Hyperthyroïdisme (Tapazole)

Has your older cat suddenly become more active and alert? Has he lost weight even though he eats more than usual? Does he drink more and produce more urine? Is he grooming himself more and bringing up food or hairballs?
These symptoms could reflect hyperthyroidism. A small subset of cats can have opposite signs such as lethargy and loss of appetite.

Hyperthyroidism is the term for an overactive thyroid gland, resulting in overproduction of circulating thyroid hormones.
Thyroid hormone functions as a stimulus to metabolism and is critical to normal function of the cell. In excess, it overstimulates metabolism and speeds up various body systems such as heart, blood pressure, gastro-intestinal tract.
Often hyperthyroidism will mask the presence of kidney disease. This happens because hyperthyroidism will improve kidney perfusion so before treatment the blood results for kidney function look better.
During the physical exam, we will usually note weight loss. Sometimes animal will be stressed or over anxious. By feeling the neck area, we might detect enlarged thyroid glands.

Upon auscultation, we will often notice an increased heart rate and a heart murmur. Sometimes even after treatment, these won’t resolve and there can be permanent changes in the heart muscle that could lead to heart disease.
All these changes coupled with the history will make us suspicious of hyperthyroidism.
A full blood test including a CBC, biochemistry profile and thyroid levels are needed to make the diagnosis.
The goal of treatment is to reduce circulating levels of thyroid hormone to a normal range. There are 3 met
hods of controlling the disease.

One of these options is administering methimazole orally. This drug inhibits formation of the thyroid hormone and has to be given orally twice a day long term. We can usually achieve good control but is not curative. Side effects are possible and appear within the first two months; they are mostly loss of appetite and vomiting. Less frequently, more severe problems such as low platelet count, low neutrophil count and anemia can occur. In some cats, this medication can affect the liver and cause an increase in liver enzymes and signs of not feeling well. Any of these problems, would lead to stopping the medication and going to another form of therapy.

The major drawback of methimazole is that control of the condition can be somewhat erratic at times. There are variations of thyroid levels within a 24 hour period. Also, some cats are difficult to pill which contributes to poor control. Compounding pharmacies can hide this medication in different preparations; they provide us with chicken flavored liquid suspensions, flavored medicated chews and transdermal gels. The gel is applied on the skin of the ear and absorbed through the skin, great for hard to pill cats.

We usually start with a lower range dosage. After 3-4 weeks, we do another physical exam and repeat blood work and thyroid levels. Based on results, we may recommend a change in methimazole dosage; we would have to check blood levels again in another month. Once the Thyroid levels are normal and there are no side effects, we will recommend rechecks every 6 months.

Another treatment alternative is surgical removal of the affected thyroid glands. This treatment is usually curative although sometimes the condition can recur if thyroid tissue has been left in or if thoracic thyroid tissue is present. For these reasons, it is recommended to check thyroid levels every 6 months as well. Surgery is not an option if the cat has an increased anesthetic risk or if there is evidence of renal disease. Complications from this surgery include life-threatening hypocalcemia; this occurs when blood supply to the parathyroid gland has been compromised. After surgery, calcium levels have to be closely monitored. Hypothyroidism could also occur following surgery but only needs to be treated if clinical signs are present: lethargy, excessive weight gain and loss of appetite.

The 3rd option is a sub-cutaneous injection of I 131, an radioactive form of iodine that will selectively destroy abnormal thyroid cells. Usually, one dose is enough but sometimes the injection may need to be repeated a few years later. This option is the safest for these older cats. Some cats may develop hypothyroidism temporarily but this tends to resolve on its own. Higher costs of this procedure might be prohibitive for some owners as well as needing to board the cat at an approved facility because cats are considered radioactive for 7-10 days.

No matter what long term treatment is chosen, you will need to start oral medication first in order to see how correction of the hyperthyroid state will affect kidneys. This has to be done before surgery to stabilize heart rate and function. If kidney function worsens, we could always reduce dose which would not be possible with surgery or I 131 because they are irreversible.

Prognosis for the hyperthyroid cat as well as choice of treatment will be influenced by overall health and body condition at the time of diagnosis.