Radioactive iodine finds its use in medicine both as a diagnostic and therapy tool. The thyroid gland which is located at the base of throat is a factory of harmones that help the body function normally. The thyroid harmone commands rate of production and use of energy in our body. If harmone level goes too high, it leads to over stimulation and eventually produces veriety of symptoms. It includes nervousness, restlessness, fatigue, sweaty, sleeplessness, abnormal heart rhythm, trembling hands etc. The cause i.e, exces of thyroid gland activity and thyroid harmone in the blood is termed as hyperthyroidism. The thyroid gland uses iodine to produce thyroid harmone. Hence, by monitoring and mapping uptake of iodine, it is possible to diagnose the problem with the gland.

In normal situation, thyroid harmone production is always under thyroid feedback mechanism which controles the production of the harmone. There are several causes for hyperthyroidism, in which the feedback mechanism fails, among which Graves disease and thyroid cancer are very important.

Graves disease is a type of autoimmune disease in which, the immune system produces antibodies against normal tissues. In this case, thyroid stimulating antibodies attach sites in the thyroid gland called TSH reciptors and prompt to produce thyroid harmone. These antibodies being not under the control of feed back mechanism, over stimulate the thyroid glands leading to hyperthyroidism.

In case of thyroid cancer, one or many thyroid nodules may start producing thyroid harmone independent of thyroid feedback mechanism. Such nodules are called autonomously functioning thyroid nodules. When there are several such nodules, the condition is called toxic multi-nodular goiter.

Radioactive iodine isotope viz, ¹²⁵I and ¹³¹I has been used in medicine as a routine tracer tool for diagnosis and therapy (^{127I is the only naturally occurring stable isotope). The tracer will be introduced into the patients body orally in the form of liquid or capsule. Thyroid gland takes-up the tracer. However, rate of uptake, and accumulation of the tracer in the gland depends on its condition. For example, if it is affected by Graves disease, it accumulates more iodine throughout the gland. On the other hand, if it is thyroid cancer at the initial stage, a small part of the gland may accumulate the tracer in large quantity compared to other areas and in a larger area if it is advanced stage.}

Thyroid gland when imaged using a gamma camera which detects gamma rays emitted by the tracer and reconstruct the image of the source i.e, accumulated part of the gland, gives clear image about type of disease and stage of advancement. For example, in case of Graves disease, the whole gland takes up the tracer and is visible when imaged where as it reveals certain spots in the gland which are more active depending on the stage of advancement of cancer.