Each element, such as iodine, is composed of a set number of electrons, protons, and neutrons that give it its chemical and physical characteristics. The number of neutrons in the nucleus may vary and provide a number of different iodine atoms that chemically act the same but have different physical properties. The nucleus may be stable or unstable. An unstable nucleus that spontaneously losses subatomic particles from its nucleus and emits energy in the form of radiation is considered radioactive. This loss of subatomic particles, such as beta particles (electrons), is called radioactive decay and there is a predictable time in which half of the unstable nucleuses will decay. This is called the physical half-life of the radioisotope.

Two unstable forms of iodine, i.e., the radioactive isotopes I-123 (13 hour half-life and photon (x-ray) emitter is good for imaging the location of iodine in the body and harmless to thyroid cells) and I-131 (8 day half-life and both photon (gamma) and beta particle emitter can both image where iodine is in the body and destroy thyroid cells) are used in the diagnosis and treatment of thyroid disease. This radioactive iodine is used in the form of sodium iodide and because of the extremely small amounts used for imaging or destroying cells, RAI is safe to use in individuals who have had allergic reactions to seafood or X-ray contrast agents. RAI is given by mouth in pill or liquid form.

Stable iodine, an essential trace element, is used by the thyroid gland to produce two thyroid hormones (T3 and T4). In papillary and follicular thyroid cancers (differentiated thyroid cancer) and hyperthyroidism, the thyroid and cancer cells produce too much thyroid hormones. In these cases, radioactive I-131 can be used to replace stable iodine and kill both the normal and abnormal thyroid cells regardless of where they are located.

Radioactive I-131 emits both gamma and beta radiation. The gamma radiation allows it to be easily detected and located with radiation survey meters and imaged by nuclear medicine imaging techniques. The gamma radiation from I-131 in the patients is the major source of external radiation to others after an I-131 procedure. The beta particles emitted are used to kill, destroy, or ablate cells that use iodine and makes it a good therapeutic agent. The beta radiation is the primary concern when there is an oral uptake of I-131 indirectly from contaminated surfaces or directly by transfer from the patient.

For the radioactive iodine to be effective, the availability of stable iodine has to be reduced so more radioactive I-131 will be used by the cells. Americans in major parts of the United States in the 1900's received too little iodine in their diets and suffered from a disease called hypothyroidism. At that time a decision was made by public health officials to have iodine added to table salt (iodized salt) and other food staples to reduce the incidence of hypothyroidism. One effect of this is that when thyroid cancer patients need to reduce the amount of stable iodine in their bodies before receiving radioactive I-131, they have to go on low iodine diets and avoid many foods. A combination between a low iodine diet and increased thyroid stimulating hormone is used to increase the use of radioactive I-131 by the cells.

Radioactive I-131 emits both gamma and beta radiation and a person receiving an I-131 procedure will continue to emit both gamma and beta radiation until the I-131 is gone. The amount of I-131 will decrease over time as the unstable I-131 decays or as the body processes the iodine and eliminates the I-131 from the body. For hyperthyroid patients, the thyroid tissue uses and holds on to the radioactive I-131. Therefore, while the patient receives smaller doses than a thyroid carcinoma patient, the radioactive Iodine is retained longer by the person.

Thyroid carcinoma patients, receiving radioactive I-131, have already had surgery to remove their thyroid gland but some "thyroid tissue" may remain. Therefore, they have less thyroid tissue to use the radioactive 1-131 and most of the radioactive I-131 remains in circulation as the body processes it and gets rid of it. It is processed by the parts of the body that produce fluids, such as the kidneys that produce urine, the sweat glands that produce sweat, and the salivary glands that produce saliva, and the mammary glands that produce milk if the person is lactating. Radioactive I-131 is also volatile so it may be released as a gas during normal breathing. If the person's kidney function is normal (and they are not lactating), removal of most of the free radioactive I-131 occurs within the first few days through the production of urine.

Contamination of the area the patient is in can happen in a number of ways. Patients that vomit before the radioactive I-131 has been absorbed through the digestive track may lose a significant amount of the iodine they received in the vomited fluid. Their clothing and bed linens may become contaminated by sweat or other bodily fluids. Hands, coughing, or sneezing can contaminate objects in a room and normal room surfaces and trash. Saliva can contaminate eating utensils, partially eaten foods such a chicken, beef or pork bones and result in contaminated trash. Saliva and urine can cause contamination in bathrooms.

Basic radiation safety associated with radioactive I-131 involves using the principles of time and distance to reduce exposure to others as the basis for precautions to take after treatment.

The risk to pregnant women is in the delivery of radioactive I-131 to the developing fetal thyroid. The risk to a lactating mother is the increased radiation dose to breast tissue and transferal of radioactive iodine to the nursing infant.

The risk to young children is both from their lack of understanding that they have to maintain a safe distance from an I-131 patient and their tendency to put fingers and things in their mouth that may be contaminated with I-131. Their thyroid glands are smaller and the same amount of radioactive will deliver a larger radiation dose to the child's thyroid than to an adult thyroid.