Key Points

  • Both federal and state regulations require that hyperthyroid cats treated with radioiodine (131I) be hospitalized in a licensed facility for an interval of time following treatment that is designed to ensure that the risk from radiation posed to their family at home is minimized.
  • During the process of its radioactive decay radioiodine (131I) emits 2 different types of radiation, namely gamma radiation and beta radiation
  • Limiting gamma radiation exposure from your cat involves limiting the time you spend close to your cat.
  • Limiting beta radiation exposure from your cat involves ensuring against your internalization of the radioactive iodine excreted by your cat.
  • A detailed explanation of the risks associated with radiation exposure can be found in a separate discussion entitled Risks Associated with Radioiodine Therapy for Feline Hyperthyroidism.

Radiation Basics

Radiation is energy. It can come from unstable atoms that undergo radioactive decay, or it can be produced by machines. Radiation travels from its source either in the form of energy waves called gamma (γ) rays or as energized particles including alpha (α) particles, beta (β) particles and neutrons. The different forms of radiation have different properties and effects. Radioiodine (131I) emits both gamma rays and beta particles. 

A gamma ray is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. The gamma rays emitted by 131I travel relatively long distances allowing them to escape the body of the patient and potentially expose nearby individuals.

A beta particle is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus during the process of beta decay. The beta particle emitted by 131I travels relatively short distances preventing them from escaping the body of the patient and preventing the direct exposure of nearby individuals. The extremely short distances traveled by beta particles in the body of the patient limit the exposure from beta particles to direct interaction with the radioiodine (131I) excreted in the patient’s urine or feces. 

As a result reducing exposure from gamma radiation is accomplished by managing distance from the patient, while reducing exposure from beta particles is accomplished by avoiding internalization of the patient’s urine and feces. 

The principles of radiation exposure reduction include time, distance, shielding and containment

Distance and the inverse square law

The single most practically useful tool to reduce radiation exposure is distance. The utility of distance for reducing radiation exposure largely has to do with something called the inverse square law. The inverse square law describes the principle of dose reduction as the distance from the source increases. Radiation travels equally in all directions from its source following the path of an expanding sphere. As the distance from the source (i.e., the radius of the sphere) increases, the area over which the dose is distributed increases according to the formula for the area of a sphere


where A is the area of the sphere and r is the radius of the sphere or the distance from the source. As a result, the exposure from a source of radiation is proportional to the inverse of the square of the radius. Thus doubling the distance will reduce the exposure by a factor of four. This conflicts with most people’s erroneous, intuitive expectation that doubling the distance from a source of radiation will reduce the exposure by 50%. The exposure at a distance of 2 inches is only 25% of the exposure from the same source at a distance of 1 inch. This property of radiation exposure becomes extremely useful when the distance changes from inches to feet and makes it an extremely useful tool that we can use to reduce our radiation exposure. Indeed the simple use of distance is all that the dental technologist is using when they take a single step outside of the room before triggering the x-ray machine that takes your dental radiograph. Similarly maintaining a relatively small distance between you and your cat (3 – 6 feet) will ensure you receive a minimal exposure. 

Time: quality vs quantity

The next tool we can bring to the goal of reducing radiation exposure is time. Unlike distance, the impact that time has on our exposure is linear and hence more intuitive. Reducing the time we spend near a source of radiation by 90% will reduce our exposure by 90%. No fancy math involved here. 

Our mutual goal to reduce your exposure to radiation from your cat does not necessitate that you completely isolate your cat but does require you to be aware of your cat’s distance from you. Obviously time you are outside of your home including while at work, and while shopping or recreating will not require any additional effort on your part. While at home during the day, most people feel like they can actively be aware of their cat’s relative proximity and manage their location or their cat’s location accordingly.

At night when asleep, no one is a competent judge of their cat’s distance from them and so a physical barrier like a closed bedroom door is likely to be needed. The use of physical barriers like closed doors can also be very useful in households with a work from home schedule or with small children who can not be relied upon to maintain a distance from the cat. Most clients find that they can spend a few quality minutes in close proximity to their cat multiple times each day to ensure the cat still feels the bond of affection that we all want you to maintain while keeping the total time in close proximity to a minimum. 

Shielding: A heavy concept

By this point many of our clients will be thinking “To heck with this time and distance stuff, I intend to get my hands on a lead lined blanket like they use at my dentist’s office. Then I can spend all day with my cat sitting on my lap.” 

The lead lined blanket/shield used at your dentist’s office is useful because the energy level of the x-ray produced by the dental x-ray machine is relatively low, typically 60-70 kV. The gamma ray emitted by 131I has a much higher energy level of 364 kV. One would need quite a few lead lined blankets to achieve the same reduction in exposure as can be achieved by a relatively short distance. 

Shielding can be very useful to supplement the use of time and distance. We have 1/4 and 1/2 inch thick lead in the walls surrounding our feline and canine radiotherapy wards, respectively. This allows employees to be relatively close to the outside of those wards with a significantly reduced exposure. The weight of that shielding however precludes the practical use of this option in the home. The cost alone for that kind of exposure reduction would be orders of magnitude more than the cost of the radioiodine therapy itself. 

Containment: This could get messy

The previous topics including the use of time, distance and shielding all work to reduce radiation exposure from an external source usually by means of emitted gamma rays. At this point we need to address the potential exposure we can get from the beta particles emitted by 131I. Just like the stable iodine (i.e., 127I) that your cat consumes in its diet, the large majority of radioactive iodine (i.e., 131I) is actively concentrated in your cat’s thyroid gland where it is used to make the thyroid hormones T4 (thyroxine) and T3 (triiodothyronine). The iodine that escapes thyroid uptake and therefore remains in circulation, as well as the iodine that returns to circulation following breakdown of thyroid hormones is rapidly removed from the circulation by the kidneys for elimination from the body in the urine. The iodine that is incorporated into the thyroid hormones T4 and T3 by a process called organification is eventually removed from circulation by the liver and sequestered into the bile for elimination from the body in the feces.

As a result of the preceding features of iodine metabolism, hyperthyroid cats treated with radioactive iodine will produce radioactive urine and feces for an interval following the administration of the 131I. The oral consumption or transcutaneous absorption of radioactive iodine by family members of hyperthyroid cats treated with radioiodine will lead to radiation exposure that is largely from the beta particle emission. Once a radioactive form of iodine is in your body the utility of time, distance and shielding are markedly reduced. As a result it is important to properly handle and manage the source of the potential radiation exposure, namely the cat’s urine and feces. 

The virtually universal goal of cat owners to avoid the consumption of cat feces dictates the behaviors we bring to the experience. Those behaviors typically include the appropriate use and handling of the scoop and most importantly the critical step of washing one’s hands following management of their cat’s litter box(es). You should manage the process of cleaning your cat’s litter box just like you manage the process of cleaning your toilet(s).