Final answer:
In comparison with I-131, I-123 is a radioactive iodine isotope with a shorter half-life of 13.2 hours and emits less penetrating radiation, making it useful for diagnostic purposes where minimal exposure is wanted or quick clearance from the body is needed.
Step-by-step explanation:
When performing a radioactive iodine uptake, it should be remembered that in comparison with I-131, I-123 has a shorter half-life and emits less penetrating radiation. I-131 has a half-life of approximately 8.04 days, while I-123 has a much shorter half-life of 13.2 hours. Hence, the radioactive decay of I-123 is much quicker which leads to a more rapid decrease in radioactivity.
The activities of I-131 and I-123 used in thyroid scans are given as 50 and 70 µCi, respectively. To compare the masses of I-131 and I-123 in such scans, one must take into account that although they have different activities, the mass of radioactive material will also depend on the half-life and specific activity of the isotope. However, typically the dosages used are so small that these isotopes are mixed with stable iodine to ensure normal chemistry and distribution in the body.
I-131 may be preferred in certain therapeutic instances due to its longer half-life, which allows it to deliver a higher cumulative radiation dose. In contrast, I-123's shorter half-life might be advantageous for diagnostic purposes where minimal radiation exposure is desired or when rapid clearance from the body is beneficial.