Anti-thyroid Drugs (Tapazole / Methimazole)

At first glance, it appears that drugs are a more economical form of treatment than radioiodine (RAI). However, drugs must be given for life as they do not cure the thyroid cancer. When we add up the cost of drugs and blood tests (necessary to check for side effects and to ensure the dose is correct), we find that drug therapy can quickly equal the cost of RAI. Studies have shown that once daily therapy is not enough to control hyperthyroidism. Drugs should be given at least twice daily for effective control of thyroid levels throughout the day.

If monitored as recommended, the cost of medication and blood tests will equal that of radioiodine treatment in 12-18 months, and will continue to cost $500 to $1000 a year for the rest of your cat’s life.

As the level of antithyroid drug in the body varies throughout the day, this means that at some points in the day cats are likely hyperthyroid. This occurs even if blood tests show that thyroid levels are in normal range. This incomplete control of hyperthyroidism is unavoidable with drug therapy and contributes to ongoing damage. For example, many cardiologists believe that cats with heart disease should not be on drug therapy as they have documented progressive heart damage. Many owners whose cats were initially on drug therapy tell us that although drugs seemed to halt the disease where it was diagnosed, the cats only returned to normal until after they were cured with RAI. This also reflects incomplete thyroid control throughout the day when on tapazole.

Side effects are common (15-20% of cats), and include nausea, vomiting, diarrhea, hair loss, itchiness, scabbing, bone marrow damage, liver damage, immune mediated diseases and bleeding tendencies which could be fatal. Frequent visits to your veterinarian and blood tests are required to monitor T4 levels and to check for potential side effects. Giving bitter pills to your cat on a twice daily can lessen the bond you share with your cat.

As the tumour continues to grow, it produces more and more thyroid hormone requiring higher and higher doses of drugs.  As the dose of drug is increased the risk of a side effect is increased.

In many cases eventually the thyroid level cannot be controlled. For these reasons, many cats require radioiodine within 2 years of diagnosis if drugs are chosen as the initial treatment.

When the tumour is not eliminated in many cases it will progress to a malignant form of thyroid cancer. In about 20% of cats hyperthyroid for 2 or more years their tumour will transform from an easily cured and relatively benign adenoma to an invasive and hard to cure adenocarcinoma.

A recent study showed that cats who were treated with RAI lived twice as long as cats that were treated with methimazole (4 years vs 2 years). The shorter lives of cats on drug therapy reflects both the difficulty in medicating cats on a long term basis and that even with twice daily medication there is ongoing health damage from incomplete control of hyperthyroidism.