Sodium Iodide I131

Pharmacology

Mechanism of Action

Oral sodium iodide I¹³¹ is rapidly absorbed and distributed within the extracellular fluid of the body. Iodide is concentrated in the thyroid via the sodium/iodide symporter, and subsequently oxidized to iodine. Beta emission of sodium iodide I¹³¹ destroys thyroid tissue.

Pharmacokinetics/Pharmacodynamics

Absorption

Oral: Rapidly (90% within 60 minutes)

Distribution

Extracellular space; primarily trapped by the thyroid

Metabolism

Iodide is rapidly oxidized to iodine in the thyroid

Excretion

Urine (37% to 75%); feces ~10%

Protein Binding

None

Use: Labeled Indications

Diagnostic agent: Diagnostic use in performance of radioactive iodide (RAI) uptake test to evaluate thyroid function and for thyroid imaging. Note: Alternative agents (eg, sodium iodide I¹²³, technetium-99m pertechnetate) are recommended for routine thyroid scintigraphy due to increased thyroid radiation exposure with sodium iodide I¹³¹ (ACR-SPR 2014)

Therapeutic agent: Treatment of hyperthyroidism and select cases of thyroid carcinomas (if the lesions take up iodide); palliative effects may be observed in patients with advanced thyroid malignancy if the metastatic lesions take up iodide

Use: Off Label

Whole body imaging for thyroid cancer metastasesyes

According to the American College of Radiology-Society for Pediatric Radiology practice parameter for the performance of scintigraphy and uptake measurements for benign and malignant thyroid disease, sodium iodide I131 is recommended for diagnostic whole-body imaging for thyroid cancer ACR-SPR 2014.

Contraindications

Preexisting vomiting and diarrhea (treatment); concurrent antithyroid medication (discontinue 3 to 4 days prior to administration); women who are or may become pregnant; breastfeeding; treatment of thyroid malignancies shown to have no iodide uptake, which include the majority of medullary and anaplastic carcinomas

Dosage and Administration

Dosing: Adult

Note: If dose or dose within range exceeds 1,000 microcuries, dose will be expressed as millicuries. Adhere to radiation safety precautions regarding radioactive iodine treatment. Discontinue antithyroid medications 3 to 4 days prior to administration of sodium iodide I¹³¹.

Diagnostic procedures: Oral (based on a 70 kg patient): Note: Consult manufacturer potency tables when applicable. All doses should be individualized; general ranges are listed here:

Thyroid uptake: 5 to 30 microcuries (or 0.185 to 1.1 megabecquerels) 24 hours prior to uptake measurement; alternatively, a maximum dose of 10 microcuries (0.37 megabecquerels) has been recommended (ACR-SPR 2014)

Dosage adjustment if used in conjunction with technetium Tc 99m sodium pertechnetate: 4 to 5 microcuries (0.15 to 0.185 megabecquerels) (ACR-SPR 2014).

Thyroid imaging: 50 to 100 microcuries (or 1.85 to 3.7 megabecquerels) 16 to 24 hours prior to imaging. Note: Alternative agents (eg, sodium iodide I¹²³, technetium-99m pertechnetate) are recommended for routine thyroid scintigraphy due to increased thyroid radiation exposure with sodium iodide I¹³¹ (ACR-SPR 2014)

Whole-body imaging for thyroid cancer metastases (off-label): 1 to 5 millicuries (37 to 185 megabecquerels) 24 to 72 hours prior to imaging (ACR-SPR 2014).

Treatment: Individual dose based on thyroid uptake and gland size. Oral:

Hyperthyroidism: Manufacturer's labeling recommends 4 to 10 millicuries (or 148 to 370 megabecquerels); however, due to high failure rates, RAI therapy with low activities is generally not recommended (ATA [Ross 2016])

Graves disease (off-label dosing): 10 to 15 millicuries (or 370 to 555 megabecquerels) or 150 microcuries/g of tissue corrected for 24-hour radioactive iodine uptake (see dose equation below) (ATA [Ross 2016]).

Note: Radioactive iodine (RAI) therapy is not recommended in patients with active and moderate to severe or sight-threatening Graves orbitopathy. RAI therapy may be considered in patients with active and mild disease; however, if risk factors for worsening of orbitopathy are present (eg, smoking, high thyrotropin receptor antibodies), concomitant use of glucocorticoids is strongly recommended. In the absence of risk factors, concomitant glucocorticoid use may still be considered (ATA [Ross 2016]).

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 (ATA [Ross 2016])

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 (see dose equation below); if hyperthyroidism persists, may repeat after 6 months if needed (ATA [Ross 2016])

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 (ATA [Ross 2016])

Thyroid cancer:

Ablation of normal thyroid tissue: Initial: 30 to 100 millicuries (or 1,100 to 3,700 megabecquerels). Note: If remnant ablation is performed after total thyroidectomy for American Thyroid Association (ATA) low-risk thyroid cancer or intermediate-risk disease with lower risk features, a low administered activity of ~30 millicuries is generally favored over higher administered activities. For initial adjuvant therapy when microscopic residual disease is suspected (in the absence of known distant metastases), up to 150 millicuries has been recommended (ATA [Haugen 2015]).

Subsequent metastases ablation: 100 to 200 millicuries (or 3,700 to 7,400 megabecquerels)

Dosing: Geriatric

Refer to adult dosing.

Empiric dosing for subsequent pulmonary micrometastases ablation for patients ≥70 years: 100 to 150 millicuries (ATA [Haugen 2015])

Dosing: Pediatric

Graves disease (off-label use): Children ≥5 years of age: Oral: Dose is dependent upon gland size; >150 microcuries/g of thyroid tissue is necessary to induce hypothyroidism; if gland is between 50 to 80 g, 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 (ATA [Ross 2016])

Reconstitution

Radiopharmaceutical; use appropriate precautions for handling and disposal. Wear waterproof gloves for preparation, handling, and administration. Maintain adequate shielding during the radiation-emitting life. Measure the appropriate dose using a suitable radioactivity calibration system immediately prior to administration.

Hicon: Using a shielded syringe, withdraw the required volume and transfer appropriate amount to shielded empty vial. Dilute with purified water containing sodium thiosulfate 0.2% (as reducing agent). For solution, provide dose in shielded container. For capsules, place unopened small capsule (contains dibasic sodium phosphate as absorbing buffer) into bottom half of opened large (empty) capsule; inject appropriate volume of diluted sodium iodide I131 into the center of the unopened small capsule, cover and seal with upper half of large capsule. Refer to manufacturer's labeling for additional details.

Administration

Ensure adequate hydration before and after treatment; instruct patients to void frequently to enhance elimination of radioiodide not absorbed by thyroid. Adhere to radiation safety precautions regarding radioactive iodine treatment.

Hicon: Administer on an empty stomach; patient should fast at least 2 hours before and 2 hours after administration to ensure adequate absorption. Solution must be diluted and prepared prior to administration; do not administer concentrated solution. Provide solution dose in a shielded container with a straw for administration.

Radiopharmaceutical; use appropriate precautions for handling and disposal. Waterproof gloves should be worn while handling and administering sodium iodide I¹³¹.

Dietary Considerations

The manufacturer recommends maintaining a low iodide diet two weeks before administration and to continue for several days during the uptake and imaging process.

In the treatment of Graves disease, it has been recommended that a special diet prior to RAI therapy is not required, but nutritional supplements containing excess iodine and seaweeds be avoided for at least 7 days; a low-iodine diet may benefit those with relatively low RAI uptake to increase the proportion of RAI trapped (ATA [Ross 2016])

Some dietary sources of iodine include cow's milk and dairy products, fish, seaweed, eggs, chocolate, and iodized salt.

Storage

Diagnostic: Store at 15°C to 30°C (59°F to 86°F). Products should be adequately shielded. Measure dose immediately prior to administration.

Therapeutic: Store at 20°C to 25°C (68°F to 77°F). Products should be adequately shielded. Measure dose immediately prior to administration.

Hicon: Prior to use, store solution at 2°C to 25°C (36°F to 77°F). Prepared capsule should be stored in suitable polypropylene container inside a lead pot; use within 7 days of preparation.

Drug Interactions

Amiodarone: May diminish the therapeutic effect of Sodium Iodide I131. Consider therapy modification

Antithyroid Agents: May diminish the therapeutic effect of Sodium Iodide I131. Management: Discontinue antithyroid therapy 3-4 days prior to sodium iodide I-131 administration. Avoid combination

Thyroid Products: May diminish the therapeutic effect of Sodium Iodide I131. Avoid combination

Adverse Reactions

Frequency not defined

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)

Warnings/Precautions

Concerns related to adverse effects:

• Hypersensitivity: Hypersensitivity reactions (including rash and hives) have been reported following sodium iodide I¹³¹ administration. Sodium iodide I¹³¹ solution may contain sodium bisulfite, a sulfite that may cause allergic-type reactions, including anaphylactic symptoms and life-threatening or less severe asthmatic episodes. The overall incidence of sulfite sensitivity in the general population is unknown and probably low. Sulfite sensitivity is seen more frequently in asthmatic than in nonasthmatic people. During pretherapy assessment, question patients about a history of hypersensitivity to sulfite.
• Infertility: Transient, dose-related testicular function impairment after sodium iodide I¹³¹ use; consider sperm banking for men who are anticipated to receive a high cumulative dose (eg, >14 GBq). Transient ovarian failure has been observed in females after sodium iodide I¹³¹ treatment.
• Radiation-induced thyroiditis: Sodium iodide I¹³¹ may cause thyroiditis (with gland enlargement) and thyroid hormone release, particularly if used in the treatment of hyperthyroidism. Thyroiditis may cause or worsen hyperthyroidism, and may lead to thyroid storm. When treating hyperthyroidism, consider pretreatment with antithyroid medication to help deplete thyroid hormone content within the gland. Discontinue antithyroid medication at least 3 days prior to sodium iodide I¹³¹ administration. Consider beta-blocker therapy prior to sodium iodide I¹³¹ administration to minimize the risk of hyperthyroidism and thyroid storm. Thyroiditis may result in gland enlargement with neck tenderness and swelling, pain with swallowing, sore throat, and cough; may require management with anti-inflammatory medications or analgesics.
• Radiation toxicities: Radiation-induced toxicities, including dose-dependent fatalities, have been reported following sodium iodide I¹³¹ therapy. Postmarketing reports have identified an increased risk for neoplasia, as well as a risk of hematopoietic suppression (transient neutropenia/thrombocytopenia; onset 3 to 5 weeks). Salivary and lacrimal gland toxicity is relatively common and may manifest as conjunctivitis, xerophthalmia, epiphora, sialadenitis, and xerostomia.
• Thyroid-stimulating hormone/thyroid enlargement: Enhanced thyroid-stimulating hormone (TSH) secretion (eg, following discontinuation of antithyroid medications, or the administration of TSH to enhance sodium iodide I¹³¹ uptake) may cause thyroid enlargement and obstructive complications of the trachea, esophagus, or blood vessels in the neck. Evaluate patients at high risk of obstructive complications prior to preparative treatments known to cause thyroid enlargement.

Disease-related concerns:

• Graves orbitopathy: Radioactive iodine therapy is not recommended in patients with active and moderate to severe or sight-threatening Graves orbitopathy. Radioiodine therapy may be considered in patients with active and mild disease; however, if patients have additional risk factors for worsening orbitopathy (eg, high thyrotropin receptor antibodies, smoking) concomitant glucocorticoid therapy is strongly recommended. In the absence of risk factors for worsening orbitopathy in patients with mild disease, glucocorticoid coverage may still be considered (ATA [Ross 2016]).
• Hyperthyroidism/thyrotoxic cardiac disease: May be aggravated by radiation thyroiditis; consider pre- and post-treatment with antithyroid agents and/or beta-blockers.
• Hypochloremia: May increase thyroid uptake of sodium iodide I¹³¹.
• Renal impairment: Sodium iodide I¹³¹ is eliminated predominantly renally; patients with renal impairment may have decreased clearance and increased radiation exposure. Sodium iodide I¹³¹ is dialyzable and hemodialysis may be used to decrease total body radiation exposure. Evaluate renal function prior to treatment. Nephrosis may increase thyroid uptake of sodium iodide I¹³¹.

Concurrent drug therapy issues:

• Drug-drug interactions: Potentially significant interactions may exist, requiring dose or frequency adjustment, additional monitoring, and/or selection of alternative therapy. Consult drug interactions database for more detailed information.
• Iodine/thyroid medications: Concomitant use of iodine, thyroid, or antithyroid medications may interfere with the uptake of radioiodide; medications should be discontinued for an appropriate time prior to dosing.

Special populations:

• Elderly: Elderly patients are more likely to have decreased renal function and comorbid conditions; may require close evaluation, dose selection, and follow-up compared with younger patients. When treating hyperthyroidism in geriatric patients at risk of developing cardiac complications, pre- and post-treatment with antithyroid drugs and/or beta-blockers may minimize the risk of excessive post-treatment hyperthyroidism due to radiation-induced thyroiditis.
• Pediatric: The thyroid gland may be more sensitive to the effects of sodium iodide I¹³¹ in pediatric patients; safety and efficacy have not been established.

Special handling:

• Radiopharmaceutical: Use appropriate precautions for handling, disposal, and minimizing exposure to patients and health care personnel. Use only under supervision of individuals with experience/training in the handling of radioactive materials approved by the applicable regulatory authority. Unwanted radiation exposure can occur from handling and administration of radiopharmaceuticals or from contaminated waste products, including urine and feces. Patients must be instructed in measures to minimize exposure of others.

Other warnings/precautions:

• Appropriate use: Patients should be adequately hydrated prior to and for at least 1 week following administration. Frequent voiding is recommended to enhance radioiodide excretion. Stimulate salivary flow (gum or sugar-free candy) to reduce radiation exposure to salivary glands. Avoid close contact with others (especially pregnant women and children) and contamination of other persons or the environment with body fluids.

Monitoring Parameters

Thyroid function tests (TSH, free T₄, total T₃) within 1 to 2 months post-treatment and at 4 to 6 week intervals for 6 months, or until hypothyroid and stable on thyroid hormone replacement (Graves disease, toxic multinodular goiter, toxic adenoma) (ATA [Ross 2016])

Pregnancy

Pregnancy Considerations

Use is contraindicated in pregnancy. Iodine-131 crosses the placenta and may cause severe and irreversible hypothyroidism in neonates. Monitor thyroid function of infants born following inadvertent in utero exposure. Elective diagnostic procedures should be delayed until after delivery (Parker 2004).

Pregnancy should be ruled out prior to therapy. Effective contraception is recommended (for females and males) during treatment and for at least 6 months following administration Transient infertility may occur in males and females (dose related).

Patient Education

• Discuss specific use of drug and side effects with patient as it relates to treatment. (HCAHPS: During this hospital stay, were you given any medicine that you had not taken before? Before giving you any new medicine, how often did hospital staff tell you what the medicine was for? How often did hospital staff describe possible side effects in a way you could understand?)
• Patient may experience nausea or vomiting. Have patient report immediately to prescriber signs of thyroid problems (change in weight without trying, anxiety, agitation, feeling very weak, hair thinning, depression, neck swelling, difficulty focusing, inability handling heat or cold, menstrual changes, tremors, or sweating), chest pain, fast heartbeat, dizziness, shortness of breath, bruising, bleeding, painful swallowing, edema, chills, sore throat, severe loss of strength and energy, dry eyes, dry mouth, mouth pain, persistent cough, neck tenderness, or neck edema (HCAHPS).
• Educate patient about signs of a significant reaction (eg, wheezing; chest tightness; fever; itching; bad cough; blue skin color; seizures; or swelling of face, lips, tongue, or throat). Note: This is not a comprehensive list of all side effects. Patient should consult prescriber for additional questions.

Intended Use and Disclaimer: Should not be printed and given to patients. This information is intended to serve as a concise initial reference for health care professionals to use when discussing medications with a patient. You must ultimately rely on your own discretion, experience, and judgment in diagnosing, treating, and advising patients.