section name header

Introduction

  1. Pharmacology. Iodine 131 is a product of fission reactions and a likely major form of internal radioactive contamination after a major nuclear reactor accident or weapon detonation. Potassium iodide (KI) blocks thyroid gland uptake of the radioactive isotopes of iodine by both diluting the radioactive iodine and filling the gland with nontoxic iodine. The radioactive molecules are subsequently excreted in the urine.For optimal protection, KI should be administered before or at the time of exposure to radioactive iodines but will have protective effects if initiated up to 4 hours after exposure. Daily administration is indicated until the risk for exposure to radioactive iodines no longer exists.
  2. Indications. Potassium iodide is indicated for prevention of uptake of radioactive iodine isotopes by the thyroid gland. The highest risk groups for radioiodine-induced thyroid cancer include infants, children, and pregnant and nursing females. The lowest risk group is persons older than 40 years. Note: KI should be used only when and if directed by federal, state, or local public health officials.
  3. Contraindications
    1. Known iodine allergy. Persons with the rare disorders of dermatitis herpetiformis and hypocomplementemic vasculitis are at increased risk for sensitivity.
    2. Patients with heart disease accompanied by nodular thyroid disease should not take KI.
    3. Patients with multinodular goiter, Graves' disease, and autoimmune thyroiditis should be treated with caution, especially if dosing exceeds a few days.
  4. Adverse effects
    1. Gastrointestinal upset, diarrhea, burning of throat, metallic taste in mouth, sore gums, and rarely inflammation of the salivary glands. These effects are more common as duration of therapy and dose increase.
    2. Allergic reactions ranging from skin rashes to respiratory distress may occur, although life-threatening reactions are very uncommon.
    3. Iodine-induced thyrotoxicosis, hypothyroidism, and goiter may occur, but incidence is less than 2%, even if therapy is used for longer durations.
    4. A bluish skin discoloration involving the sweat glands may occur after large doses of iodine-containing products.
    5. Use in pregnancy. FDA Category D. KI crosses the placenta and can suppress thyroid function in the fetus. The FDA recommends that pregnant women avoid repeated dosing unless other protective measures are not available. Risk is minimal with short-term use (<10 days) and when given long before term.
    6. Use in neonates. Increased risk for hypothyroidism in infants, especially in neonates less than 1 month of age. Thyroid function tests should be monitored in neonates given more than a single dose of KI.
    7. Use in breast-feeding. KI and radioiodines both pass into breast milk. Lactating mothers should be cautioned not to breast-feed infants unless there is no other alternative.
  5. Drug or laboratory interactions
    1. Synergistic hypothyroid activity with lithium.
    2. Thyroid-stimulating hormone (TSH) and free thyroxine (T4) monitoring of thyroid function is reliable in the setting of standard dosing of KI. Recommended in all neonates treated with KI.
    3. Risk for hyperkalemia with prolonged use along with other potassium supplements and potassium-sparing medications (eg, spironolactone). However, the daily dose of potassium from KI is only 3-4 mEq.
  6. Dosage and method of administration
    1. There are various dosing guidelines, including those recommended by the US Food and Drug Administration (FDA) and the World Health Organization (WHO). Public health officials should decide on the regimen they will use in a specific situation. A guidance document from the CDC is available at https://www.cdc.gov/nceh/radiation/emergencies/ki.htm
    2. A single dose provides 24 hours of protection. Once-a-day dosing is recommended. Potassium iodide oral solution can be made from crushed KI tablets for use in children and adults unable to swallow tablets. Crush a 130-mg tablet and mix with four teaspoons (20 mL) of water until dissolved, then add four teaspoons (20 mL) of chocolate milk, orange juice, soda, or baby formula. This results in a solution containing 3.25 mg/mL. Plain water or low-fat milk may not adequately mask the salty, unpleasant taste of KI tablets. The solution will keep for up to 7 days in the refrigerator. The FDA recommends that the solution be prepared weekly and the unused portion discarded.
    3. Dose by age group:
      1. Adults older than 18 years: 130 mg orally once a day.
      2. Adolescents and children (3-18 years): 65 mg daily. (Adolescents weighing 150 lb or more should be given the adult dose of 130 mg.)
      3. Infants (1 month-3 years): 32 mg daily.
      4. Neonates (0-1 month): 16 mg one-time dose with protective measures (evacuation, avoiding breast milk and local cow's milk) put in place.
    4. Duration of therapy may be from 1 day to many weeks, depending on public health recommendations. Prolonged prophylaxis may be required for protection from radioactive iodine-contaminated produce and milk. The study of childhood thyroid cancers following Chernobyl suggests that continued dosing long after the initial accident may result in decreased cellular proliferation and reduced risk for thyroid cancer.