Final answer:
When the extrathyroidal iodine pool is increased, the percent radioiodine uptake by the thyroid generally decreases as the thyroid does not need to concentrate as much iodine from the bloodstream.
Step-by-step explanation:
If the extrathyroidal iodine pool is increased, the percent radioiodine uptake by the thyroid will generally decrease. This is because the thyroid gland has a mechanism to concentrate iodine from the bloodstream into its follicular cells, facilitating the synthesis of thyroid hormones T3 and T4. When there is an abundance of iodine available outside the thyroid, as seen with a larger extrathyroidal iodine pool, the gland doesn't need to take up as much radioiodine. The concentration of iodide ions "trapped" in the follicular cells is typically many times higher than the concentration in the bloodstream, a process vital for the synthesis of these hormones.
Moreover, thyroid functioning alters iodine uptake. In conditions such as hyperthyroidism, where the thyroid is overactive, there would be a higher uptake of iodine than in a healthy thyroid. Conversely, an underactive thyroid, or hypothyroidism, results in lower iodine uptake. Factors such as urinary elimination, which varies with iodine intake and metabolic factors like exercise, also affect the amount of iodine in circulation and its uptake. For instance, higher urinary elimination rates could indicate lower iodine retention and vice versa.
In summary, the activity of the thyroid gland and the size of the extrathyroidal iodine pool interact to determine radioiodine uptake levels. An increased iodine pool outside the thyroid results in less need for the gland to absorb iodine, thus lowering the percent of radioiodine uptake.