Radioactive iodine concentrates in the thyroid because the thyroid gland has a unique biological mechanism that actively takes up iodine from the bloodstream to produce thyroid hormones. This process is essential for normal metabolism and growth, and it is the reason why iodine, including radioactive forms, naturally accumulates in the thyroid.
The thyroid gland contains specialized cells called follicular cells that have a protein called the sodium-iodide symporter (NIS) embedded in their membranes. This symporter actively transports iodide ions (I⁻) from the blood into the thyroid cells against a concentration gradient, meaning it can concentrate iodine inside the gland at levels much higher than in the bloodstream. This active transport is crucial because iodine is a rare element in the body but indispensable for synthesizing thyroid hormones such as thyroxine (T4) and triiodothyronine (T3).
Once inside the thyroid cells, iodide is oxidized to iodine by the enzyme thyroid peroxidase. The iodine then binds to the amino acid tyrosine residues on a protein called thyroglobulin, forming iodinated compounds that are precursors to thyroid hormones. These hormones are stored in the thyroid follicles and released into the bloodstream as needed to regulate metabolism, growth, and development.
Because radioactive iodine isotopes, such as iodine-131, chemically behave identically to stable iodine-127, the thyroid cannot distinguish between them. When radioactive iodine is present in the bloodstream, the sodium-iodide symporter actively transports it into the thyroid cells just like normal iodine. This selective uptake allows radioactive iodine to concentrate in the thyroid, making it useful both diagnostically and therapeutically in medicine.
In diagnostic nuclear medicine, radioactive iodine isotopes that emit gamma rays, such as iodine-123, are used to image the thyroid gland. The concentrated radioactive iodine emits signals that can be detected by gamma cameras, providing detailed pictures of thyroid function and structure.
Therapeutically, radioactive iodine-131 is used to treat conditions like hyperthyroidism and thyroid cancer. Because the radioactive iodine accumulates in the thyroid, it delivers targeted radiation that destroys overactive or malignant thyroid cells while sparing most other tissues. This selective destruction is possible precisely because of the thyroid’s natural iodine-concentrating mechanism.
The thyroid’s affinity for iodine is so strong that in cases of radioactive iodine exposure, doctors can administer a large dose of stable iodine to saturate the thyroid and block further uptake of radioactive iodine, protecting the gland from radiation damage.
In summary, radioactive iodine concentrates in the thyroid because the gland’s follicular cells actively and selectively transport iodine from the blood to produce essential thyroid hormones. This biological mechanism does not differentiate between stable and radioactive iodine, allowing radioactive iodine to accumulate in the thyroid and enabling its use in medical imaging and treatment.