Pros and Cons of Radiotherapy vs Oral Medication for Hyperthyroidism

Radiotherapy (radioactive iodine therapy)

In the normal human or feline body, iodine and the amino acid tyrosine are joined together in the thyroid gland to make T4. T4 is the hormone that we measured to diagnose hyperthyroidism; it is one of the main products of the thyroid gland and controls a myriad of processes in the body, including general metabolic rate. Too much T4 and we have hyperthyroidism.

In order to treat the disease we take advantage of the enormous iodine-concentrating ability of the thyroid. Almost all of the iodine we ingest is taken straight to the thyroid gland and put to use. In radiotherapy we give a single injection of radioactive iodine (I-131) which is transported immediately to the thyroid gland. The I-131 emits high-speed electrons that penetrate the surrounding abnormal tissues. Because these electrons only travel a fraction of a millimeter, the radiation is confined to the thyroid gland and the rest of the body is spared. This is highly focused therapy and the rest of the cat is in no danger.

The cat experiences no side effects with this treatment. In fact, if not for radiation safety laws most cats could go home the day of treatment. Unfortunately, there is a legal requirement to keep cats hospitalized until much of the I-131 is eliminated from the body. There is also a high level of I-131 in the urine and feces, which must be collected and disposed of in special radioactive waste containers for about a week after treatment. For this reason your cat will have to board for about a week at the nuclear medicine facility. The good news is, there are no dogs there! The staff take very good care of the patients.

T4 levels are checked 1 and 3 months after treatment. If all is normal at the last check, we will probably never have to check your cat's thyroid level again. If there is evidence that the initial treatment did not work, a second course will be needed. As of this writing, the treatment facility that we use (Northwest Nuclear Medicine for Animals) will repeat the treatment at no cost.

Advantages of radiotherapy:

1. Treatment is a one-time event. Only 1-2% of cats require a second treatment.
2. No ongoing treatment is required no pills.
3. The disease is cured rather than managed.
4. No anesthesia required, non-stressful to older animals.

Disadvantages of radiotherapy:

1. You are separated from your cat during the "quarantine" period after treatment. There are no visitors allowed in the treatment area.
2. There will be some minor restriction on your interaction with the cat for one month after treatment.
3. Cats with kidney disease may not be good candidates for radiotherapy due to the potential to worsen the disease.
4. The initial expense is higher than with oral medication.

Medication (methimazole)

Methimazole (brand name: Tapazole) is the most commonly used anti-thyroid medication. It blocks the production of T4, thus lowering the amount that is circulating in the bloodstream and decreasing the adverse effects of too much T4 (ie. the clinical signs of hyperthyroidism).

Advantages of methimazole:

1. Relatively inexpensive (initially) compared to radiotherapy.
2. Control of the thyroid disease happens only when the cat is on the medication. In cats with kidney disease this means that if the kidneys worsen when the hyperthyroidism is being treated, we can stop the treatment.
3. No hospitalization is required.
4. If an occasional dose is skipped no harm is done.
5. If after the first 3 months no side effects are encountered, the chance of side effects occurring thereafter (at that dose of medication) is substantially reduced.

Disadvantages of methimazole:

1. Medication need to be given at least once daily (usually twice).
2. 15-35% of cats experience some kind of side effects (lethargy, loss of appetite, and vomiting). Medication must be discontinued until side effects resolve.
3. Facial itching is a more serious side effect and happens in about 4% of cats on methimazole. If this occurs another treatment form must be chosen.
4. Serious liver failure happens in a small number (less than 4%) of cats on methimazole.
5. Bone marrow changes may occur. Blood tests for evaluating white blood cells should be done periodically.
6. Every 4-6 months blood tests should be done for white blood cell patterns, kidney function, and liver enzymes.