Whats the difference in Radioactive iodine uptake and radioactive iodine in Graves Disease?

I am doing a project on Graves disease and can not tell the difference bettween the two. If radioactive iodine uptake measures how much radioactive iodine is taken up by the thyroid gland then how does doses of radioactive iodine destroy cells in the thyroid gland. Also doesn't the gland need iodine to make thyroid hormones and if absorbing large amounts of iodine its producing to much hormone which causing the hyperthyroidism. So how can you be tested and treated with radioactive iodine,