I-131 Radioiodine therapy - 150 mCi

I-131 Radioiodine Therapy is a targeted nuclear medicine treatment primarily used in the management of differentiated thyroid cancer, such as papillary and follicular carcinoma, especially after surgical removal of the thyroid gland (thyroidectomy). The thyroid gland has a natural tendency to absorb iodine for the production of thyroid hormones. Leveraging this physiological function, radioactive iodine (I-131) is selectively absorbed by any remaining thyroid cells, including normal remnant tissue and potentially malignant or metastatic thyroid cancer cells. The standard therapeutic dose varies depending on the clinical objective—ablation of remnant tissue, adjuvant treatment for possible microscopic disease, or treatment of known metastases. A dose of 150 mCi (millicurie) is typically recommended for ablation or treatment of intermediate-risk thyroid cancer, or in cases of suspected residual or minimal metastatic disease. This therapeutic intervention is designed to reduce recurrence and assist in long-term disease monitoring and control.

How I-131 Works

Radioiodine therapy uses Iodine-131, a radioactive isotope that emits beta radiation, which destroys thyroid tissue and cancer cells by causing DNA damage. When administered orally in capsule or liquid form, I-131 enters the bloodstream and is actively taken up by thyroid cells through the sodium-iodide symporter.  Because healthy tissues in the body do not absorb iodine in significant quantities, I-131 provides a highly selective and localized form of radiation therapy, minimizing exposure to other organs.

Indications for I-131 Radioiodine Therapy – 150 mCi

This therapeutic dose of I-131 is commonly indicated in the following clinical scenarios:

• Post-thyroidectomy ablation of remnant thyroid tissue
• Treatment of regional lymph node involvement
• Minimal distant metastasis (e.g., lungs)
• Intermediate-risk thyroid carcinoma
• Elevated thyroglobulin levels with no evident disease on imaging
• Adjuvant therapy to eliminate microscopic residual disease

By eliminating any residual functioning thyroid tissue, the therapy not only lowers recurrence risk but also improves the effectiveness of future thyroglobulin monitoring and whole-body radioiodine scans.

Preparation for I-131 Radioiodine Therapy

Before undergoing radioiodine therapy, certain preparations are essential to ensure the treatment is effective:

1. Elevation of Thyroid Stimulating Hormone (TSH) Levels

To enhance uptake of radioactive iodine by thyroid cells, the TSH level must be elevated above 30 mIU/L. This can be achieved in two ways:

• Thyroid Hormone Withdrawal
  • Discontinuation of thyroid hormone replacement (e.g., Thyronorm or Eltroxin) for 3–6 weeks
  • Induces hypothyroidism, which naturally increases TSH
  • May cause temporary symptoms: fatigue, depression, cold intolerance, constipation, and weight gain
• Recombinant Human TSH (rhTSH or Thyrogen®) Injection
  • Synthetic TSH injection administered over two consecutive days
  • Allows patients to continue their thyroid hormone therapy
  • Avoids hypothyroid symptoms
  • May have limited accessibility or be costlier

Both approaches are effective; the choice depends on clinical judgment, patient preferences, and resource availability.

2. Low-Iodine Diet

• Patients are advised to follow a low-iodine diet for 1–2 weeks prior to the therapy
• This enhances I-131 uptake by depleting the body's iodine reserves
• Avoid iodized salt, dairy products, seafood, eggs, soy products, and certain food colorings

3. Baseline Tests and Imaging

Before therapy, your physician may request:

• Serum thyroglobulin and anti-thyroglobulin antibody levels
• TSH and Free T4 levels
• Neck ultrasound
• Whole-body iodine scan (pre-therapy diagnostic scan, if required)

These assessments help evaluate the extent of disease and establish a baseline for post-therapy monitoring.

Procedure: What to Expect on the Day of Therapy

• The I-131 dose (150 mCi) is administered orally, either as a capsule or liquid
• Patients are required to fast for 2 hours before and after the ingestion
• Minimal contact with family members, especially children and pregnant women, is recommended for 3–7 days following therapy due to radiation safety

Post-Therapy Instructions and Isolation Guidelines

Since I-131 is excreted through urine, sweat, saliva, and feces, patients are advised to follow radiation safety precautions:

• Maintain distance from others, especially vulnerable individuals
• Use separate bathrooms if possible
• Wash hands frequently and flush toilets twice
• Sleep in a separate room for a few days
• Avoid public places or workplaces temporarily
• Avoid conception (pregnancy) for at least 6 months post-therapy

These measures help protect others from unnecessary radiation exposure.

Possible Side Effects

Though generally well-tolerated, some patients may experience:

• Neck pain or swelling due to inflammation of remnant tissue
• Dry mouth or altered taste (from salivary gland involvement)
• Nausea or fatigue for a few days
• Temporary changes in menstrual cycles
• Rarely, low blood cell counts or infertility with repeated high doses

Most of these effects are transient and resolve without medical intervention.

Follow-Up After Therapy

About 5–7 days after therapy, a post-therapy whole-body scan may be performed to assess:

• Extent of I-131 uptake in residual thyroid tissue or metastasis
• Efficacy of ablation or therapy
• Guidance for further monitoring or treatment planning

Long-term follow-up includes:

• Periodic blood tests (TSH, thyroglobulin, anti-Tg antibodies)
• Repeat imaging as indicated
• Adjustments in thyroid hormone dosage for optimal suppression of TSH (to reduce cancer recurrence)

Conclusion

I-131 Radioiodine Therapy – 150 mCi plays a critical role in the comprehensive management of differentiated thyroid cancers, particularly after thyroidectomy. It allows for selective destruction of residual thyroid or cancerous tissues, reducing recurrence and enabling more effective long-term surveillance. This therapy is tailored based on individual patient risk factors, staging, and thyroglobulin levels. The 150 mCi dose is considered optimal for intermediate-risk patients or those with suspected minimal residual disease. With appropriate preparation, radiation safety protocols, and follow-up care, I-131 therapy is a safe and effective modality offering long-term disease control and peace of mind to thyroid cancer patients.