Treatment Options for Hyperthyroid Cats

Jun 22, 2023

There are four ways of treating feline hyperthyroidism, each of which has different advantages/ disadvantages.  These include:


The two medications prescribed to treat feline hyperthyroidism in Australia are methimazole or carbimazole.  All these medications block the production of T4 and T3. It takes 2 to 4 weeks before thyroid blood tests will show the effect of treatment.

Advantages of Methimazole or Carbimazole
  • Medication is inexpensive relative to radiotherapy or surgery, at least in the short term.
  • Control of thyroid disease is achieved only while the pet is on medication so that if there is any problem with increased poor kidney function, treatment can be discontinued.  In other words, the effects of these drugs are reversible.
  • No hospitalization is required.
  • Side effects are relatively uncommon.
  • If no side effects are encountered after the first 3 months of therapy, the chance of side effects occurring is unlikely.
Disadvantages of Methimazole or Carbimazole 
  • Medication must be given at least once daily (often twice daily is required). Some cats are difficult to tablet but there is a formulation of methimazole that can be applied to the inside of the ear (transdermal methimazole).
  • Approximately 15% of cats will experience some kind of side effect. The more common side effects are facial itching, liver disease, bone marrow disease, lethargy, loss of appetite, and vomiting. Some of these side effects mean that the medication can’t be used again.
  • Pre-existing kidney failure can be masked, as hyperthyroidism results in an increase in blood flow to the kidneys, making the kidneys seem more efficient. Once treatment is started, the blood flow to the kidneys returns to normal, and any kidney disease is unmasked or made worse. Approximately 15-22% of cats treated for hyperthyroidism will show kidney disease that was not evident prior to treatment.
  • Cats treated with these medications, have been shown in studies to have a significantly shorter survival time than those treated with radioiodine (4 years vs. 2 years), when excluding cats with chronic kidney disease.

Diet- Hills Y/D

Iodine is needed to make thyroid hormones in the thyroid gland. So hyperthyroid cats need more iodine to make the additional thyroid hormone. If iodine intake is restricted, it reduces the amount of thyroid hormone made. Hill’s Pet Nutrition has developed a diet called y/d, that has reduced Iodine content. After diet has been commenced, cats will usually have normal thyroid levels in the first two months of diet use; however, a full response can take up to 12 weeks. By 12 weeks on the diet, 90% will show normal thyroid levels. The diet is ineffective in approximately 10% of cats.

Advantages of Hills Y/D 
  • No tableting required
Disadvantages of Hills Y/D 
  • The cat cannot be fed any other food other than y/d or this treatment will not be effective. Therefore, this diet is unlikely to be effective in cats that go outdoors and hunt.
  • Other cats in the household (that are not hyperthyroid) should not be fed this diet.
  • In 10% of cats strictly fed this diet, the thyroid hormones do not reduce to normal after 12 weeks, and therefore an alternative treatment will need to be pursued.


Surgery can be performed to treat hyperthyroidism by removal of the thyroid gland(s). It is common (70% cases) for cats hyperthyroidism to have disease in both glands even if one gland larger than the other. Hyperthyroid cats can be difficult anaesthetic patients and therefore cats are usually managed medically with methimazole or carbimazole prior to surgery, which also allows the cats kidney function to be assessed when the cat has a normal thyroid level.

Advantages of Surgical Treatment 
  • Treatment is permanent. Unless complications arise or only one side is removed, no further treatment is needed.
  • A regular veterinarian can perform surgery, specialist surgeons are not required.
Disadvantages and Possible Complications of Surgical Treatment 
  • Performing surgery and general anaesthesia on a geriatric patient with potential heart disease has inherent risk.
  • If abnormal thyroid tissue is left behind, hyperthyroidism is likely to recur within six to 24 months.
  • The laryngeal nerve is located near the thyroid gland. If it is damaged during surgery, the cat can experience a voice change. This change may be permanent.
  • The sympathetic trunk (nervous tissue) is located near the thyroid gland. If it is damaged during surgery, the eye on that side may develop Horner’s syndrome. This syndrome may be permanent.
  • It takes one to three months for thyroid blood levels to stabilize following surgery. Some cats become hypothyroid (low blood thyroid levels) after surgery and must take thyroid supplementation tablets either temporarily or permanently.
  • Hypoparathyroidism. A potentially serious complication of when both thyroids are removed is hypoparathyroidism. Near or inside the thyroid glands are 4 parathyroid glands. These tightly control bloods calcium levels. If they are damaged, or removed during surgery, development of low blood calcium is a serious possible risk. Low blood calcium can develop 1- 5 days following surgery. If this develops, admission to hospital is often required for calcium and Vitamin D supplementation.
  • Thyroid Storm: – This refers to the phenomenon where the thyroid gland releases large amounts of thyroid hormone. The result is a life-threatening increase in heart rate, blood pressure, and body temperature. The hyperthyroid cat is potentially always at risk for this situation, but surgery may be an inciting factor.


Radioiodine is generally considered the safest and most effective method of treatment for feline hyperthyroidism. This involves the cat receiving an oral dose of radioactive iodine (I-131) followed by a period of 5 days in isolation in the hospital, until the radiation levels have reduced adequately to allow the cat to return home.

Radioiodine is taken by the thyroid gland, preferentially by the abnormal thyroid tissue. The Radioiodine emits radiation that damages the thyroid tissue from within.

Advantages of Radioiodine Treatment
  • Treatment is a one-time event (only about 5% of cats require a second treatment) and usually no on-going therapy is required.
  • No anaesthesia  or daily tableting is required.
  • Cats treated with I-131 survive longer than cats treated with oral medication.
Disadvantages of Radioiodine Treatment 
  • Owner and pet are separated during the isolation period.
  • Following treatment, the cat must be confined indoors and have limited close contact with owners for a 2-week period after discharge. Children and pregnant women can have no contact with the cat for two weeks after therapy.
  • There is a chance (less than five percent) that the cat will become hypothyroid after treatment, requiring daily oral thyroid hormone supplementation.
  • If the cat has not received medical treatment of hyperthyroidism prior to I131, there is the possibility of unmasking a previously unknown, renal insufficiency

If you have any questions about what treatment option is best for your cat, please call WAVES on 08 94125700 or email 

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