I-131 Radioiodine therapy - 100 mCi

I-131 Radioiodine Therapy – 100 mCi is a nuclear medicine treatment used for the management of differentiated thyroid cancers, such as papillary and follicular carcinoma, especially after complete or near-total thyroidectomy. This therapy utilizes the thyroid gland’s natural affinity for iodine to deliver targeted radiation to any remaining thyroid tissue or microscopic cancerous cells.  After surgical removal of the thyroid, small remnants of thyroid tissue or possible cancerous cells may still remain in the neck or, in some cases, elsewhere in the body. Administering radioactive iodine (I-131) allows these residual cells to be selectively destroyed, a process known as radioiodine ablation. The 100 mCi dose is typically used for low to intermediate-risk patients where limited residual thyroid tissue is suspected, or to ablate remaining thyroid tissue for effective long-term monitoring.

What is I-131 and How Does it Work?

Iodine-131 (I-131) is a radioactive form of iodine that emits beta radiation, which is capable of destroying thyroid cells. Since the thyroid naturally absorbs iodine to produce thyroid hormones, I-131 gets absorbed in any residual thyroid tissue or thyroid cancer cells that express iodine-absorbing capabilities. Once administered, I-131 circulates through the bloodstream and is taken up by these cells. The beta emissions from I-131 cause localized cell death, thereby reducing the risk of recurrence and allowing for more accurate monitoring of thyroid cancer markers like thyroglobulin in the future.

Why is 100 mCi I-131 Therapy Given?

The dose of 100 mCi of radioactive iodine is commonly used in the following clinical indications:

• Post-surgical ablation of thyroid remnants in low to intermediate-risk patients
• Adjuvant therapy in patients with small-volume microscopic disease
• Early-stage thyroid cancer with no distant metastasis
• Thyroglobulin-positive but imaging-negative patients
• Preparation for future surveillance scans or thyroglobulin monitoring

This dose provides a balance between efficacy and safety, ensuring sufficient ablation while minimizing radiation exposure to the rest of the body.

Preparing for I-131 Radioiodine Therapy

Successful I-131 therapy relies on adequate preparation, particularly ensuring that thyroid cells are primed to absorb iodine. This is achieved through the elevation of Thyroid Stimulating Hormone (TSH) levels and following a low-iodine diet.

1. Elevating TSH Levels

TSH stimulates thyroid tissue to absorb iodine. For effective radioiodine uptake, TSH levels should be elevated (>30 mIU/L). This can be done in two ways:

• Thyroid Hormone Withdrawal
  • Stop taking thyroid hormone (e.g., Thyronorm) for 3–6 weeks
  • Allows the body to naturally elevate TSH due to hypothyroidism
  • May cause symptoms like fatigue, depression, cold intolerance, and weight gain
• Recombinant Human TSH (Thyrogen®)
  • Administered as intramuscular injections over two days
  • Allows continuation of thyroid hormone therapy
  • Avoids hypothyroid symptoms
  • Preferred in patients with comorbidities or quality-of-life concerns

Both methods are clinically effective; the decision is personalized based on medical history and patient preference.

2. Low-Iodine Diet

To increase the body’s uptake of radioactive iodine, patients are advised to follow a low-iodine diet for 1–2 weeks prior to therapy. The diet excludes:

• Iodized salt and sea salt
• Dairy products
• Seafood and seaweed
• Egg yolks
• Soy products
• Certain multivitamins and baked goods with iodate

This preparation helps "starve" the body of iodine, making any remaining thyroid tissue more eager to absorb the radioactive iodine.

Procedure of I-131 Radioiodine Therapy – 100 mCi

• The I-131 dose is administered orally, usually as a capsule (or occasionally a liquid)
• Fasting is required for 2 hours before and after swallowing the capsule
• The patient may be asked to remain isolated for 2–4 days, depending on local radiation safety guidelines
• No anesthesia or hospitalization is typically required unless there are special medical conditions

Following the procedure, the patient begins to emit low levels of radiation, and radiation safety precautions must be followed.

Post-Therapy Guidelines and Radiation Safety

Because I-131 is excreted in urine, sweat, and saliva, patients are advised to follow specific precautions to limit exposure to others:

• Maintain a safe distance from children and pregnant women for several days
• Use separate bathroom facilities if possible
• Flush toilets twice and wash hands thoroughly
• Sleep in a separate room for 3–5 days
• Avoid sharing utensils or personal items
• Stay home from work or public places as advised by your physician

These safety measures are temporary and usually apply for up to one week, depending on how quickly the body clears the radiation.

Potential Side Effects of 100 mCi I-131 Therapy

I-131 is generally safe, especially in this dose range, but some side effects may include:

• Temporary neck discomfort or swelling
• Mild nausea on the day of therapy
• Dry mouth or altered taste (rare at this dose)
• Temporary changes in menstrual cycles
• Fatigue or mild fever in some cases

Most of these effects are short-lived and resolve without intervention. Drinking plenty of fluids and sucking on sour candies may help reduce salivary gland discomfort and aid in clearing radiation.

Post-Therapy Imaging and Monitoring

After I-131 therapy, a post-therapy whole-body scan may be scheduled within 5–7 days to check:

• Whether the residual thyroid tissue has absorbed the iodine
• Presence of any unexpected metastasis
• Distribution of I-131 in the body

Regular follow-up visits with your endocrinologist will include:

• Thyroglobulin blood levels
• TSH and Free T4
• Ultrasound of the neck
• Periodic whole-body scans (as needed)

The primary goal of post-treatment monitoring is to assess response to therapy and detect any recurrence early.

Conclusion

I-131 Radioiodine Therapy – 100 mCi is a vital component in the post-operative management of differentiated thyroid cancer. It is a targeted, effective, and safe way to destroy residual thyroid tissue and reduce the risk of recurrence. This dosage is commonly recommended for patients with early-stage disease or minimal residual tissue, balancing efficacy with patient safety. With proper preparation—including TSH elevation and a low-iodine diet—this therapy provides a crucial step in ensuring long-term cancer control and accurate future surveillance through thyroglobulin testing and imaging. When guided and monitored by experienced medical professionals, patients can expect high rates of success with minimal discomfort. If you are advised to undergo I-131 therapy at 100 mCi, understanding the process and adhering to preparatory steps and safety guidelines will ensure the best possible outcome.