Radioactive iodine (I-123) is used in the diagnosis of some thyroid diseases. I-123 is described as a non-tissue damage isotope with a half-life of 13 hours, emitting x-ray and gamma photons. In cases where I-123 is not available, low-dose I-131 can also be used. Radionuclide imaging is also performed using technetium together with iodine-123.
In practice, radioactive iodine is given orally, and iodine activity in the thyroid gland is displayed at the 4th, 6th and 24th hours. Precautions may be required, as iodine metabolism may be affected in patients who take iodine in their diet. These patients need to make dietary restrictions before the radioactive iodine-123 test. Similarly, the use of iodine-containing drugs, algae or seaweed diet with high iodine content should be interrupted. At the same time, it is preferred that other imaging tests containing iodinated contrast material are not performed until one or two months before this test. It may also be necessary to discontinue anti-thyroid drugs such as methimazole, carbimazole, propylthiouracil, and thyroid hormone replacement therapy, including levothyroxine (LT4) and liothyronine (LT3), prior to testing.
Since iodine is the most important element in the mechanism of thyroid hormone production in the thyroid gland, the activity of the thyroid gland tissues against oral iodine is observed. This allows interpretation of iodine metabolism in the patient’s body. It also contributes to possible disease evaluations according to the effects of thyroid gland diseases on this metabolism.
The iodine we take in the normal diet is in the form of iodide. It is usually taken as sodium iodide and reaches the thyroid gland through the blood. In the thyroid gland, it is stored in a colloid substance called thyroglobulin in the thyroid follicles. Then iodide is processed and used in the production of T3 and T4 hormones. Up to 40 times the amount of iodine in the blood can be stored in the thyroid gland.
In regions with high dietary iodine intake according to nutritional habits, the uptake of oral iodine by the thyroid gland may remain at the level of 15%, while in regions with high dietary iodine intake, the uptake of oral iodine by the thyroid gland may increase up to 60%.
Radioactive iodine given to the patient has a unique technique and measurement method. These methods may vary depending on the disease being investigated. The half-life of I-123 is 13 hours, while the half-life of I-131 is 8 days. I-131 causes much more radiation exposure. For this reason, I-131 can sometimes be used for therapeutic purposes. I-123 is used more often in scanning. In fact, I123 can be used in the preparation phase before I-131 treatment. Therefore, radioactive iodine-123 intake should not be confused with iodine-131 ablation therapy.
Radioactive iodine uptake test is used to evaluate thyroid metabolism in the thyroid gland. It can be determined that the thyroid metabolism is normal, slow or fast, either in the whole or in a specific region of the thyroid gland.
When thyroid nodules with high uptake of radioactive iodine-123 are detected, these nodules are considered active. Since active nodules may rarely be malignant, it is preferable not to biopsy these nodules.
In some cases, the T3 suppression test is called whether the radioactive iodine given to the patient is taken in the thyroid glands or the suppression level after the T3 hormone given to the patient. This may indicate an autonomously functioning thyroid nodule in the thyroid gland if the thyroid gland still receives radioactive iodine and the radioactive iodine is not suppressed.
Radioactive iodine uptake test is a specialty that requires detailed evaluation according to the level and location of iodine uptake in the body. It requires a combined analysis of the evaluations of the thyroid gland with other imaging methods and laboratory techniques.
Increased radioactive iodine intake is more common in patients with hyperthyroidism with elevated thyroid hormones. In diseases such as Graves’ or toxic nodular goiter, evaluation is made according to the level of radioactive iodine uptake. Radioactive iodine intake is increased in Graves’ disease and Marine-Lenhart syndrome, a variant of Graves’ disease.
Similarly, radioactive iodine uptake can change in many thyroid diseases. Although radioactive iodine intake provides information about the current activity of the thyroid and thyroid metabolism, it may not be sufficient for definitive diagnosis or decision mechanisms. It helps to see the whole picture and supports the diagnosis.
Radioactive iodine uptake test is not applied to pregnant or lactating mothers. It is also done in children with the lowest possible dose.