2665 5 Mile Rd. NE Grand Rapids, MI 49525

Phone : (616) 364-1211

Fax : (616) 364-9571


Hyperthyroidism Client Review

  1. Thyroid hormone production by the thyroid is controlled by the brain in a negative feedback loop under ordinary circumstances. The level of thyroid hormone in the blood fluctuates up and down throughout the day within a certain typical range.


  1. With hyperthyroidism, thyroid tumor tissues produce and release thyroid hormone independently of the brain’s control. Consequently, thyroid hormone levels start increasing in the blood as the body cannot use up the hormone as quickly as it is being produced. With time, thyroid hormone levels can become very high. These levels of hormone interact dangerously with the body’s tissues and with other hormones such as cortisol and epinephrine, causing weight loss, muscle wasting, increased appetite and activity, and cardiovascular problems such as hypertension and heart failure.

  2. Radioactive iodine is taken up only by thyroid tissue producing thyroid hormone at the moment of dosing. With hyperthyroid cats, only the tumor tissues will absorb the iodine. The normal cells are dormant since the brain is not releasing any stimulating hormones that would activate the normal cells.

  3. Consequently, the tumor cells are destroyed but the normal dormant thyroid cells are spared. The blood thyroid hormone levels then start going down as they are degraded by the body. With time the levels of hormone become low enough that the brain starts releasing thyroid stimulating hormones again. This reactivates the normal dormant thyroid cells and starts the normal thyroid cycle again. Therefore, unless surgery was previously performed to remove hyperthyroid tissue, most cats have enough normal tissue left to avoid needing any thyroid supplements.


A follow up blood sample and exam by your veterinarian two to three weeks after going home is needed after treatment to evaluate effectiveness of treatment and to look for any complications.

  1. The radioactive iodine decays quickly. More than 90% of the dose of radioactive iodine will be gone before your cat leaves our hospital. More than 98% will be gone after one week of home quarantine (adults only in the household). More than 99% will be gone after two weeks of home quarantine (when children or pregnant women are in the household). Bonfire analogy: the amount of radiation when first treated is hot; candle flame analogy is small amount of radiation by the time they go home.

  2. Your cat cannot make anything radioactive. If there is a spill of body fluids (vomit, urine, blood or saliva) or stool on a surface, the surface may be contaminated temporarily with a surface layer of radioactive material. (Paint analogy) Clean the affected area with warm water and dish soap and toilet paper and flush down the toilet. Bedding can be washed twice in the washing machine if soiled.

  3. Distance is your friend with handling cats treated with radiation. They need to be quarantined away from the rest of the household in a separate room with a door.

  4. Radioactive iodine releases two types of energy that we are concerned about:

    1. Gamma radiation: could cause a slight increase in cancer with long term high level exposure. No immediate damage, penetrates extensively (reason for 9 layers of lead in our radiation room wall.)

    2. Beta particles: destroy the tumor, cannot penetrate wall. Wearing exam or latex gloves blocks the beta particles from your skin. Wash hands to remove if you get urine or stool on your hands.

    3. Follow the quarantine rules and you will receive less radiation from your cat than if you flew from here to San Diego and back.

  5. Bonfire analogy: when your cat is first treated here, it is similar to a bonfire—get too close and you get burned. When your cat leaves us, it is equivalent to a candle flame—you only get burned if you stick your finger into the flame too long. By the time it is out of quarantine, the “fire” is out and the level of radiation is too low to be of any harm.

  6. Please call us if you have any questions or concerns.

Tammy P. Sadek, DVM & Dr. Alyson Coppens
Cat Clinic North & Kentwood Cat Clinic | ABVP Feline Practice

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