Guidelines for Thyroid Dose Calculation in Radiation Exposure
For radiation exposure involving radioiodines, potassium iodide should be administered as soon as possible (within 6 hours) with specific dosing based on age and weight to protect the thyroid gland from radiation-induced cancer. 1
Thyroid Protection in Radioiodine Exposure
When to Use Potassium Iodide
- Potassium iodide (KI) is specifically indicated for exposure to radioiodines (131I, 125I)
- KI is NOT a generic antiradiation drug and should NOT be used if radioiodines are not part of the exposure 1
- KI should be administered as soon as possible after exposure (≤6 hours) to be most effective 1
- When administered within 4 hours of exposure, KI reduces radioiodine uptake by approximately 50% 1
Dosing Guidelines for Potassium Iodide
The FDA-approved dosing for potassium iodide in radiation emergencies is as follows 2:
- Adults over 18 years: 130 mg (2 mL) daily
- Children 12-18 years weighing ≥150 pounds: 130 mg (2 mL) daily
- Children 12-18 years weighing <150 pounds: 65 mg (1 mL) daily
- Children 3-12 years: 65 mg (1 mL) daily
- Infants 1 month to 3 years: 32.5 mg (0.5 mL) daily
- Newborns to 1 month: 16.25 mg (0.25 mL) daily
Administration Protocol
- Take KI only when directed by public officials during a radiation emergency
- Take only one dose every 24 hours; additional doses will not provide further protection and may increase side effects 2
- Continue KI until the risk of exposure to radioactive iodine through breathing or ingestion stops 2
- For pregnant women in the second and third trimesters, KI is critical to protect both maternal and fetal thyroid glands 1
Thyroid Dose Calculation in Occupational Exposure
Effective Dose Calculation for Medical Personnel
For healthcare workers exposed to radiation, the National Council on Radiation Protection and Measurements (NCRP) recommends the following formulas to estimate effective dose (E) 1:
Single Badge Method
When wearing a single radiation badge at the collar level:
- E(estimate) = H(col) / 21 (when wearing a thyroid collar)
- E(estimate) = H(col) / 14 (when not wearing a thyroid collar)
Where H(col) is the collar badge reading in mSv.
Dual Badge Method
When wearing two radiation badges (one under the apron at waist level and one over the apron at collar level):
- E(estimate) = 0.5 H(u) + 0.025 H(col)
Where H(u) is the under-apron badge reading and H(col) is the collar badge reading, both in mSv.
Special Considerations
Pregnant Women
- Any pregnant woman exposed to >0.25 Gy of radiation should have fetal dose determined 1
- Fetal thyroid dose may be higher than maternal dose in radioiodine exposure due to increased iodine avidity 1
- Pregnant women in first trimester with near-field exposure should receive KI to protect the mother 1
- Pregnant women in second and third trimesters should receive KI for both near and far-field exposures to protect both maternal and fetal thyroid glands 1
Children and Adolescents
- The thyroid gland in children is particularly radiosensitive and at higher risk for malignant transformation 1
- Thyroid cancer resulting from radiation exposure may be more aggressive than de novo forms 1
- Children require appropriate weight-based dosing of KI 2
Pitfalls and Caveats
Inappropriate KI Use: KI is not effective against other forms of radiation and should only be used when radioiodines are present. For example, KI provides no benefit in "dirty bomb" scenarios (which typically don't contain radioiodines due to their short 8.5-day half-life) and may cause unnecessary side effects including anaphylaxis 1
Timing of Administration: KI effectiveness decreases significantly if not administered promptly after exposure. Maximum protection occurs when given before or within 4 hours of exposure 1
Allergic Reactions: Caution must be exercised in individuals with known iodine allergies as severe allergic reactions have been reported 1
Regulatory Compliance: Regulatory agencies may adopt different rules for calculation of effective dose that differ from NCRP recommendations. Always verify local regulatory requirements 1
Badge Placement: Incorrect placement of radiation badges can lead to inaccurate dose estimates. The collar badge should be worn on the left side at collar level, and the under-apron badge at waist level 1