Understanding Your Cats Hyperthyroid Diagnosis

The most common endocrine disorder in cats is a syndrome called hyperthyroidism. Hyper- means these cats have an overactive thyroid gland. Many people are familiar with hypothyroidism which is when the thyroid gland does not produce enough thyroid hormone. But with hyperthyroidism the thyroid gland is producing too much thyroid hormone.

In cats this is most often due to a benign (not cancerous) increase in size of the thyroid gland. This is called hyperplasia. Hyperthyroidism is much more uncommon in dogs, but when it does occur this is usually due to a cancerous type of growth called carcinoma. Cats have a much better prognosis with treatment because you are not battling cancer. The goal is simply to decrease the amount of active thyroid hormone being produced.

Symptoms of Hyperthyroidism

If your cat has hyperthyroidism you may notice that they have an increased appetite. Often they are ravenous. But despite eating very well these cats are losing weight and losing muscle mass.
The most noticeable and common side effect of hyperthyroidism is weight loss.

Your cat also may be drinking more water and as a side effect urinating more in their litter box.

They can also have behavior changes. Some owners have reported that their cat is suddenly up all night meowing. Some hyperthyroid cats also become hyperactive or act more aggressive than normal.

If you notice that your cat is losing weight I recommend taking them in to your vet for an exam.

The Vet Visit

The first thing your vet will do is a physical exam. They will weigh your cat and find out if there is any weight loss. A cat we saw recently had been living with hyperthyroidism for 2 years without treatment, and had lost 11 pounds! (started at 16 pounds and was down to 5) That’s a whole cat in weight loss!

This cat can still be helped with treatment but there is going to be long term damage to some of his organs due to disease.

Some other abnormalities we can detect during the examination are an increased heart rate, a new heart murmur or increased blood pressure. These heart changes can resolve with treatment, but for the previous cat he may have permanent damage to the heart muscle.

Many of these cats will also have dry skin and will stop grooming. This causes matted fur and an unkempt appearance.

But we can’t diagnose hyperthyroidism just from the examination. Unfortunately I have had many patients who have looked like a classic case of hyperthyroidism, only to find that their bloodwork is normal or shows a different issue.

The next step for the vet visit will probably be to collect a blood and urine sample. We will use the blood sample to check kidney and liver values, red and white blood cells and check if the thyroid hormone levels are elevated.

An elevation in the thyroid values, specifically the total T4 and the free T4, will confirm that your cat has hyperthyroidism. But there are also some other abnormalities that can be present which is why we want to check things other than just the thyroid values.

Many times there are increases in some of the liver values. With successful treatment these liver values should return to normal so it’s important to know the starting value so we can monitor if they are improving with treatment.

It is also common for many of these cats to have concurrent kidney disease. This will not improve with thyroid treatment and sometimes the renal values will actually increase more after treatment.

I mentioned before that cats with hyperthyroidism have an increased heart rate and increased blood pressure. This causes increased blood flow through the kidneys which filters out the renal values faster. But as we treat the thyroid disease this blood flow will decrease back to normal which causes the renal values to start increasing.

It’s still important to treat the thyroid disease because that increase in blood pressure and blood flow through the kidney can cause more damage. If the kidney values are getting too high during treatment we can either lower the thyroid treatment to balance the two issues, or start other treatments that will help the kidneys.

Once we have done our exam and bloodwork that confirms your cat has elevated thyroid hormones, then we can discuss treatment options.

1. Methimazole

This is a medication which blocks one of the enzymes needed to produce thyroid hormone. This medication needs to be given by the owner every day at home for the rest of the cats life. One benefit of this treatment is that it is affordable. It is a life long treatment but in elderly cats this is still the cheapest option.

The dosage can also be altered easily. For cats that also have kidney disease we can decrease the dosage slightly if we find the blood renal enzymes are getting too high. The goal in that situation is not 100% resolution of the thyroid disease. But instead the goal is to manage it enough that they can maintain weight while keeping the renal enzymes from getting too high. We are actually using some of the side effects of the thyroid disease to “mask” the kidney disease.

There are some possible side effects which would make us discontinue methimazole. One is that some cats will become very itchy, especially around their faces. These cats have an allergic reaction to the medication which can cause facial swelling and they will even scratch their faces so badly that they cause open bleeding sores.

It tends to occur within the first couple of weeks of starting the medicine so if you notice your cat itching and scratching stop the medicine and contact your vet.

Another possible side effect is a liver reaction. It is important to take your cat back in for the recheck bloodwork 4-6 weeks after starting methimazole, because if your vet sees an increase in liver values they will have you stop the medication. If this reaction is not caught early enough it could result in permanent liver damage. Thankfully the liver can heal if we discover the issue early and stop the medicine.

If your cat has either of these reactions, or if they develop vomiting or diarrhea, I would recommend asking your vet for another treatment option.

Methimazole comes in a pill form and most cats will take between 1/2 tablet to 1 full tablet twice daily. One of the issues with this is many cats are very resistant to taking pills. This does not mean you can’t use methimazole as the treatment for your cat. There are many compounding pharmacies in the United States which can create a different form of this medication to give. I am sure this is probably true in many other countries as well.

One option is a flavored liquid that needs to be given orally. If you think your cat will eat food with this liquid mixed in, or if you think you can give a liquid medication directly in to your cats mouth twice a day then this is a good option.

And thankfully, for the very difficult to medicate cat, this medication can also be compounded in to a transdermal ointment. To give your cat this version of the methimazole you will rub a small amount of the ointment on the inner hairless part of your cats ear flap. The medication is then absorbed through the skin with no stress to you or your pet. You should always where a glove when administering a transdermal ointment.

One possible issue with transdermal medications is that it doesn’t absorb as well through the skin. Your vet may have to increase the strength of the ointment for the thyroid values to return to normal. But I personally believe partial absorption of the transdermal medication is better than no medicine at all if you can’t give an oral pill.

After starting methimazole in any of the above forms, your vet will need to recheck bloodwork to see if a dosage increase or decrease is needed. I recommend this bloodwork 4-6 weeks after starting medication or after any dosage change.

Once a cat is determined to be on a stable dose I monitor bloodwork every 6 to 12 months. 6 months is better, but if a cat is extremely stressed at the vet, or cost is a concern, 12 months is fine.

2. Radioactive Iodine Treatment

This is a great treatment because it does provide a cure in about 99% of cats. A single subcutaneous injection of radioactive iodine is administered which then destroys thyroid tissue.

It tends to target the thyroid tissue that is increased in size. This means normal thyroid gland is left unharmed which allows these cats to return to normal thyroid function. In very very rare cases all thyroid tissue will be destroyed which makes these cats hypothyroid.

One of the downsides of this treatment is the cost. Prior to treatment cats are required to have baseline bloodwork, urinalysis, chest x-rays and blood pressure check. This will be done at your regular vet and will cost a couple hundred dollars. Then the actual treatment is also expensive. In my high cost of living area I believe the cost has increased above $1500.

For this reason, unless cost is no concern, many people with elderly cats do no pursue this option. With elderly patients there is increased risk for another disease such as cancer to develop, which makes the benefit to a very expensive treatment seem less worthwhile.

This treatment will also make your cat radioactive for a short period of time. This means the treatment is only offered in specialized facilities with the ability to protect their staff from the radiation. These cats also need to stay at the facility for about 3 days after injection until their radioactive level decreases to a safe level.

But even then owners need to take special precautions. Children and pregnant women should not interact with these cats for about 2 weeks as they are more susceptible to the risks of radiation. Anyone who is caring for these cats should not pet them for long periods of time. The recommendation is no more than 30 minutes of contact per day.

Their urine and feces also needs to be discarded in a safe way. For 2 weeks you will need to use flushable cat litter so all urine and feces can be flushed. I have also heard of places recommending you bag all litter and then bury it but this is not what my local facility recommends.

If your cat spends some time outdoors they should be kept indoors for 2 weeks so they are not contaminating the outdoor environment with radioactive urine or feces.

I chose this treatment for one of my own cats about 5 years ago. I diagnosed him at a pretty young age of 6 years. The initial cost was cheaper long term then potentially 8-10 years of medication. He is also extremely difficult to medicate so I would have to compound the methimazole which is a little more expensive each month. And even then he would have made it difficult.

So I contacted my nearby facility which is called RadioCat. I have linked to their website because they have locations all across the US. I dropped him off on a Monday and picked him back up on a Thursday when he was declared safe to go home.

At home I had purchased a cheap litter box for him to use as it was recommended to dispose of the box after the 2 week period. He used the new box and flushable litter well but we did have some issues. He does not board well so was very stressed while there. I sent some anxiety meds to the hospital with him but still he had signs of stress when he got home.

He urinated outside the box several times which was stressful considering the urine was “radioactive”. He also had some blood in his urine which told me he had stress induced cystitis. I discuss how stress can cause bladder issues more in a separate post.

I continued to use the anxiety meds at home which helped and eventually we could return to normal. Despite the couple weeks of stress for him and I, it was the best decision for us. I recently did annual bloodwork on him and his thyroid value is still normal!

After treatment with radioactive iodine you will need to check some blood work 1 month and 3 months after treatment to check if it worked. If everything is normal you should not have to worry about this disease again. It is rare for them to develop new hyperplasia of the thyroid glands after treatment.

3. Diet Therapy

If radioactive therapy is too expensive or if methimazole can’t be given, then there is a special diet which can work. It was produced by Hill’s and is their y/d diet. This diet is much lower in dietary iodine than other diets. Since iodine is necessary in the synthesis of thyroid hormone this low iodine diet will lower blood thyroid levels.

If you want to try this diet to treat your cats thyroid disease it needs to be the only food offered. If they get some regular food this will provide enough iodine for the hyper plastic thyroid tissue to produce excess thyroid hormone.

But for the cat who isn’t a picky eater it’s a great option. Just like with methimazole this diet will need to be continued life long. If your cat stops eating the y/d diet, or is getting in to another cats food, the thyroid hormone will start increasing again and symptoms will return. In that case another treatment will need to be tried.

4. Surgery

Surgery to remove the thyroid glands is the last option. The thyroid gland is located on the outside of the trachea and there is tissue on both the left and right side. The thyroid gland needs to be removed bilaterally for successful treatment of hyperthyroidism.

There is also a parathyroid gland located next to each thyroid gland. If both parathyroid glands are removed as well during surgery this can cause hypoparathyroidism. This results in low blood calcium levels. So the surgeon needs to be careful to not remove the parathyroid glands.

In many animals there can also be ectopic thyroid tissue. This means thyroid gland tissue forms elsewhere in the body such as along the sternum in the chest. If this tissue is present and is producing excess thyroid hormone than surgery will not be successful.

For this reason I do not usually recommend surgery. The risk for complications and cost for surgery in my area makes radioactive iodine the better option for most patients who can afford more expensive treatment. Surgery and radioactive iodine are considered the two permanent solutions. But if cost is the same, radioactive iodine is painless with fewer complications.

Long Term

Hyperthyroidism is a common disease of older cats but carries a very good prognosis with treatment. Some treatments such as methimazole or diet will need to be continued life long. These cats need to have at least yearly bloodwork to monitor the thyroid and kidney values.

If surgery is successful or the patient receives radioactive iodine most cats will be cured of their thyroid disease. These patients will need to have bloodwork checked several times after treatment to be sure they are responding well.

With treatment we will see many of the symptoms completely resolve. These cats will start to gain weight and the behavior changes will resolve. On bloodwork liver values will return to normal. And if they have developed heart changes due to thyroid disease these changes will often resolve.

If you care for multiple cats as pets throughout your lifetime there is a good chance you will eventually encounter this disease. I hope this post is helpful in recognizing the symptoms and knowing what treatment options you may have.