Hyperthyroidism – One of the Most Common Cat Diseases

Hyperthyroidism is one of the most common diseases in cats

Last week I wrote about Hypothyroidism in Dogs; this week I will discuss another important topic that also seems to cause much confusion. Hyperthyroidism is a condition mostly seen in cats and it is treated with a completely different set of medications. It is extremely important to know the difference between these two conditions and which medication treats which condition. I will again start with a few basic definitions and also attempt to keep things as visual and clear as possible:


a prefix used when describing something as being less than or lower


a prefix used when describing something as being greater than or more


overactive thyroid


underactive thyroid

Thyroid Gland

generally found in the neck and its function is to produce T3 and T4


Triiodothyronine, produced by the thyroid gland


Thyroxine, produced by the thyroid gland and along with T3 regulates many important body functions such as metabolism

The thyroid gland has the role of converting iodine into thyroxine (T4) and triiodothyronine (T3). The T4 and T3 circulate throughout the body and affect almost every single organ. Thyroxine regulates growth, metabolism, immune function and heart function. Thyroid stimulating hormone (TSH) from the pituitary gland is what signals the thyroid gland to secrete thyroxine. Hyperthyroidism is the most common endocrine disorder diagnosed in cats. It usually occurs because of a tumor of the thyroid gland that ends up increasing the secretion of the thyroid hormones.


More common in cats


More common in dogs

Predisposing factors to hyperthyroidism

  • More common in middle age to older cats
  • Several potential nutritional and environmental causes have been suspected
  • No real known predisposing factors.

Signs and Symptoms

  • Weight loss
  • Decreased appetite
  • Increased thirst, and increased urination
  • Vomiting
  • Diarrhea
  • Nervousness
  • Irritability
  • High heart rate
  • Poor hair coat
  • Muscle weakness
  • Irregular or slowed weak breathing (dyspnea)

If not detected early, hyperthyroidism in cats can proceed to something called the “Thyroid Storm,” an emergency condition that needs immediate treatment. Signs and symptoms of Thyroid Storm are extremely elevated heart rate, open-mouthed breathing, appearance of a panic attack, possible aggression and/or other hysterical behavior when handling.

Feline hyperthyroidism signs can look like other diseases such as chronic renal failure, diabetes, inflammatory bowel disease, and other intestinal disorders.

The veterinarian may do a complete blood count (CBC), run a serum biochemistry to detect for possible presence of increased liver enzymes, a baseline T4 test to get an estimate of the thyroid levels, and something called a T3 Suppression test. In the T3 Suppression test, T3 is administered and if it causes a decrease in T4 then the cat is normal; if the cat has hyperthyroidism then administering T3 will cause no decrease or a slight decrease in T4. The veterinarian may also want to do a scan of the thyroid gland to get a better idea of what is going on.

The goal of treatment is to lower and normalize circulating levels of thyroid hormone. There are three treatment options available for hyperthyroidism in cats:

Methimazole is a is administered as a long term maintenance treatment for hyperthyroidism in cats
  1. Drug Therapy: The antithyroid drug of choice, methimazole (Tapazole), is administered as a long term maintenance treatment for hyperthyroidism. It may also be administered temporarily until treatment through surgery or radioiodine is completed. The usual dose is 5mg by mouth every 8 to 12 hours. Blood tests are usually completed every 3 to 4 weeks until the proper maintenance dose is reached. A transdermal gel formulation may also be compounded by the pharmacist and can be administered to the hairless skin of the inner ear flap. Propylthiouracil (PTU) can also be used, but has higher incidences of side effects and is therefore no longer recommended in use for cats.
  2. Surgery: The thyroid gland can be removed. This is relatively safe if performed by an experienced surgeon. Like any surgery there are certain risks involved with the anesthetics or from the procedure itself.
  3. Radioiodine: In radioiodine treatment, the cat is given a one-time injection of radioactive iodine. The radioactive particles concentrate in the thyroid and selectively irradiate and destroy only the thyroid tissue that is malfunctioning.

There are other treatment options that your veterinarian may discuss with you during the exam. There is no known way to prevent hyperthyroidism in cats since the cause is not known. Monitoring weight loss and having regular blood work done in older cats may help in early detection. The best way to keep our cats healthy and safe is by developing a good relationship with your veterinarian and by asking questions when something is not clear.

It is important to note that keeping medications separate and understanding these conditions and the reason for each medication that is used could actually save your pet’s life. If you have any questions about your pet’s medication you may give one of the 1800PetMeds pharmacists a call for assistance. Under no circumstance is it a good idea to give a medication that you are not understanding and fully clear about.

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  1. presantly my cat take Methimazole Transdermal gel
    perscrition is 5mg/0.1 mil 7mL

    dosage is .2 mil per day in ear passage.

    I presently pay $85 for the meds, for a monthly supply.
    I would like to buy this med at a local Pharmacy, such as Wla-Mart, cvs, Target

  2. Dr. Michael Dym, VMD veterinarianMarch 20, 2014 at 9:18 pm

    I dont think Walmart and/or Target have transdermal meds available, but you can check with them.

  3. My vet (no name given) is prescribing Transdermal Methimazole Pen 5mg* for one of my male cats, cost factor $56.35, twice per month. I am not sure if this is over priced or what? I do know that I get my females meds at less then 1/2 price through your site and am wondering if I will be able to get the new meds cheaper or what!!

    I also am not sure this vet will ok my using your service for med’s since I am new to the clinic. I feel that if I go elsewhere I may cause the clinic to drop us due to a conflict of trust.

    I also am not sure that the for life issue was explained with reference to side effects as explained in your synopsis of the drug and its side effects.

    Please advise as I trust in Petmeds.

    Sincerely; Mr. Augustine June 11, 2014

  4. Dr. Michael Dym, VMD veterinarianJune 13, 2014 at 6:49 pm

    Honestly Mr Augstine, I am not sure how this product should be priced. You may want to ask 1800petmeds pharmacist Eddie.

  5. VINCENT AUGUSTINEJune 14, 2014 at 7:35 am

    Good morning ROn:

    Did you get to either of the stores and did they carry the script that you need? If so, please reply and let me know if they will do this or not… As I too am looking to save on the cost! Did your vet tell you that this is a life time treatment to do with your pet. Or did I get the life time sentence as punishment for my pet for being a male cat.


  6. I am using the transdermal medication at a cost of $25 per month from Wedgewood Pharmacy. 18003318272. You can also call Diamondback Drugs in the Phoenix area 18666462223. They are the leading compounding pharmacy for vet medications in the US. I’ve used them also as my elderly cat has both IBD and Hyperthyroidism.

  7. I just started using the transdermal gel on my cat, and she has some swelling on her face (above eyes) and I see red bumps. I sent pics to my vet and he told me to continue giving her the meds, monitor her and let him know if it gets worse. Is this a common side effect?

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