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INTRODUCTION

  1. Pharmacology. Iodine 131 is a product of fission reactions and likely to be a 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 isotopes of iodine 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 who have 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 become 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, and lactating mothers should be cautioned to not breast-feed infants unless no other alternative is available.

  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 ...

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