Radioactive Iodine (Feline Hyperthyroidism)

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Feline Hyperthyroidism Protocols
Frequently Asked Questions

Feline Hyperthyroidism usually results in marked weight loss in affected cats, and the associated loss of muscle mass may reduce serum creatinine (SCr) concentration and complicate the diagnosis of underlying renal disease. Thyrotoxicosis increases glomerular filtration rate (GFR) and renal plasma flow (RPF), which also may contribute to reduced SCr concentrations and interfere with recognition of underlying renal disease. Treatment of hyperthyroidism may reduce GFR and RPF and allow emergence of azotemia.1

Hyperthyroid cat with a unilateral thyroid adenoma.
Planar thyroid scintigraphy of a normal cat.

Symptoms of Feline Hyperthyroidism may include:
weight loss despite excellent appetite.
changes in behavior - anxiety or nervousness
excessive appetite or decreased appetite
increased water intake
hyperactivity or lethargy
excessive shedding, hair loss (alopecia), poor coat condition
diarrhea or vomiting
increased urination
cardiac symptoms - rapid heart rate, arrhythmia

Feline Hyperthyroidism is a disorder that occurs when the thyroid produces and excess of thyroid hormone. Hyperthyroidism occurs in middle aged and older cats. Both sexes and all breeds are equally at risk.

In the normal cat, the lobes of the thyroid gland are butterfly-shaped and located in the neck region cannot be felt with one's fingers. In the hyperthyroid cat at least one lobe is usually prominent and may be detected by your veterinarian during a physical exam. Seventy percent of cats have both lobes affected.

What is thyroid hormone?
Active thyroid hormone (nicknamed "T3," short for "triiodothyronine") sets the body's metabolic rate, sort of like a volume dial. One might say T3 determines how hard or how fast each cell works to do its job. Every cell of the body is affected by T3.

The thyroid glands (there's one gland on each side of the windpipe) do not produce T3. Instead, they produce an inactive form called "T4." Tissues of the body absorb T4 andconvert it to T3.

Doctors will readily make references to T3 and T4. It is a good idea to know what they are referring to. While T3 is the active hormone, it turns out that more meaningful information is gained by measuring T4. Your veterinarian will probably mention monitoring your cat's T4 level.

What causes hyperthyroidism?
Hyperthyroidism is caused by a benign growth in the thyroid gland which are over-producing T4. It is important to realize that these tumors are almost always benign and represent a form of goiter rather than a form of cancer. Only 3-5% of hyperthyroid cats have a cancerous thyroid growth.

Why is it so important to treat hyperthyroidism in cats?
Hyperthyroid cats frequently experience reduced quality of life through weight loss, muscle deterioration, chronic vomiting or chronic diarrhea. Not all cats experience these signs at the time of diagnosis but there are less visible reasons to treat: heart disease and high blood pressure. These problems can result in heart failure, sudden blindness, or sudden death and all can be prevented with timely treatment for thyroid disease.

Thames Valley Veterinary Services
London, Ontario
Copyright © 2006