Why is radiopharmaceutical therapy useful in thyroid disease?
The most widely used form of radiopharmaceutical therapy is radioiodine for thyroid disease, the discovery and development of which was largely the birth of the specialty of nuclear medicine. Thyroid cells utilize dietary iodide to synthesize thyroid hormone, which is then stored in colloid adjacent to the cell. The body cannot differentiate radioactive iodide from radiostable iodide, and so by administering radioactive iodine a portion of the administered dose is concentrated and stored for prolonged periods in or very near the thyroid cells. Because the rest of the body has short retention times of radioactive iodine, the therapeutic ratio is very high. Thus, incredibly high radiation doses can be delivered to thyroid cells (much higher than could be safely delivered with external beam radiotherapy) with acceptable toxicity. Iodine-131 ( 131 I) is the primary therapeutic iodine isotope with an 8-day half-life and both beta and gamma emissions. The gamma emissions permit imaging for detection of sites of disease and confirmation of dose delivery