What is Apathetic Hyperthyroidism?

Hyperthyroidism is an endocrine disease that is very common in senior cats. This disease is caused by an overproduction of circulating thyroid hormone by the thyroid gland. There are many possible clinical signs; animals will present a few or many of the signs. The most common signs seen in cases of hyperthyroidism are often absent in what is known as Apathetic HyperthyroidismThe typical signs for hyperthyroidism are many and varied. Rarely does a cat show all of the signs, but the most common clinical signs are:

• Weight loss, often despite an increase in appetite
• increased appetite, demanding food more often
• Vomiting and diarrhea
• Increased energy or irritability
• Drinking and urinating more
• Increased heart rate (difficult to assess for owners)

Treatment options Thyroid disease is common in senior cats, specifically,hyperthyroidism. Typical signs may include weight loss (despite increased appetite), increased energy or irritability, and increased thirst and urination to name a few. Rarely, the opposite signs are seen: decreased activity, decreased appetite, and weakness. This is termed apathetic hyperthyroidism. Treatment options are the same for either presentation, however all of the variables must be evaluated prior to selecting the best treatment option for your cat.


Hyperthyroid Treatment Option#1:

Methimazole (trade name is Tapazole®)
This medication is in the form of a pill that needs to be given by mouth one to three times daily, depending on each cat's case. Alternatively, compounding pharmacies can create a tasty liquid medication or a paste that is applied to the inside of the ears for absorption if pilling the cat is difficult or not an option.

This drug works by suppressing the thyroid gland's production of thyroid hormone, but does not cure the disease. If treatment is stopped, the hyperthyroid condition will recur.

Methimazole Pros:
• Does not require hospitalization or anesthesia
• Initial costs are less
• Can adjust the dose up or down fairly easily to control signs
• Side effects are usually mild and resolve over time (lethargy, anorexia, vomiting)
Methimazole Cons:


• This medication is not a cure; signs will recur if the medication is stopped
• Blood test monitoring and the cost of pills can add up
• Some cats can experience more severe side effects (itching, liver failure, blood changes)
• Administering pills twice daily may be too difficult for the owner or the cat

Hyperthyroid Treatment Option #2: 

Thyroid removal surgery
The thyroid gland consists of two parts, or lobes. Some vets remove only the visibly diseased lobe, others recommend removing both, since there is a high probability of the other lobe becoming diseased. This option requires a skilled veterinary surgeon, and the patient will be under a general anesthesia for the surgery. Careful pre-operative evaluation must be completed prior to surgery to assess kidney, liver and heart function.

Surgery Pros:
• Is often curative for hyperthyroidism
• No daily medications to administer
Surgery Cons:


• Anesthetic risk is higher for senior patients (and who often have compromised health of the heart or kidneys)
• More expense at one time and for post surgery monitoring
• The parathyroid gland function may be compromised*
• If any thyroid tissue is left behind, it may become hyperthyroid (including ectopicthyroid tissue, located in other, non-typical places in the body)

Hyperthyroid Treatment Option #3: 

Radioiodine therapy treatment is via a single injection of radioiodine (131I) under the skin. The hyperactive thyroid tissue takes up large amounts of this substance via the bloodstream, and the diseased thyroid cells die.

Radioiodine Therapy Pros:
• Is curative for all diseased thyroid tissue in the body, even in atypical locations
• No daily medications to administer
• Safe - very few side effects
• Parathyroid glands left intact
• No anesthesia or surgery necessary
Radioiodine Therapy Cons:


• Treatment must be given at a special facility
• Cat must be boarded for a number of days post treatment due to radioactive wastes in litter box
• Special disposal of litter box waste required for a period of weeks post treatment at home
• A few cats may become hypothyroid post treatment
• Cats with underlying or latent problems, such as kidney failure, may have a rapid exacerbation of signs
• Treatment is permanent -- dose cannot be titrated later on (i.e. as with methimazole)

* = The parathyroid glands are intimately associated with the thyroid gland. They regulate calcium.