses in the diet.

Surgical Removal (Thyroidectomy)

Although this surgery can also be curative, we at Killarney Cat Hospital do not perform this surgery as the risk of also removing the parathyroid gland (a very important next-door neighbor to the thyroid gland) is very high. Likewise, the hyperthyroid cat is not a very good surgical candidate and they can be difficult to manage under anesthesia.

My Veterinarian keeps talking about unmasked kidney disease, what does that mean?

Many cats that develop hyperthyroidism can also develop concurrent kidney disease. When the T4 levels are high, and the hyperthyroid cat is not being managed very well, blood pressure increases and the kidney is happily perfused with lots of blood. It is the kidney’s job to filter this blood and remove toxins and excess sugars/proteins/fluids to make urine. The more blood that pumps through the kidneys, the better they do their job. Once the hyperthyroid cat begins treatment and T4 levels normalize, so does the blood pressure. The amount of blood reaching the kidneys declines and often a blood test will now detect kidney disease, it is said to be “unmasked”.

It is unknown if hyperthyroidism causes kidney disease or if the kidney disease was already present but the cat was just coping. Not all hyperthyroid cats will have concurrent kidney disease, but it is a phenomenon that happens commonly enough that we at Killarney Cat Hospital feel it is important to re-tests kidney and thyroid values soon after establishing treatment of hyperthyroidism so that any unmasked kidney disease can also be treated, thus improving your cats overall health and longevity.