Final answer:
After approximately one week (which is close to one half-life of I-131), about 50% of the initial dose of radioactive iodide will remain in the thyroid from the thyroid scan, as I-131 has a half-life of roughly 8 days.
Step-by-step explanation:
The amount of iodide remaining in a thyroid scan using radioactive iodide (I-131) after one week can be calculated using the concept of half-life, which for I-131 is approximately 8 days. After one week, which is almost a full half-life, roughly half of the initial amount of I-131 would have decayed. Due to this decay, only about 50% of the radioactive iodine initially used in the scan would remain in the thyroid. While small doses of I-131 are used for imaging the thyroid, the isotope is also used for treatment, where it is absorbed by the thyroid gland to produce necessary hormones or to destroy excess thyroid tissue. Since I-131 is also mixed with stable iodine, its distribution and chemistry in the body are maintained.