Radioactive iodine therapy
What is feline hyperthyroidism?
Hyperthyroidism is a commonly diagnosed hormonal disease of geriatric cats (usually over 10 years of age). The disease is caused by a benign growth of cells that secrete thyroid hormone in excess of the normal levels. If left untreated, a hyperthyroid cat can exhibit many (if not all) of the following signs: extreme weight loss, anorexia, muscle weakness, heart disease (increase in the size of the heart, increased heart rate, changes in heart rhythm, cardiac arrest), intolerance to stress, and eventually death.
How does radioactive iodine treatment work?
The thyroid gland is the only tissue in the body that concentrates iodine actively. However, the glands cannot differentiate between normal dietary iodine and radioactive iodine ( I-131). The overactive thyroid tissue concentrates the radioactive iodine while the atrophied, less-functional normal tissue is spared.
Administration
The preferred route of administration is a single subcutaneous (under the skin) injection very similar to a vaccination. Sedatives are generally not necessary unless the cat is overly fractious
Hospitalization
The hospitalization period varies between cats depending on the rate of radioactive decay. The effective half-life of the radioactive iodine is variable depending upon each cat’s ability to excrete it via the kidneys and how long the radioactive iodine is bound to the thyroid. Cats with preexisting kidney disease may have to stay longer because the iodine will remain bound to the tissue longer.
Each cat is monitored daily to determine when they can be released. The patient can be released from the hospital once the exposure rate from the radioactivity in the thyroid gland reaches a level that is deemed to be safe to the general public. This level is determined by the Louisiana Department of Environmental Quality – NOT Southeast Veterinary Specialists. The anticipated time of hospitalization is approximately 1 week.
Cats will receive treatment at least twice daily while in the radiation ward. Only qualified technicians and doctors are allowed in the radiation ward and their access is limited. Contact with the cats is limited to basic feeding, watering, litter changes and cleanup. Any required medical treatments while hospitalized are limited to basic care, and must be performed in the radiation ward since the cat cannot be removed from that location. You may bring bedding from home, however these items cannot be returned.
Home care after discharge
Litter should be collected in a separate bag from the household garbage and held for 2 weeks in a garage or other out of the way part of the house prior to disposal in the normal household trash. This is done to prevent others (garbage collection personnel) from being exposed to radiation. Most garbage facilities have radiation detectors at their plants and will return the garbage to you if radiation is detected. Flushable litter can be flushed normally.
Pregnant women and children under the age of 18 should have minimal contact with the patient for a minimum of 2 weeks. These two groups of people are the most susceptible to the hazards of radiation, even at these small doses.
The cat should be kept in a confined area (laundry room or bathroom works well) and close contact with family members should be avoided for 2 weeks. It is important to always wash hands after any contact with the cat or their litter. This will help to prevent the spread of radiation to other regions of the house as well as to decrease the exposure to family members from any possible contamination.
Monitoring and follow-up
The thyroid level (T4), BUN and creatinine should be measured at 1 month and 3 months after treatment. After treatment, it is possible for cats to have a period of hypothyroidism (low thyroid hormone levels). This is almost always asymptomatic and does not require treatment in the majority of patients. During this period, thyroid-stimulating hormone (TSH) secretion resumes, the atrophied thyroid tissue becomes functional, and the cat's thyroid level returns to a more normal level. Patients that continue to have high levels of thyroid hormone (T4) 3 months after treatment will probably require retreatment. This has been noted in only 5 to 15% of cases.
