Locally-Owned in Oakdale and St. Paul, Minnesota

Hyperthyroidism in Cats

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If your pet is experiencing respiratory distress, this is considered a “RED” – or true emergency – on our Fast Track Triage system. We advise you to seek immediate veterinary care. Please call ahead of your arrival so the veterinary team knows to expect you!

 If your pet is experiencing persistent/severe vomiting, appetite loss for more than 24 hours, or diarrhea paired with vomiting or loss of appetite, these are considered “ORANGE” – or urgent cases – on our Fast Track Triage system. We recommend having your pet see your family veterinarian or local animal emergency hospital within the next 12 hours. Please call ahead of your arrival so the veterinary team knows to expect you!

If your pet is experiencing vomiting (two episodes or fewer) or acute diarrhea without vomiting these are considered “YELLOW” – or semi-urgent cases – on our Fast Track Triage system. We recommend having your pet evaluated by your family veterinarian or local animal emergency hospital within 24 hours. Call ahead of your arrival so the veterinary team knows to expect you, and if your pet’s condition worsens, call the team back to inform them of the status change.

If your pet has a chronic illness/disease with no recent change in condition, this is considered a “GREEN” case on our Fast Track Triage system. This means emergency care isn’t needed, but your pet should be evaluated by your family veterinarian within the next few days.


Elevated thyroid hormone production, also known as hyperthyroidism, is the most common hormonal disease we see in cats, occurring in 10% of cats ages 10 and older.  Most often it’s secondary to a benign (non-cancerous) tumor of the thyroid gland in the neck. Rarely, it can be secondary to a cystic (fluid-filled) mass or cancer (carcinoma). In dogs however, hyperthyroidism is quite rare and typically obtained from eating raw foods that contain excess animal thyroid gland or cancers of the thyroid gland (carcinoma).  

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Clinical Signs 

Symptoms originate from the effects of excess thyroid hormone on the body. These hormones regulate metabolism, heart rate and rhythm, and numerous other functions in the body.  Please note, however, these signs are not specific to hyperthyroidism and can indicate other diseases.   

  • Swelling or growth on the neck
  • Weight or muscle loss despite a normal to ravenous appetite
  • Increased activity and vocalization
  • Increases in thirst and urination
  • Vomiting and/or diarrhea
  • Coarse or irregular hair coat
  • Less commonly, cats can present with poor appetite and low energy
  • Difficulties breathing secondary to heart failure
  • Acute blindness or weakness (stroke) secondary to elevated blood pressure  

Diagnosing Hyperthyroidism 

Blood testing of the thyroid hormones (total thyroid hormone or TT4 test) often can confirm hyperthyroidism in a cat with compatible clinical signs.  If testing is inconclusive, more comprehensive lab work (TT3 test and thyroid-stimulating hormone (TSH) level count), and nuclear imaging of the thyroid gland known as nuclear scintigraphy can be performed through a veterinary referral center.  Supportive lab work including a physical examination, blood work, urine sampling, and blood pressure is also recommended. 

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Cats receiving I-131 treatment at Animal Emergency & Referral Center of Minnesota.

Treatment Options 

The goal of treatment of hyperthyroidism is to improve clinical signs, as well as decrease the most catastrophic risks of heart disease or stroke.  Each treatment varies in risks and costs, and some treatments are irreversible.   

1. Radioactive Iodine or I-131 Treatment 

This involves a single injection of radioactive iodine, which is picked up exclusively by overactive thyroid tissue, sparing the normal thyroid tissue. This therapy destroys the diseased tissue, and in most cats can “cure” hyperthyroidism.  This means their thyroid levels are normal and they do not require further treatment.  

  • This treatment is irreversible.   
  • Approximately 10% of cats become hypothyroid (low thyroid hormone) following treatment and require oral thyroid supplementation for life.  
  • Approximately 5-10% of cats require repeated therapy as they remain hyperthyroid after treatment.  
  • Concerns with radioactive iodine include expense and isolation during treatment for about a week.  
  • Cats are intensively screened for diseases that could make them unstable for isolation and I-131, but this is not a guarantee of safe/successful treatment. For this reason, we will always want to ensure that your cat is as healthy as possible (apart from the hyperthyroidism) before treatment with radioactive iodine.
  • This treatment is not recommended for cats that have other serious disease (i.e. diabetes mellitus, moderate to severe kidney disease, heart failure, cancer) due to the required isolation and our inability to medicate in isolation.
  • Any iodine restricted diets or methimazole therapy (see below) must be discontinued prior to I-131 therapy.
  • Learn more about Animal Emergency & Referral Center of Minnesota’s premiere I-131 treatment facility here.   
2. Oral or Topical Methimazole 

This drug works by decreasing the thyroid gland from producing thyroid hormone. The drug is readily available and any effects are reversible. It doesn’t affect the thyroid gland directly so the tumor can grow and rarely transform to cancer over time.   

  • The major disadvantage of this drug is it must be administered for life, usually twice daily.  
  • It may cause side effects (e.g. vomiting, loss of appetite), as well as scratching at the face.   
  • Less common but more concerning side effects can include liver damage and bone marrow damage, resulting in decreases in red and white blood cells, and platelets (cells that help blood clot).  
  • Most cats tolerate treatment with methimazole well with successful control of their thyroid levels in ~95%.   

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3. Surgical Removal of the Abnormal Thyroid Tissue.  

Because the thyroid gland is located near major blood vessels and other hormonal structures, this surgery carries risk of bleeding, nerve damage, and low calcium levels due to removal of the parathyroid gland.   

  • Methimazole and I-131 therapy is often recommended before surgery due to risks, but surgery can be considered if these treatments are unsuccessful.   
  • This treatment is irreversible.   
4. Iodine Restriction with a Prescription Veterinary Diet (Hill’s y/d).   

This diet is formulated in both dry and canned formulations. Iodine is necessary for the production of thyroid hormone. By limiting the amount of iodine in the diet, we decrease the amount of thyroid hormone produced. To be effective, this diet needs to be the sole food that the patient receives (no other treats).  

  • This diet can be difficult in multi-cat households or in cats with concurrent medical conditions that also warrant specific prescription diets.  
  • This treatment is reversible, but like methimazole therapy, it doesn’t affect the size of the thyroid gland. 
  • Studies report 82% of cats respond favorably to this diet, but it takes several weeks to months to work.  

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Without treatment of hyperthyroidism, cats can decline or pass away from this disease.  Monitoring of kidney values before and after treatment is recommended, as many hyperthyroid cats have underlying chronic kidney disease that is “unmasked” with treatment and resolution of hyperthyroidism.  However, with treatment, these cats will often live for an extended period (years) in a properly managed case.  Hyperthyroidism secondary to thyroid cancers carries a more guarded prognosis. 

 If you have questions or concerns about your pet’s health or hyperthyroid treatment questions, talk to your family veterinarian or ask about a referral to our Internal Medicine Service to consult with one of our board-certified veterinary internists.    

 Source: JFMS 2016 AAFP Guidelines for the Management of Feline Hyperthyroidism 

Corie Fitschen, DVM, DACVIM, Animal Emergency & Referral Center of Minnesota, veterinary internist, board-certified veterinarian, Twin Cities specialty vet, Internal Medicine

 

 

 

 

 

 

 

 

 

 

 

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