Hypothyroidism in Cats

Causes, Symptoms and Treatment of Feline Hypothyroidism

Photo of Cat Joey After Radioactive Iodine Therapy
Joey After Radioactive Iodine Therapy. photo © Franny Syufy

Hypothyroidism is an endocrine gland disorder. Hypothyroidism is relatively rare in cats, and it is not considered fatal, although it can affect a cat's quality of life.

What is the Thyroid?

The thyroid gland is an essential gland in the body, producing a number of hormones, including T3 (liothyronine) and T4 (levothyroxine), both of which are required for normal metabolism in the body.

The thyroid gland is found in the neck of humans and animals and secretes glands responsible for metabolic rate, calcitonin, and others.

What causes hypothyroidism in cats?

Although there are studies which indicate hypothyroidism might be an inherited disorder in dogs, most cats develop hypothyroidism in one of two ways:

  • Injury to the gland through surgery
  • As a result of radioactive iodine therapy for hyperthyroidism. Although conventional thinking was that only 2 to 5 percent of cats were affected this way, it is now believed that the odds may be much higher, as much as 30 percent or more.

Hypothyroidism may also be associated with thyroid tumors if a cat's diet is low in iodine. This is relatively rare today, as most commercial pet foods contain a sufficient quantity of iodine.


Hypothyroidism may present with one or more of these symptoms:

  • Weight Gain
    The cat will have an increased appetite and may also drink more water.
  • Dull, Dry Coat
    This may also be accompanied by hair loss and excessive grooming.
  • Listlessness and Lethargy
    The cat may lose interest in playing or socializing with other cats.
    • Mental dullness
    • Weakness
    • Unkempt appearance
    • Hair matting
    • Loss of hair (alopecia)
    • Delayed teeth eruption
    • Constipation
    • Low body temperature


    A medical history and complete physical examination are important for the diagnosis of this disease. You will need to provide your veterinarian with a thorough history of your cat's health leading up to the onset of symptoms.

    Finding the exact cause of hypothyroidism may require a thorough investigation. Routine laboratory testing will include a complete blood count, biochemistry profile, and urinalysis. Your doctor may be able to make an initial diagnosis based on the results of these tests, but endocrine testing is also an important panel for the diagnosis of hypothyroidism. The levels of T3 and T4 will be measured to determine if these are in the lower ranges. Radiographic studies may also be conducted to examine your cat internally for abnormalities that may be causing the dysfunction of the thyroid glands.

    Hypothyroidism is diagnosed through a blood panel and testing of thyroxine levels through T-3 and T-4 tests.


    In cats, usually no treatment is required for hypothyroidism, as this condition is frequently transitory in cats. Fortunately, this condition is easy to treat in cats that require treatment, although treatment is usually lifelong. Thyroxine supplement is given, usually through pills in synthetic form, although gels or flavored formularies may also be available. Dosage is adjusted periodically based on your cat's individual physical condition and progress. 

    Compliance with the prescribed drugs required for successful therapy.

    To avoid complicating the condition, do not change the type or dosage of the drug yourself, and never give anything new to your cat without first consulting with your veterinarian. Most cats respond well to therapy, with activity levels and mental alertness increasing significantly in a short amount of time. Cats with controlled hypothyroidism may continue to live long, happy lives.