Prophylactic Use of Potassium Iodide (KI) in Radiological Emergencies*: Information for Physicians

* Based on FDA's guidance document on use of KI as a thyroid blocking agent in radiation emergencies

1. What is potassium iodide and what is it used for?

Potassium iodide (KI) is a chemical compound that can be used to protect the thyroid gland from possible radiation injury caused by radioactive iodine (radioiodine). Some radiological emergencies may release large amounts of radioiodine to the environment. Since iodine concentrates in the thyroid gland, inhalation or ingestion of food contaminated with radioiodine can lead to radiation injury to the thyroid, including increased risk of thyroid cancer and other thyroid diseases.

2. How does KI work?

Taking KI saturates the thyroid gland with stable (non-radioactive) iodine. This prevents or reduces the amount of radioiodine that will be taken up by the thyroid.

3. What are the risks to the thyroid from radioiodine?

The radiation dose to the thyroid that results from the uptake of radioiodine increases the risk of thyroid cancer, especially among children. Observations in Europe following the Chernobyl reactor accident in 1986 suggest that the younger the child at the time of exposure, the greater the risk of thyroid cancer. Risk may accrue at very low levels of radioiodine exposure, especially in young children. High radiation doses to the thyroid can also induce hypothyroidism, both in children and adults.

4. How effective is the use of KI?

Potassium iodide when taken before, or shortly after exposure to radioiodine, is effective in reducing radioiodine uptake by the thyroid gland. Reducing this uptake decreases the risk of thyroid cancer in individuals or populations at risk for inhalation or ingestion of radioiodine. KI does not protect against radiation doses received from external sources of radiation or from other radionuclides (except radioactive isotopes of iodine) that may be ingested or inhaled. It also does not protect body organs or tissues, other than the thyroid.

5. What are the side effects of KI?

The risks of stable iodine administration include sialadenitis (inflamation of the salivary gland), gastrointestinal disturbances, allergic reactions and minor rashes. In addition, persons with known iodine sensitivity should avoid KI, as should individuals with dermatitis herpetiformis and hypocomplementemic vasculitis, extremely rare conditions associated with an increased risk of iodine hypersensitivity. Thyroidal side effects of stable iodine include iodineinduced thyrotoxicosis. This is more common in older people and in iodine deficient areas, and usually requires repeated doses of stable iodine. Iodide goiter and hypothyroidism are potential side effects more common in iodine sufficient areas, but they require chronic high doses of stable iodine. Therefore, individuals with multinodular goiter, Graves' disease, and autoimmune thyroiditis (most likely to be adults) should be treated with caution, especially if dosing extends beyond a few days. Transient biochemical hypothyroidism was observed in 0.37% (12 of 3214) of neonates treated with KI in Poland after the Chernobyl reactor accident in 1986, with no reported sequelae to date. FDA has determined that short-term administration of KI at thyroid blocking doses is safe and, in general, more so in children than adults.

6. What precautions are recommended if KI is used?

The FDA determined that the benefits of KI treatment to reduce the risk of thyroid cancer outweigh the risks of such treatment in neonates. However, in light of the potential consequences of transient hypothyroidism for intellectual development, FDA recommends that neonates treated with KI be monitored for this effect by measurement of TSH (and FT4, if indicated). Thyroid hormone therapy should be instituted in cases in which hypothyroidism develops.

7. What are the recommended prophylactic doses of KI?

FDA recommendation for administration of KI based on age/weight, predicted thyroid exposure, and pregnancy and lactation status are given in the table on the reverse side.

Threshold Thyroid Radioactive Exposures and
Recommended Daily Dosages of KI for Different Risk Groups
*Adolescents approaching adult size (>=70 kg) should receive the full adult dose (130 mg).
  Predicted
Thyroid
Exposure (cGy)
KI dosage (mg) Number of ml
liquid
(65 mg/ml)
Number of 65-mg
tablets
Number of 130-mg
tablets
Adults over 40 years >=500 130 2 2 1
Adults over 18 through 40 years >=10
Pregnant or lactating women >=5
Adolescents over 12 through 18 years* 65 1 1 1/2
Children over 3 through 12 years
Over 1 month through 3 years 32 0.5 1/2 1/4
Birth through 1 month 16 0.25 1/4 1/8

KI is currently FDA-approved and available overthe- counter in 65-mg and 130-mg tablets and liquid form. The 65-mg tablets are larger than the 130-mg tablets and scored in quarters. Dosing at the FDA recommended level is much easier with the 65-mg tablets. For children or babies who cannot take pills, parents and caregivers can cut or crush the pill to make lower doses, or give the liquid form of KI. Efforts should be made to dose at the FDArecommended level, especially for neonates. Because of the difficulty of splitting a large number of tablets, the New York State Commissioner of Health endorses the administration of an entire tablet for children in settings such as schools or childcare centers in the event of emergencies. If the 65-mg tablet is not available, the New York State Commissioner of Health supports the administration of the 130-mg tablet in the event of emergencies. This dose is safe and well within the recommended therapeutic range of KI for other indications. The blocking effect of iodide on the thyroid lasts only a few days and any suppressive effect of KI on thyroid function has been shown to be minimal, even in young children.

6. How often should KI be administered?

The protective effect of KI lasts approximately 24 hours. For optimal prophylaxis, KI should therefore be dosed daily, until a risk of significant exposure to radioiodines by either inhalation or ingestion no longer exists. FDA indicates that across populations at risk for radioiodine exposure, the overall benefits of KI far exceed the risks of overdosing, especially in children, although it continues to emphasize particular attention to dose in infants. Pregnant women should be given KI for their own protection and for that of the fetus, as iodine (whether stable or radioactive) readily crosses the placenta. However, because of the risk of blocking fetal thyroid function with excess stable iodine, repeat dosing with KI of pregnant women should be avoided. Lactating females should be administered KI for their own protection. KI to the mother is not a means to deliver KI to infants, who should get their KI directly. As with direct administration of KI, stable iodine as a component of breast milk may also pose a risk of hypothyroidism in nursing neonates. Therefore, repeat dosing with KI should be avoided in the lactating mother, except during continuing severe contamination. If repeat dosing of the mother is necessary, the nursing neonate should be monitored as recommended above.

For additional information contact:

New York State Department of Health Infoline 1-800-458-1158 or 518-402-7550 or at berp@health.state.ny.us

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