The Senior Cat - Thyroid Disease
The thyroid glands play a vital role in regulating the body's metabolic rate. Hyperthyroidism is a disorder in older cats (>9 years) characterized by the overproduction of thyroid hormone resulting in an increase in the metabolic rate. The 'classic' signs of hyperthyroidism are weight loss with increased appetite, increased heart rate, irritability and restlessness (sometimes owners note 'night howling'). Other signs may include vomiting, mild diarrhea (bulky soft stools), twitching and an unkept coat.
Diagnosis is confirmed by measuring thyroid hormone levels in the blood. Ideally a full 'Senior Profile' including serum chemistries, CBC, blood pressure measurement, urinalysis and culture should be performed when thyroid disease is suspected. Elderly cats often have more than one problem and a balanced approach to management is needed.
Secondary complications of this disease include heart disease - thyrotoxic cardiomyopathy - and hypertension (high blood pressure). If a cat with elevated thyroid levels also has chronic renal insufficiency signs of hypertension and kidney disease may become apparent after treatment normalizes thyroid hormone levels. This is because the kidneys have become accustomed to the higher blood flow stimulated by elevated thyroid hormone. For this reason we repeat measurement thyroid hormone, blood pressure, creatinine, and urine concentration 4-8 weeks after beginning treatment.
There are currently four treatment options for managing hyperthyroidism:
Methimazole, given twice a day, blocks iodine uptake by the thyroid gland stopping production of excess hormone. This medication does not change the thyroid gland itself , so the drug must be given for the remainder of the cat's life. Periodic blood tests should be done to check for adequate control and to detect other issues that can occur in elderly cats. Methimazole is also used for a few weeks to stabilize a cat before surgery. Side effects are either mild and transient or reversible when medication is discontinued if more serious.
Iodine Restricted Diet (YD Hills Pet Nutrition)
Thyroid hormone production can be normalized by feeding a prescription diet that is restricted in iodine (an essential component of thyroid hormone). No other food can be fed. Dietary management works best in a single cat household or in a household with only hyperthyroid cats (this can occur when all cats in the household are elderly).
A very effective way to treat hyperthyroidism is radioactive iodine (I131). Radioactive iodine is given by injection and destroys all abnormal thyroid tissue without endangering other organs. This treatment requires no anesthesia, but can only be given at a veterinary hospital licensed to administer radiation therapy. The cat must be hospitalized for 5 to 10 days while radioactivity dissipates to safe levels. Up front cost is high, but this treatment is curative, non-invasive, and safe. One treatment is generally sufficient.
- Thyroidectomy (surgical removal of thyroid gland) Surgery can be curative and it provides a tissue sample that can be examined for the presence of neoplasia. This is important when the thyroid gland is very large or growing. We used to do many of these surgeries before radio-iodine therapy was locally available. Cost is equivalent to 3 - 4 years of medication. Risks include anesthesia, and potential disturbance of calcium homeostasis if the parathyroid gland which sits on the capsule of the thyroid gland is damaged or removed.
Early intervention can keep little problems from turning into big problems!