Radioactive Iodine

What is Radioactive Iodine (RAI)?

RAI (I131) is a radioactive form of the more common iodine that can be found in foods (especially shellfish). Thyroid cells take up iodine much more than any other cell in the body in order to make thyroid hormone. When RAI is taken up by the thyroid, the radioactive iodine destroys the thyroid cells.

What are the reasons to use RAI?

The three main reasons to use RAI are:

  1. To destroy or ablate hyperactive thyroid cells in hyperthyroidism (such as Graves’ disease)
  2. To destroy or ablate remaining normal thyroid tissue and thyroid cancer cells after a total thyroidectomy
  3. To destroy or ablate thyroid cancer recurrences or metastases

In cases of differentiated thyroid cancer, RAI may reduce the chance that thyroid cancer will come back (i.e. recur) for patients with more aggressive disease, but has not been shown to decrease overall survival for the majority of patient.

In cases of papillary thyroid cancer, RAI is presently recommended after total thyroidectomy for high-risk and some intermediate risk tumors (based on the American Thyroid Association risk stratification system). This includes tumors with aggressive features (extrathyroidal extension, disease in lateral lymph nodes, large burden of central node disease, aggressive pathology such as tall-cell variant, BRAF+/TERT+ combination, distant metastases, etc…) RAI is usually not recommended for well circumscribed cancers <4cm that has no known or microscopic only lymph nodes involvement. For some controversial cases, the use of RAI will be determined in discussion with your treating physicians.

RAI is recommended for follicular or Hurthle cell cancers after total thyroidectomy (but not “minimally invasive follicular cancer”).

RAI is NOT recommended for medullary cancer.

When and how is RAI performed?

RAI is usually given 2-6 months after surgery, even up to one year post-op. Recent studies have shown that the timing of RAI does not affect outcomes. There is no longer a need for thyroid hormone withdrawl, as patients can be given Thyrogen (recombinant TSH, see below) to stimulate their thyroid in preparation for RAI. RAI is given by mouth as a pill, with the dose determined by your physicians and will depend on your disease burden and the goal of RAI (ablation or treatment of gross disease). One week after receiving the dose, a whole body scan will be performed to show where the iodine collects in the body. Uptake is normally seen in the salivary glands, gastrointestinal tract, and urinary bladder. Normal uptake may also be seen in the liver. Seeing these areas light up on scan does NOT mean that thyroid cancer has spread to these organs.

Often, uptake is seen in the mid portion of the neck where residual thyroid tissue (i.e. the remnant) is found. (Figure 4) Uptake in the sides of the neck may indicate that cancer has spread to lymph nodes. (Figure 5) Uptake in the upper-middle part of the chest (i.e. mediastinum) may be normal uptake in the esophagus or thymus or may indicate spread of thyroid cancer to the lymph nodes in the chest. (Figure 6) Uptake in the lungs or bones may indicate spread of thyroid cancer to these areas as well. (Figure 7)

Will I require multiple treatments?

The initial treatment after surgery is a single dose. You may receive additional doses of RAI in the future if you are found to have persistent or recurrent disease, though surgical resection is the first line of treatment for recurrent disease, except for distant metastases.

How do I prepare for RAI?

In order for RAI to work, residual thyroid tissue must be stimulated to take up as much iodine as possible. This is accomplished by increasing the TSH level which tells the thyroid take up iodine to make thyroid hormone. Though there are two main ways to stimulate thyroid cells that are equally effective, recombinant TSH is more ‘patient friendly’, better tolerated, and now readily available as the standard of care:

Thyroid hormone withdrawal

By purposely not taking thyroid hormone replacement after total thyroidectomy for 4 to 6 weeks, the pituitary will naturally respond by increasing the TSH level. It does not “know” that there is no thyroid to make thyroid hormone and keeps trying to stimulate it more and more. In short, the patient is hypothyroid. Towards the end of the withdrawal phase, patients will be very hypothyroid and may feel very tired, fatigued, unmotivated, etc. Many physicians will put patients on the shorter acting thyroid hormone (T3, Cytomel) for 2 to 4 weeks followed by no hormone for 2 weeks in order to reduce the amount of time the patient is symptomatic. The TSH level is then checked to ensure adequate stimulation before RAI treatment. A TSH >30 is recommended before RAI. Thyroid hormone is restarted 2 to 3 days after taking the dose of RAI.

Recombinant TSH

Injections of recombinant TSH (or Thyrogen stimulation) involves giving a man-made form of TSH to stimulate the thyroid. This method allows patients to avoid the symptoms of hypothyroidism seen with thyroid hormone withdrawal therapy. Typically two doses of Thyrogen are given on two consecutive days followed by the RAI dose on the third day.

To maximize iodine uptake, a low iodine diet is recommended for 1-2 weeks prior to RAI. This is very important. Information about low iodine diets may be obtained on the Thyroid Cancer Survivors Association website (www.thyca.org).

Do I have to be in the hospital to be treated with RAI?

Generally, treatment can be performed on an outpatient basis provided the patient can stick to certain radiation precautions (see below), though laws can vary by state.

What radiation precautions are required?

In the first week after treatment, the RAI comes out of the body in the urine, saliva, and sweat. It also comes out in breast milk, so breast-feeding after RAI is not recommended.

Typical recommendations for the first 3 to 5 days after RAI include:

  • Do not share a toilet/bathroom. Sit down with urination and flush twice with the lid down.
  • No exchange of saliva through kissing or food sharing.
  • No prolonged close contact (<3 feet) with others, particularly pregnant women and children.
  • Sleep alone.
  • Refrain from sexual activity.
  • Wash clothes, towels, and linens separately.
  • Drink plenty of fluids, suck sour candy and chew gum to clear the unused iodine from your body.
  • Shower/bathe and wash hands frequently.

What are the potential side effects of RAI?

Mild nausea, salivary gland swelling, dry eyes and mild pain may occur within the first 24 hours after RAI. Some patients also complain of dry mouth, or change/loss of taste. RAI will NOT cause hair loss.

Does RAI cause cancer?

There is a small risk of developing a second, different cancer (i.e. other than thyroid) after higher doses of RAI. This risk increases with cumulative doses of RAI greater than 400 to 500 mCi and has not been seen after small, ablative doses (ie. <30-50 mci). One benefit of Thyrogen stimulated RAI is that it decreases the radiation dose delivered to unwanted areas of the body, compared to RAI after thyroid hormone withdrawal.

Can I get pregnant after RAI?

Yes. There is no increased risk of infertility or birth defects after RAI. The typical recommendation is to wait 6-12 months after RAI before getting pregnant. There may, however, be an increased risk of miscarriage or menstrual irregularities during that 6-12 month period.