This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
[{'content': 'Which of the following radio of Iodine is used for thyroid scan: September 2008 \nSelect and explain. \nA. 1-123, B. 1-125, C. 1-127, D. 1-131', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. A: 1-123 The radionuclides that are used in thyroid scans are two isotopes of iodine, 1-131 and 1-123, and an isotope of technetium known as 99m Tc. Technetium scanning is preferred for some diagnostic workups because it is relatively fast and does not require the patient to fast beforehand. Many prefer to reserve 1-131 for follow-up evaluations of cancer patients, and use 1-123 for thyroid uptake tests and routine thyroid scans. The reason for the distinction is the higher radiation burden of 1-131.', 'role': 'assistant'}]