Endocrinology Book

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Radioiodine

Aka: Radioiodine, Radioactive Iodine, I-131
  1. Indications
    1. Management of choice for Grave's Disease of all ages (most common treatment in U.S.)
    2. Recurrent Hyperthyroidism after Antithyroid Drugs
    3. Toxic Multinodular Goiter
    4. Toxic Nodule in patient over age 40 years
  2. Contraindications
    1. Pregnancy
    2. Moderate or Severe Graves Orbitopathy, esp. Tobacco users
      1. Radioactive Iodine may worsen orbitopathy
      2. Mild Graves Orbitopathy may be concurrently treated with Corticosteroids
  3. Pharmacokinetics
    1. Half-life of I-131: 9 days
    2. Effects nos seen for over 3-4 weeks
  4. Mechanism
    1. Concentrates in and destroys Thyroid tissue
  5. Precautions
    1. Background
      1. Radio-Iodine excreted in urine, Saliva, feces
      2. Most is eliminated in first 48 hours
    2. For first 3 days
      1. Avoid contact with children and pregnant women
    3. For first few weeks
      1. Double flush toilet
      2. Wash hands carefully
  6. Adverse Effects
    1. No increased risk of Cancer Death
    2. Radiation-Induced Thyroiditis
      1. Acute Thyroiditis in first 1-2 weeks post-I131
    3. Slight increased risk of Thyroid Cancer
      1. Graves Disease predisposes to Thyroid Cancer
      2. Cancers may have been present before I-131
    4. Graves Ophthalmopathy may worsen on I-131
      1. Occurs more commonly in Tobacco abuse
      2. Prednisone used to minimize this effect
        1. Dose: 40-80 mg daily
        2. Taper dose over 3 months
  7. Efficacy: Euthyroid or Hypothyroid after symptoms
    1. Euthyroid at 6-8 weeks after I-131: 50 to 75%
    2. Euthyroid after one dose I-131: 80 to 90%
  8. Labs
    1. Urine Pregnancy Test
      1. Obtain within 48 hours prior to Radioactive Iodine administration
  9. Protocols
    1. Stop all medications containing Iodine before Iodine (at least 2-3 weeks)
      1. Avoid large Iodine doses (e.g. iodinated contrast or Amiodarone) within 3 months prior to Radioiodine ablation
    2. Avoid pregnancy for 6 months after Radioiodine exposure
      1. Obtain Pregnancy Test within 48 hours of administration
  10. Management: Adverse Effects
    1. Beta Blockers control symptoms around radio-Iodine
    2. Concurrent use with Antithyroid Drugs (e.g. PTU, Methimazole)
      1. Indications
        1. Consider if on antithyroid medication prior to Radioiodine
        2. May be helpful in severe Hyperthyroidism
      2. Protocol near time of radio-Iodine treatement
        1. Stop Antithyroid Drugs 5 days before radio-Iodine
        2. Restart Antithyroid Drugs 3-5 days after radio-Iodine
        3. Continue Antithyroid Drugs for 3 months then taper
  11. Management: Follow-up
    1. Obtain Free T4 and Free T3 at 4-8 weeks after ablation and then every 8-12 weeks until stable
    2. Expect permanent Hypothyroidism at 2-6 months after Radioiodine ablation
    3. Initiate low dose Thyroid Replacement as Hypothyroidism develops
  12. References

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