That Crazy Thyroid!

There are a lot of diseases that dogs and cats have in common with people. Diabetes, allergies, inflammatory bowel disease, and obesity, to name a few. Both species also can have thyroid gland problems. The interesting thing is dogs and cats are at different ends of the thyroid disease spectrum—cats only develop an overactive thyroid gland and dogs almost exclusively develop an underactive thyroid gland. People can have either, but not both.

The thyroid gland is found in the neck. Dogs and cats have a lobe on either side of the trachea, but it can’t be felt unless it’s enlarged. Thyroid hormone (the gland that produces T4, which is converted into the active hormone T3) regulates the metabolic rate of every cell in the body. If there is more T4, the cells work faster. Less T4, and the cells slow down. Hyperthyroidism is just that—the cells are hyper and working overtime. The metabolic rate speeds up, leading to increased energy output. The affected cat is burning more calories all the time, even when sleeping, so it’s very hungry, losing weight, and shedding more. Overactive cells in the intestines can cause vomiting or diarrhea. Hyper cells in the heart lead to a faster heart rate and thicker heart muscles, which can eventually cause heart failure.

Hyperthyroidism is the most common hormonal disease in cats and is caused by a small tumor in the thyroid gland. It’s most commonly seen in cats over 13 years of age. There isn’t an exact cause for the disease found, but it’s probably due to multiple factors. Excess iodine in the diet may play a role in development of hyperthyroidism. Iodine levels in cat foods vary widely, sometimes up to 10 times the daily recommended level. Cats who eat mostly a canned-food diet, particularly with fish flavors or from pop-top cans, have three and a half times the risk of developing the disease over cats eating just dry food. Fire retardant chemicals (PBDEs) and persistent organic pollutants (PFAS) are found in higher levels in hyperthyroid cats. Genetics may also play a part, as some purebred cats, such as Siamese and Persian breeds, have a decreased risk of developing hyperthyroidism.

I recently had a cat patient come in because she was losing weight and vomiting more. The owner said she was ravenously hungry, but just kept getting thinner. I looked back in the cat’s record and saw that she had been diagnosed with hyperthyroidism six months previously and started on daily medication, but the owner had only gotten a month’s worth of tablets and never followed up. When asked, the owner said she didn’t realize it was a lifelong disease.

Treatment consists of either twice-daily medication, feeding a special prescription diet exclusively, or radioactive iodine treatment. Most people choose to medicate their cat daily. Methimazole blocks the production of T4, which decreases the amount in the body, slowing down the metabolism. It works well and doesn’t have too many side effects, but periodic blood work is needed to make sure the dose is correct and the cat isn’t becoming hypothyroid.

The prescription diet Hill’s Y/D is formulated to be very low in iodine, containing just enough so that the cat doesn’t develop an iodine deficiency. The food works well in about 90 percent of cats, but it’s the only food the cat can eat, making its use more difficult in multicat households.

Radioactive iodine is the most definitive treatment, but the most costly up front. The cat spends up to a week at a specialty hospital (in Madison, UW vet school) and has radioactive iodine injected into a vein. The iodine attaches to the thyroid gland and emits high-speed electrons that destroy the thyroid tumor (which takes up more iodine than normal thyroid tissue). In 97 percent of cases, the cat is cured and requires no further treatment. This is great news if the cat is difficult to medicate and picky about food!

Years ago, I had a hyperthyroid cat and had surgery done to remove the affected gland. My cat never developed hyperthyroidism after that, but surgery isn’t performed very often anymore. It can be difficult to tell if one or both glands are affected. If one gland is removed, there is a chance the other gland will eventually start producing too much T4. Removing both glands can lead to not enough hormone being produced as well as other complications. Ultimately, the cost of radioactive iodine treatment is comparable to surgery with fewer risks and a better long-term outcome.

About the only way for a cat to develop hypothyroidism is by removing the thyroid glands, but an underactive thyroid gland is one of the most common hormonal imbalance in dogs. Rather than a tumor, an autoimmune disease called thyroiditis is at play. The thyroid gland is overtaken with inflammatory cells, which eventually destroy the normal thyroid cells. Over 75 percent of the thyroid gland needs to be destroyed before signs of hypothyroidism appear, which can take months to years. Hypothyroidism is considered a heritable disease. Some of the most common breeds affected include beagles, border collies, boxers, cocker spaniels, dachshunds, and golden retrievers.

Since the thyroid gland is underactive, the cells in the body aren’t working very fast at all, so metabolism slows down. Clinical signs of hypothyroidism include weight gain, thinning fur because the hair follicles stay dormant longer than normal, a slow heart rate, sluggishness, and weakness. Unfortunately, just like when I go to the doctor and sort of hope my thyroid level is low to explain any weight gain, most overweight dogs are not hypothyroid.

Diagnosis can be challenging, especially early on. If there are some clinical signs that fit, your veterinarian will check a T4 level. But T4 can be low if the dog is ill for any other reason. This is called sick euthyroid. Further testing may include checking the level of T3, FT4, and cTSH. The cTSH is the thyroid-stimulating hormone produced by the pituitary. If the amount of T4 is low, the brain sends out hormone signals to tell the thyroid gland to produce more T4. But if the gland is diseased and can’t produce more, the pituitary just keeps cranking out cTSH. So if the T3 and FT4 are also low, the cTSH is high, and the dog has signs consistent with an underactive thyroid, we have a diagnosis.

Treatment of hypothyroidism is very straightforward—we give the dog a thyroid supplement every day for the rest of its life. Some dogs do best on twice-a-day supplementation and some on once a day; it does require blood tests to determine the best dose and yearly testing to make sure the dog is continuing to do well.

Annual blood work for both cats and dogs can help pick up thyroid issues before they cause any damage to internal organs (particularly in the case of heart disease in cats). If your pet is diagnosed with a thyroid disease, keep in mind that it will require daily treatment for life (excepting if you choose radioactive iodine treatment for your cat). As always, if you have any questions or concerns about your pet, please talk to your veterinarian.

Lori Scarlett, DVM, is the owner and veterinarian at Four Lakes Veterinary Clinic. For more information, visit .

Photograph by Brenda Eckhardt