Thyroid Disease

This discussion will be about Thyroid disease. This will be for both dogs and cats. Both dogs and cats get thyroid disease, however, it is opposite thyroid disease. Dogs get hypOthyroid(low) while cats get hypERthyroid(high). As always there are a few exceptions but this is the general rule.


The thyroid is a gland that sits along the trachea(windpipe) in the neck. It is located in the same spot for both dogs and cats(and people!). Simply put, the thyroid is one of the main controls for our pets’ metabolism. Through controlling the metabolism it will also control heart rate, energy, mentation and skin/hair coat. As dogs and cats get older we can see issues with the thyroid gland.


Dogs will most commonly become hypothyroid(or under active thyroid). This also occurs most commonly in bigger dogs – dogs over about 50 pounds. Very rarely will they become hyperthyroid(over active thyroid), but it is possible. To become hypothyroid, the dogs own immune system attacks the thyroid for an unknown reason or the thyroid gland atrophies, or breaks down. When this happens, the thyroid gland can no longer release adequate hormone. We will begin to see the physical signs of hypothyroidism.

Signs for hypothyroidism in dogs:

These dogs will be overweight, have a dry, flaky coat and have a dull or lethargic mentation. Now, JUST because a dog is overweight does NOT mean they have a thyroid issue. If you feed your dog a bunch of treats, no matter how organic or natural they are, and the dog is overweight, it is likely the treats or excessive food causing the problem. If you are truly trying to get some weight off and the dog is not losing any, then we can discuss testing for thyroid disease.


Testing for thyroid disease is not complicated. There are a few different tests to run and your vet may have a different opinion about which test or how much to test for. There are two forms of the thyroid hormone which we test for. Some people also like to test for the hormone that tells the thyroid to release more hormone, but I feel that is personal preference. Most of the time I am fine testing what’s called the T4 and Free T4(free meaning not bound to anything, has nothing to do with cost!). If both of these values come back low, that is enough for me to say we have hypothyroid, as long as there is no other significant disease occurring.


Treatment is very simple. It involves a pill that you give at home every 12 hours. Treatment is for the life of the pet. This is not a disease that is cured or goes away, once we diagnose it the pet will need supplement for the rest of its life. Now, a VERY important point regarding treatment. If you or anyone you know is on Thyroid medication for hypothyroid you may find out that their dose is something like 0.1-0.2 mg per day. If you have a 70 pound lab, your pet will be on 0.7 mg every 12 hours! This is very important because if you decide to get the medication from a local pharmacy, the pharmacist may not know this and may try to change the medication. Make sure the prescription your vet gives you is what you take home and if not sure call your vet!

After a couple weeks of being on the treatment, another thyroid level will need to be checked to make sure your pet is not hypERthyroid now! If the value looks good then we can check the thyroid every 6 months to a year depending on your vet.


Cats will most commonly get the opposite of dogs. We see a lot of hyperthyroidism in cats. This is an over active thyroid gland. This is most commonly caused by a benign tumor on the thyroid that is continually pushing out excessive thyroid hormone. We see this in older cats. All of our domestic breeds are at risk for this.

Signs for hyperthyroidism in cats:

The most common thing we see these cats for is weight loss despite a good appetite. These cats will love to eat but will just slowly lose weight. You may also notice them drinking and urinating a lot. In the exam room, even though they could be a little nervous, we will hear an increased heart rate. These cats still have a decent amount of energy, but they are on edge/anxious most of the time.


To diagnose hyperthyroid in cats we just need one test. If we test the T4 and it is elevated, we have our diagnosis of hyperthyroid. Of course we don’t know if there are any other issues but to diagnose hyperthyroid, we only need one test.


There are a few different options for treatment in cats. They involve Radioactive iodine, Methimazole(oral and topical), and a Prescription diet.

The Radioactive iodine works by traveling to the thyroid gland and destroying the tissue. The thyroid will no longer release too much hormone. This treatment has a high up front cost, but you will not have the monthly medication cost. We mainly use this on younger cats. One drawback is the possibility of the cat developing another thyroid tumor years down the road, so I would take this into consideration as well.

Methimazole is a medication that blocks the creation of the thyroid hormone. This involves either giving your cat a pill every 12 hours for the remainder of its life(not many cats enjoy taking pills), or it can be compounded into a topical medication that is rubbed on the inner ear of your cat. Our most commonly chosen option is the topical medication. This is applied to the inner ear flap every 12 hours and you HAVE to wear gloves giving the medication as you can absorb it through your skin!

The last option we have is a special food. This food is made to be so low in iodine(which is what the thyroid needs to make its hormone) that the thyroid gland has only enough to make what the cat needs and no more. This food is nice because all you have to do is feed it to your cat! Other cats can eat the food as well, but they will need to be given some extra cat treats to make sure to supplement their iodine since they have a normal functioning thyroid. The food is nice but can be expensive if having to feed it to multiple cats. You can NOT let your hyperthyroid cat eat ANYTHING else as they may get too much iodine and then still have the same issue as before.

The testing to monitor the disease is the same for dogs. After we have established if the treatment is working, we can test the thyroid every 6 months-1 year depending on your vet.


This is a general overview of hypo/hyperthyroidism in dogs/cats. It’s a disease that can sometimes be confusing to decide whether or not to test for it but once it’s diagnosed it is very manageable. If you feel your dog or cats has any of these signs you can discuss this with your vet and they can decide with you about pursuing diagnostics.