I-131 is given as an injection under the skin at the back of the neck or it can be taken as a capsule by administration. It is absorbed into the bloodstream and then gets taken up by thyroid tissue. It becomes concentrated in the thyroid, and the beta-radiation it emits kills the thyroid cells. This reduces the size of the gland and its ability to produce thyroid hormone. Cats treated with I-131 need to be hospitalized for 14 days following the injection or administration. This is when they are the most radioactive. Their level of radioactivity will be checked daily with a Geiger counter and only when it reaches acceptable low levels can they go home.