From the Guidelines
A 3 mCi dose of I-131 represents a relatively low radiation exposure, delivering approximately 1.5 to 4.5 rem (15-45 millisieverts) of radiation to the whole body, which is roughly equivalent to 1-3 years of natural background radiation 1.
Radiation Exposure Context
To put this into perspective, the thyroid gland itself receives a higher dose because it concentrates iodine, but this level of exposure is considered safe for diagnostic purposes, with minimal risk of long-term effects 1.
- The radiation exposure from a 3 mCi dose of I-131 is significantly lower than therapeutic doses, which can range from 30-200 mCi and are used to treat thyroid conditions.
- Most of the radiation leaves the body through urine within 24-48 hours, and basic precautions such as frequent handwashing, flushing the toilet twice after use, and limiting close contact with pregnant women and young children should be followed for 24 hours after receiving the dose.
Precautions and Safety
These precautions are primarily precautionary rather than due to significant danger, as the radiation from diagnostic doses like 3 mCi is much lower than therapeutic doses of I-131 1.
- It is essential to note that the use of recombinant human TSH (rhTSH) has been approved for preparation for radioiodine ablation of post-surgical thyroid remnants in patients with well-differentiated thyroid carcinoma without evidence of metastatic disease, using a fixed dose of 3700 MBq (100 mCi) of 131I 1.
- However, a recent randomized prospective study has shown that a lower dose of 1850 MBq (50 mCi) of 131I is equally effective as 3700 MBq (100 mCi), even in the presence of lymph node metastases, and further reduces radiation exposure to the whole body 1.
From the Research
Radiation Exposure from 3 mCi I131
- The radiation exposure from 3 mCi I131 can be understood by considering the absorbed dose to the thyroid gland and other tissues 2.
- The thyroidal absorbed dose is typically approximately 1,500 rad/mCi of I131 administered to a euthyroid adult 2.
- For a 3 mCi dose, the thyroidal absorbed dose would be approximately 4,500 rad.
- Extrathyroidal absorbed doses range from a minimum of 0.15 to 0.5 rad/mCi for breast and gonads to a maximum of 1.5 to 2 rad/mCi for stomach and salivary glands 2.
- The absorbed doses of the urinary bladder wall decrease with increasing thyroid uptake, from 2 to 0.6 rad/mCi 2.
Comparison to Other Radiation Exposures
- For comparison, the effective dose from a chest X-ray is typically around 0.1 mSv, which is equivalent to 0.01 rem 3.
- The total thyroid and mean extrathyroidal absorbed doses to adult family members from immediately released I131-treated patients are approximately 0.01 and 0.02 rad/mCi administered, respectively 2.
- A maximum permissible effective dose of 0.5 rem for adults is consistent with a release criterion of 30 mCi of retained I131 2.
Health Effects of Radiation Exposure
- High-dose radiation exposure can induce hypothyroidism, with threshold doses of ∼10 Gy after external beam radiotherapy and of ∼50 Gy after radioiodine therapy 4.
- Children are more sensitive to radiation-induced hypothyroidism than adults, with subclinical hypothyroidism detected in 3%-6% of cases with significant correlation to thyroid absorbed doses above 2.5 Gy 4.
- Iodine thyroid blocking (ITB) is the method of choice to keep thyroid absorbed doses low in case of nuclear emergencies, with a protective effect of 90% after 1 day, 80% after 2 days, and 50% or less after 3 days 4.