Sodium iodide I-131 (Iodine 131) is radioactive iodine that is used to treat a variety of thyroid-related disorders including graves disease and thyroid cancers.

Sodium Iodate (Iodine 131) Uses:

• Diagnostic agent:

  • Iodine 131 is used for the radioactive iodine (RAI) uptake test to evaluate thyroid function and for thyroid imaging.
  • Note: Due to increased thyroid radiation exposure with sodium iodide, alternative agents such as sodium iodide I-123, technetium-99m pertechnetate are recommended for routine thyroid scintigraphy

• Therapeutic agent:

  • It is indicated for the management of hyperthyroidism and some of the thyroid malignancies if the lesions take up iodide.

• Off Label Use of Sodium iodide, I-131 in Adults:

  • Whole-body imaging for thyroid cancer metastases

Sodium iodide (Iodine 131) dose in adults:

Note: Dose will be expressed as millicuries if dose >1000 microcuries. Radiation safety precautions are necessary during radioactive iodine treatment. Antithyroid medications should be stopped 3 to 4 days before using sodium iodide I-131.

Diagnostic procedures: Oral based on a 70 kg patient:

• Thyroid uptake:

  • 5 to 30 microcuries (or 0.185 to 1.1 megabecquerels) 24 hours before uptake measurement
  • maximum dose of 10 microcuries (0.37 megabecquerels)

• Dose adjustment if used in conjunction with technetium Tc 99m sodium pertechnetate:

  • 4 to 5 microcuries (0.15 to 0.185 megabecquerels) .

Iodine 131 Dose in the Thyroid imaging:

• 50 to 100 microcuries (or 1.85 to 3.7 megabecquerels) 16 to 24 hours before imaging.

Iodine 131 Dose in the Whole-body imaging for thyroid cancer metastases (off-label):

• 1 to 5 millicuries (37 to 185 megabecquerels) 24 to 72 hours before imaging.

Sodium iodate (Iodine 131) dose in the treatment of Hyperthyroidism:

• Manufacturer's labeling recommends 4 to 10 millicuries (or 148 to 370 megabecquerels) but it has high failure rates.

Sodium iodate (Iodine 131) dose in the treatment of Graves’ disease:

• 10 to 15 millicuries (or 370 to 555 megabecquerels) or 150 microcuries/g of tissue corrected for 24-hour radioactive iodine uptake.

Sodium Iodate (Iodine 131) dose in the treatment of Toxic multinodular goiter (off-label dosing):

• 150 to 200 microcuries/g of tissue corrected for 24-hour radioactive iodine uptake (see dose equation below), if hyperthyroidism persists, may repeat after 6 months if needed

Sodium Iodate (Iodine 131) dose in the treatment of Toxic thyroid adenoma (off-label dosing):

• 10 to 20 millicuries (or 370 to 740 megabecquerels) or 150 to 200 microcuries/g of tissue corrected for 24-hour radioactive iodine uptake, if hyperthyroidism persists, may repeat after 6 months if needed
• Dose equation corrected for 24-hour radioactive iodine uptake:
  • Activity (millicuries) = (gland weight in grams x desired dose in microcuries/gram x [1/24 hour uptake in % of dose])/1,000

Sodium Iodate (Iodine 131) dose in the treatment of Thyroid cancer:

• Ablation of normal thyroid tissue:

  • Initial: 30 to 100 millicuries (or 1,100 to 3,700 megabecquerels).

Note: 150 millicuries is advised for initial adjuvant therapy in microscopic residual disease without metastasis.

• Subsequent metastases ablation:

  • 100 to 200 millicuries (or 3,700 to 7,400 megabecquerels)

Sodium iodide (iodine 131) dose in children with Graves Disease:

• Children ≥5 years of age:

  • Oral: Dose is dependent upon gland size;
  • >150 microcuries/gm of thyroid tissue is necessary to induce hypothyroidism;
  • if the gland is between 50 to 80 gms, higher activities (200 to 300 microcuries/g of thyroid tissue) may be needed.
  • Therapy in children between 5 to 10 years of age is acceptable if the calculated dose is <10 millicuries

Pregnancy Risk Factor X

• It crosses the placenta and can cause irreversible neonatal hypothyroidism. Therefore, it is not recommended during pregnancy.
• Both males and females should use two effective contraceptives during treatment. They are both recommended for at least one year.
• It is best to delay elective diagnostic procedures until after delivery.

Sodium iodide I-131 use during breastfeeding:  

• Iodine-131 can be found in breast milk. Therefore, it is not recommended that you breastfeed or pump for at least 4 weeks before taking sodium iodide I.
• If sodium iodide Iodide has been given after delivery, breastfeeding should be avoided
• It is important to delay elective diagnostic procedures until after breastfeeding has stopped.

Sodium iodide (Iodine 131) dose adjustment in renal disease:

• There are no dosage adjustments provided in manufacturer's labeling.
• However reduced clearance and increased radiation exposure is observed in renal impairment.
• Hemodialysis: Sodium iodide I is dialyzable.

Sodium iodide (Iodine 131) dose adjustment in liver disease:

There are no dosage adjustments provided in manufacturer's labeling.

Side effects of Sodium iodide (Iodine 131):

• Cardiovascular:

  • Chest Pain
  • Tachycardia

• Central Nervous System:

  • Cerebral Edema (In Patients With Iodine-Avid Brain Metastases)
  • Headache

• Dermatologic:

  • Pruritus
  • Skin Lesion (Iododerma)
  • Skin Rash
  • Urticaria

• Endocrine & Metabolic:

  • Goiter
  • Hyperthyroidism
  • Hypoparathyroidism
  • Hypothyroidism
  • Ovarian Failure (Transient)
  • Thyroid Storm

• Gastrointestinal:

  • Gastritis
  • Nausea
  • Salivary Gland Disease
  • Sialadenitis
  • Sore Throat
  • Vomiting
  • Xerostomia

• Genitourinary:

  • Inhibition Of Testicular Function (Transient)

• Hematologic & Oncologic:

  • Anemia
  • Bone Marrow Depression
  • Hematologic Abnormality
  • Leukemia
  • Leukopenia
  • Neoplasm
  • Solid Tumor
  • Thrombocytopenia

• Hepatic:

  • Hepatic Insufficiency

• Hypersensitivity:

  • Hypersensitivity Reaction

• Local:

  • Local Pain (Pain On Swallowing)
  • Local Swelling (Thyroid Or Site Of Iodide Avid Tumor)
  • Localized Tenderness

• Ophthalmic:

  • Conjunctivitis
  • Epiphora
  • Lacrimal Duct Obstruction (Dacryostenosis)
  • Lacrimal Dysfunction
  • Xerophthalmia

• Respiratory:

  • Bronchospasm
  • Cough
  • Pulmonary Fibrosis (In Patients With Iodine-Avid Lung Metastases)
  • Radiation Pneumonitis (In Patients With Iodine-Avid Lung Metastases)

• Miscellaneous:

  • Radiation Injury (Including Radiation Sickness)

Contraindication to Sodium iodide I-131 (Iodine 131):

• Diarrhea and vomiting
• Breastfeeding and Pregnancy
• Concurrent antithyroid medication (discontinue 3-4 days before administration).
• Treatment of thyroid malignancies without iodide uptake, such as anaplastic and medullary carcinomas

Warnings and precautions

• Hypersensitivity

  • Sodium Iodide I solution could contain sodium bisulfite. This sulfite can cause anaphylactic reactions, fatal asthmatic episodes, and allergic-type reactions.

• Infertility

  • Transient,dose-related testicular impairment in males and ovarian failure in females after sodium iodide is seen.

• Radiation-induced thyroiditis

  • Thyroiditis can be caused by sodium iodide I. This can lead to hyperthyroidism that may become more severe.
  • To reduce the chance of thyroid storm and hyperthyroidism, beta-blocker therapy should be used before I use sodium iodide.

• Radiation toxicities

  • These include salivary gland and lacrimal toxicity that presents as conjunctivitis and epiphora.
  • There is an increased risk of neoplasia, hematopoietic suppression and transient neutropenia/thrombocytopenia.

• Thyroid-stimulating Hormone/thyroid Enlargement

  • Thyroid enlargement can lead to obstruction of the trachea or esophagus.

• Graves orbitopathy

  • Patients with Graves orbitopathy are not advised to receive radioactive iodine therapy.
  • Radioactive iodine therapy (RAI) is only indicated for active/mild diseases.
  • A combination of glucocorticoids may be required in cases of high thyrotropin receptor antibody and smoking. This can lead to orbitopathy.

• Hyperthyroidism/thyrotoxic cardiac disease:

  • Radiation thyroiditis can sometimes be exacerbated. This can be prevented by antithyroid drugs and/or beta-blockers prior to and after treatment.

• Hypochloremia:

  • May increase thyroid uptake sodium iodide I and 131.

• Renal impairment

  • Nephrosis can increase thyroid uptake sodium iodide I and 131.

Sodium iodide I-131: Drug Interaction

Note: Drug Interaction Categories:

• Risk Factor D: Consider Treatment Modification
• Risk Factor X: Avoid Concomitant Use

Monitoring parameters:

Thyroid function tests (TSH, free T4, free T3, and total T4 and T3 ) 1-2 months after the treatment and at 4 -6 week intervals for 6 months, or until hypothyroid and stable on thyroid hormone replacement (Graves’ disease, toxic multinodular goiter, and toxic adenoma)

How to administer Sodium iodide I-131 (Iodine 131)?

Proper hydration before treatment and radiation safety precautions are recommended.

Hicon:

• The solution should be diluted before use and kept in a shielded container with a straw. It should be given on an empty stomach.
• The patient should fast at least 2 hours before and 2 hours after administration to ensure adequate absorption.

Radiopharmaceutical:

• Use appropriate precautions for handling and disposal. Waterproof gloves should be worn while handling and administering sodium iodide I131.

Mechanism of action of Sodium iodide I-131 (iodine 131):

• The body's extracellular fluid is able to absorb and distribute sodium iodide quickly. 
• The thyroid is the site of concentration of iodide via the sodium/iodide portaler. It is then oxidized to Iodine by beta emission.

Absorption: Oral: Rapidly (90% within 60 minutes)

Distribution: Extracellular space; primarily trapped by the thyroid gland.

Protein binding: None

Metabolism: Iodide is rapidly oxidized to iodine in the thyroid

Excretion: Urine (37% to 75%), feces 10%

International Brands of Sodium iodide I-131:

• Draximage

Sodium iodide I-131 brand names in Pakistan:

Brands will be updated later.