Hyperthyroidism is the most common hormone imbalance in cats. This disease emerged in the 1980s but has become so common that a wealth of information is available. 

What is Thyroid Hormone?

Active thyroid hormone (nicknamed T3, short for triiodothyronine) sets the body’s metabolic rate, sort of like a volume dial. One might say T3 determines how hard or how fast each cell works to do its job. Every cell of the body is affected by T3.

Enlarged thyroid gland

Graphic by MarVistaVet

The thyroid glands (there’s one gland on each side of the windpipe) do not produce T3. Instead, they produce an inactive form called T4. Tissues of the body absorb T4 and convert it to T3. Doctors will readily make references to T3 and T4. It is a good idea to know what they are referring to. While T3 is the active hormone, it turns out that more meaningful information is gained by measuring T4. Your veterinarian will probably mention monitoring your cat’s T4 level.

Clinical Picture of Feline Hyperthyroidism

There are many clinical signs associated with feline hyperthyroidism. The hallmark sign is weight loss despite an excellent appetite.

Hyperthyroid cats are also extra thirsty and are sometimes brought to the vet because they are drinking so much water. They are commonly restless and especially demanding of attention and/or food. Many cats have chronic intermittent vomiting or diarrhea and may be urinating in inappropriate locations.

In a normal cat, the lobes of the thyroid gland cannot be felt with your fingers. In the hyperthyroid cat, at least one lobe is usually prominent and may be detected by your veterinarian during a physical exam.

How to Tell if a Cat is Hyperthyroid

A blood panel is often ordered in the diagnostic work-up of any significant clinical signs. The blood panel surveys hepatic, renal, hematologic, and other body systems. Although there are multiple changes that fit hyperthyroidism, mild increases in liver enzymes are most commonly observed indicating mild (usually clinically insignificant) damage to the liver and/or bones. The elevated T4 level forms the basis for diagnosis; there can be no other diagnosis in such a case.

If T4 is markedly elevated, the diagnosis is clear. Some cats have borderline results and require further testing.

When T4 Levels are Borderline

In older cats, the normal range for T4 is much lower than it is for young adult cats and thus sometimes it is not obvious whether a cat is truly hyperthyroid. Further, T4 levels are readily reduced by other disease states and there is some normal but unpredictable fluctuation in T4 throughout the day.

The easiest tests for your veterinarian to perform are called the T3 suppression test and the equilibrium dialysis-free T4. To do the T3 suppression test, the owner must be able to give the cat a pill three times a day for two days and return to the vet’s office for a blood test on the third day. In other words, some specific scheduling is necessary and the cat must be able to take pills.

The pill given is a T3 pill, an active thyroid hormone. After two days of exposure to this medication, a normal thyroid will respond by reducing its T4 production. In a hyperthyroid cat, this negative feedback mechanism is not operating and the T4 level will not drop. Even borderline cases can be sorted out.

Thyroid x ray

Scan of cat with normal thyroid glands. Photo by: Advanced Veterinary Medical Imaging

The Equilibrium dialysis-free T4 represents a certain form of thyroid hormone, the part that is not bound to blood proteins as measured by a laboratory technique called equilibrium dialysis. Unlike T3 and total T4 measurements, the equilibrium dialysis-free T4 is not as subject to the influence of non-thyroidal illness. For example, it is normal for total T4 levels to drop when you are sick; however, the free T4 portion remains unchanged. In this way, a hyperthyroid cat that is sick from another disease might have a T4 in the normal or borderline range, but the equilibrium dialysis-free T4 would still be high enough to confirm the hyperthyroid condition. The equilibrium dialysis-free T4 is a simple blood test that can be run on any serum sample, though sometimes it must be sent out to a laboratory, thus requiring a few days for results.

Thyroid gland x ray

Scan of hyperthyroid cat, showing enlarged thyroid glands. Photo by: Advanced Veterinary Medical Imaging

Of course, a nuclear medicine scan will also identify a hyperthyroid cat readily. The equipment necessary is only available in a limited group of group of radiotherapy facilities.

What Causes Hyperthyroidism?

Many risk factors are associated with hyperthyroidism in cats, although more research is needed to say definitively what may cause it. The most common abnormality is benign adenomatous hyperplasia, or an overgrowth of thyroid tissue, which leads to your cat producing too much thyroxine (T4). Currently, we don’t know for sure what causes this hyperplasia, but this type of tumor is present in 96-98% of hyperthyroid cats. These tumors are almost always benign (not cancerous) and are a form of goiter (enlargement of the thyroid gland)rather than a form of cancer.

Many people want to know why their cat’s thyroid gland grew a tumor, benign or otherwise.  

Risk factors for hyperthyroidism in cats include:

• Genetics: Several mutations and expression differences have been shown in some hypothyroid cats.  More research is needed to completely understand these changes.
  • Some purebred cats, especially the Siamese and Himalayan breeds, appear to have a decreased incidence of hyperthyroidism (meaning they are less likely to develop this condition). 
• Age: 12-13 years old is the average age for cats when diagnosed.
• Exposure to pollutants in cat litter, certain pesticides, flame retardant chemicals
• Diets with high amounts of canned food, and fish canned foods (some studies suggest 2-3x likelihood of hyperthyroidism for mostly canned food diets)

Diet and Chemical Exposure

It has been speculated that aluminum cans lined with a substance called bisphenol-a (or BPA) affect the thyroid gland and other hormone-producing organs by disrupting hormone receptors. BPA from canned diets may transfer into food containing oils or fats. In areas of the world where this type of can is not used for cat food, hyperthyroidism is not a common disease. Still, it is important to realize that 25% of hyperthyroid cats have never eaten canned foods, so there is likely more than one factor at work.

Another study by Dye et al. looked at exposure to bromated flame retardants as a factor in the development of feline hyperthyroidism. These chemicals have become largely present in homes in the last 30 years, about the same time that feline hyperthyroidism went from an extremely rare disease to an extremely common disease. They looked at PBDE levels in young cats, older normal cats, and older hyperthyroid cats and while they readily found PBDE in the blood samples tested they did not find any difference in levels between cats with or without hyperthyroidism.

We mention this study as it received a great deal of press attention when it first came out leading many people to read the "headline" that PBDEs were being investigated as a cause of hyperthyroidism in cats and miss the conclusion that no significant difference in groups was found. It should be mentioned that an indoor lifestyle is correlated to an increased chance of a cat being diagnosed with hyperthyroidism.

All of this has led to many other questions:

• Are there chemicals in cat litter involved?
• Are there other household chemicals involved?
• Does the fact that cats are living longer make diagnosis more likely?
• Are indoor cats more likely to see the vet and have diagnostic testing while outdoor cats simply go undiagnosed?

Why is Treatment Important?

Hyperthyroid cats frequently experience reduced quality of life through weight loss, muscle deterioration, chronic vomiting, or chronic diarrhea. Not all cats experience these signs at the time of diagnosis but there are less visible reasons to treat: heart disease and high blood pressure. These problems can result in heart failure, sudden blindness, or sudden death and all can be prevented with timely treatment for thyroid disease.