Sodium Iodide (I-131)

Usage

Sodium Iodide I-131 is a radioactive therapeutic agent primarily used for the treatment of:

• Hyperthyroidism: Including Graves’ disease and toxic nodular goiter.
• Thyroid Cancer: Including ablation of residual thyroid tissue after surgery and treatment of thyroid cancer metastases. It may offer palliative benefits even in advanced stages if the cancerous tissue retains iodine uptake capacity.

Pharmacological Classification: Radiopharmaceutical, antithyroid agent.

Mechanism of Action: Sodium Iodide I-131 emits beta particles and gamma radiation. The thyroid gland actively absorbs iodine, including radioactive iodine. The concentrated radiation within the thyroid tissue destroys thyroid cells, reducing hormone production in hyperthyroidism and destroying cancerous cells in thyroid cancer.

Alternate Names

• Iodine I-131
• Radioactive Iodine
• Sodium Iodide (I131)
• Radioiodide I-131

Brand Names:

• Hicon
• Iodotope

How It Works

Pharmacodynamics: Sodium Iodide I-131 targets the thyroid gland due to the gland’s natural affinity for iodine. After administration, the drug is actively transported into thyroid follicular cells via the sodium-iodide symporter (NIS). The subsequent beta emissions cause localized cell death within the thyroid, effectively destroying both healthy and cancerous thyroid tissue. Gamma emissions allow for imaging and monitoring of iodine uptake.

Pharmacokinetics:

• Absorption: Orally administered Sodium Iodide I-131 is readily absorbed from the gastrointestinal tract.
• Distribution: It concentrates primarily in the thyroid gland. Minor uptake can occur in salivary glands and gastric mucosa. During lactation, it is excreted in breast milk potentially exposing the infant to radiation.
• Metabolism: The drug is not metabolized but undergoes radioactive decay.
• Elimination: Primarily excreted through the kidneys in urine. A smaller amount may also be excreted through feces and sweat.

Mode of Action: The beta decay causes localized cell death and tissue damage.

Receptor Binding: Not Applicable (NIS is a transporter not a receptor)

Enzyme Inhibition: Not Applicable

Neurotransmitter Modulation: Not Applicable

Elimination Pathways: Primarily renal excretion. Small amounts may also be eliminated in feces.

Dosage

Dosage is highly individualized and depends on the indication, thyroid uptake, and gland size. Radioactivity is measured in megabecquerels (MBq) or millicuries (mCi).

Standard Dosage

Adults:

• Hyperthyroidism: 148 to 370 MBq (4 to 10 mCi), orally, as a single dose. Higher doses may be required for toxic nodular goiter.
• Thyroid Carcinoma:
  • Post-surgical Ablation: 1850 MBq (50 mCi), orally, as a single dose.
  • Metastases Treatment: 3700 to 7400 MBq (100 to 200 mCi), orally, as a single or multiple doses, depending on the individual case.

Children: Dosage should be carefully calculated based on body weight and thyroid uptake. Pediatric patients are more susceptible to the long-term effects of radiation exposure and therefore require special precautions and individualized dosing strategies.

Special Cases:

• Elderly Patients: No specific dose adjustments are recommended, however caution should be used as these patients are more susceptible to impaired kidney function which can increase radiation exposure.
• Patients with Renal Impairment: Dose adjustments may be necessary. Monitor radiation levels carefully.
• Patients with Hepatic Dysfunction: No specific dose adjustments are recommended, however caution is advised if there are other relevant health issues.
• Patients with Comorbid Conditions: Dose individualization is important to balance the benefit against radiation risks in patients with underlying health conditions.

Clinical Use Cases

Sodium Iodide I-131 is not typically used in clinical settings like intubation, surgical procedures, mechanical ventilation, or emergency situations such as status epilepticus or cardiac arrest. It is primarily used in outpatient settings for the treatment of hyperthyroidism and thyroid cancer. Its primary clinical use occurs within Nuclear Medicine departments.

Dosage Adjustments

Dose adjustments are essential based on renal function, thyroid uptake, and individual patient factors. Dosimetry studies may be necessary in complex cases.

Side Effects

Common Side Effects

• Nausea
• Vomiting
• Neck tenderness or swelling
• Dry mouth
• Metallic taste changes

Rare but Serious Side Effects

• Bone marrow suppression
• Radiation sickness
• Secondary malignancies (leukemia)
• Thyroid storm (in hyperthyroidism)
• Sialadenitis (salivary gland inflammation)

Long-Term Effects

• Hypothyroidism
• Increased risk of certain cancers
• Infertility

Adverse Drug Reactions (ADR)

• Severe allergic reactions (anaphylaxis)

Contraindications

• Pregnancy
• Breastfeeding
• Vomiting and diarrhea (impaired absorption)
• Lack of iodine uptake by the thyroid malignancy

Drug Interactions

• Antithyroid drugs: Discontinue several days before and after I-131 administration to optimize uptake.
• Thyroid hormone: Discontinue several weeks before I-131 administration to allow TSH levels to rise which optimizes uptake.
• Iodine-containing medications/contrast agents: These may interfere with iodine uptake and should be avoided prior to I-131 therapy.
• Lithium: Can increase the risk of hypothyroidism.

Pregnancy and Breastfeeding

• Pregnancy Safety Category: X (Contraindicated) - can cause congenital hypothyroidism.
• Breastfeeding: Contraindicated - I-131 is excreted in breast milk.

Drug Profile Summary

• Mechanism of Action: Destroys thyroid cells via beta radiation.
• Side Effects: Nausea, vomiting, neck pain, hypothyroidism, rarely bone marrow suppression.
• Contraindications: Pregnancy, breastfeeding, vomiting/diarrhea.
• Drug Interactions: Antithyroid drugs, thyroid hormone, iodine-containing medications.
• Pregnancy & Breastfeeding: Contraindicated.
• Dosage: Highly variable, depends on indication and individual patient factors.
• Monitoring Parameters: Thyroid function tests (TSH, T4, T3), complete blood counts, iodine uptake scans.

Popular Combinations

Sodium Iodide I-131 is typically used alone. Adjunctive therapies with antithyroid medications or thyroid hormone replacement may be used depending on the clinical scenario.

Precautions

• Careful patient selection and dosimetry are crucial.
• Verify absence of pregnancy.
• Patients should discontinue breastfeeding and use effective contraception.
• Monitor for signs of radiation toxicity.

FAQs (Frequently Asked Questions)

Q1: What is the recommended dosage for Sodium Iodide I-131?

A: The dosage is highly individualized based on factors such as the indication (hyperthyroidism or thyroid cancer), thyroid uptake, and gland size. It ranges from 148-370 MBq (4-10 mCi) for hyperthyroidism to potentially higher doses for thyroid cancer.

Q2: How is Sodium Iodide I-131 administered?

A: It is administered orally, typically as a single dose, in the form of a capsule or solution.

Q3: What are the most common side effects?

A: Common side effects include nausea, vomiting, neck tenderness or swelling, dry mouth, and metallic taste changes.

Q4: What are the serious side effects of Sodium Iodide I-131?

A: Rare but serious side effects include bone marrow suppression, radiation sickness, secondary malignancies (e.g., leukemia), thyroid storm (in hyperthyroidism patients), and sialadenitis.

Q5: Is Sodium Iodide I-131 safe during pregnancy or breastfeeding?

A: No, Sodium Iodide I-131 is contraindicated in both pregnancy and breastfeeding due to the risk of fetal and neonatal hypothyroidism.

Q6: What are the long-term effects of this treatment?

A: Long-term effects can include hypothyroidism, an increased risk of certain cancers, and infertility.

Q7: What medications interact with Sodium Iodide I-131?

A: Medications such as antithyroid drugs, thyroid hormone, and iodine-containing medications/contrast agents can interact with Sodium Iodide I-131. These should be carefully managed before and after treatment.

Q8: What monitoring is required after I-131 therapy?

A: Patients require monitoring of thyroid function (TSH, T4, T3 levels) and complete blood counts to assess for bone marrow suppression. Iodine uptake scans may also be performed.

Q9: How long does it take for Sodium Iodide I-131 to work?

A: The effects of Sodium Iodide I-131 may take several weeks or months to become fully apparent, depending on the condition being treated.

Q10: What precautions should patients take after I-131 therapy?

A: Patients should limit close contact with others, especially pregnant women and children, for a period of time determined by the radiation safety officer after their therapy. This period will depend on the dose received. Specific instructions will be tailored to each patient and include information on limiting potential exposure through bodily fluids.