Answer to Question #7303 Submitted to "Ask the Experts"

Category: Medical and Dental Patient Issues — Diagnostic X Ray and CT

The following question was answered by an expert in the appropriate field:

Q

I would like to know the dose limit to the thyroid gland. I know the thyroid is a radiosensitive organ. What is the limit for the dose that a thyroid gland can receive during any diagnostic examinations (e.g., CT)?
 

A

Thank you for your question. Actually, the adult thyroid is very insensitive to radiation exposure. It appears, in fact, that the risk of thyroid cancer in someone over the age of 20 exposed to radiation is either zero or too small to measure (NAS 2006).

There is no limit on the amount of radiation exposure for a patient undergoing medical diagnostic exams. This is because the patient and his/her physician should be assuring the exams are justified and beneficial to the patient.

For someone occupationally exposed (like an x-ray technologist), the annual limit to the thyroid is 50 rem (rem is a unit of effective radiation dose). For occupationally exposed individuals who might receive higher thyroid doses, we try to limit their exposure by the use of leaded thyroid collars.

Kelly Classic
Certified Medical Health Physicist

Reference
National Academy of Sciences (NAS). Health effects from exposure to low-level ionizing radiation BEIR VII Phase 2. Washington, DC: National Academies Press. 181-182; 2006.
 

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
Answer posted on 11 March 2008. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.