Feline Hyperthyroidism: Getting a Diagnosis

Even after all this time, there are vets who still think that a cat losing muscle mass is just getting old, and people joining the groups continue to report vets who are slow to test the T4, even refuse to test it on request, and vets who aren't up to date on the variables. Actually, any cat over 8-10 years old should have the T4 included as part of an annual “senior wellness” exam

I know all too well how expensive vet trips and blood tests can be, but they are still "cheap" compared to waiting until an illness is more serious. If a vet doesn't want to check for thyroid issues, remember that we are the only advocates cats have in these situations, and we must be their voice....with a smile added whenever possible.

What Symptoms Do I Look For?

Cats are weird, and I'm sure that comes as no surprise. The most common symptoms of feline hyperthyroidism are increased appetite, weight loss, increased activity/vocalization, vomiting, and decreased fur quality. However, there is also what's known as “apathetic” hyperthyroidism, which means decreased appetite and decreased activity. You might see a combination of these symptoms, or since feline hyperT comes on slowly and cats may not show it for a long time, you may see no symptoms at all.

As usual with cats, the most common symptoms for this disease are also common with other diseases, so it's often a matter of full blood tests to rule out diabetes, chronic kidney disease, or other problems along with checking the thyroid. It's not uncommon for a person to either discover the disease by accident when doing blood tests for other reasons or to suddenly notice the weight loss/muscle wasting that has been happening slowly for months.

Getting and Interpreting Results

The primary number for diagnosing hyperT is the T4, also called TT4 (Total T4). It's been standard to use the Free T4 as a follow-up test if the T4 is inconclusive. It's almost a relief when the T4 comes back high enough to confirm feline hyperT, and you're free to move on to treatment, but it's not uncommon for the diagnosis to be somewhat more complicated.

The “normal” range for T4 in older cats is 0.8-4.0 (or 4.7) on the US scale (but keep in mind that different labs have their own variations of the scale). On the international scale, normal is about 10-40 or 50. The “ideal” range is mid normal (1.5-2.5, US scale). Anything over about 3.0 and up to about 5.0 is the “gray zone”, at which level a cat might or might not be hyperT. (Our Lancelot had a T4 of 3.7 when diagnosed.)

When the T4 is in the gray zone, standard procedure has been to test the Free T4. (This can usually be done using blood from the original draw if it's less than a week since the blood was sent out.) However, Dr. Mark Peterson has confirmed that it's possible for a cat to have a slightly high T4 AND Free T4 and still not be hyperT. What causes these higher readings is not yet known, but treating such cases with methimazole can be dangerous.

As part of any physical exam of an older cat (8 years old or more), the vet should “palpate” the thyroid. This is a technique for trying to feel the thyroid. A normal thyroid can't be felt, but if there's a tumor (which is what causes feline hyperT), there is at least a 50% chance that it can be felt. If a vet is good at palpating the thyroid (which may mean using two or three variations on the technique), a tumor can be felt (if there) almost every time. Always ask that the vet palpate the thyroid during an exam, even if you don't suspect hyperT.

Of the other test results, the ALT (liver enzyme) may be high with hyperthyroidism. This can be used as additional support for the diagnosis, but the ALT doesn't typically need direct treatment if raised by hyperT. In most cases, it will come down on its own as the T4 drops, although it does sometimes lag behind a bit, another situation where patience is needed.

The creatinine (still a primary number for judging kidney condition) may be forced down by a high T4, so the precise kidney condition may be a tricky question until the T4 has been in the ideal zone for a few weeks. If there is no chronic kidney disease CKD) present, the kidneys can typically handle faster changes in the T4, even a period of mild hypOthyroidism. If CKD is being “masked” by the high T4, then the kidneys are likely to be affected much faster and stronger by drops in the T4. Since the presence of CKD may be uncertain, that is one of the prime reasons for dropping the T4 slowly.

The Nature of Feline Hyperthyroidism

Feline hyperT is caused by a granular tumor on one or both thyroid lobes. Luckily, this tumor is almost always benign and slow-growing, but that is also why the disease is so often not noticed until it's more advanced. Although benign by definition, the tumor is stubborn and has rather wide-ranging effects directly or indirectly.

The healthy thyroid tissue is forced into a dormant state and slowly atrophies. At the same time, the tumor causes the production of more T4, which causes the metabolism to run at a faster and faster rate. This forces the kidneys to work faster, which is why the creatinine may read lower. In addition, the heart works harder, so at some point the high T4 can cause high blood pressure (which can cause detached retina) and can cause heart damage.

HyperT cats are also more prone to an assortment of diseases, including diarrhea, urinary tract infections, upper respiratory infections, and asthma. This may be due to the immune system essentially getting “tired”. Even when the T4 is controlled with medication, some cats seem to remain somewhat more prone to diseases, and there have been cases of diarrhea and asthma that could not be cure until the I-131 was done.

There are times when medication is the necessary route, but the tumor does continue to grow, which means higher doses will be needed over time. There is also a small chance that a tumor will become malignant at any time, although that can still be treated with the I-131.