Why is technetium not organified by the thyroid?

Technetium is not effectively organified by the thyroid gland because of its low affinity for thyroglobulin. Thyroglobulin is a dimeric glycoprotein that acts as a precursor for the thyroid hormones thyroxine (T4) and triiodothyronine (T3). These hormones are synthesized by the sequential addition of iodine atoms to the tyrosine residues of thyroglobulin. The process is mediated by the thyroid peroxidase enzyme, which couples iodine to the tyrosine residues of thyroglobulin to form monoiodotyrosine (MIT) and diiodotyrosine (DIT). MIT and DIT then undergo further coupling to form T4 and T3, respectively.

Technetium, on the other hand, is a transition metal with atomic number 43 and chemical symbol Tc. It has a different chemical structure and does not have the same affinity for thyroglobulin as iodine. While iodine is a nonmetallic element with atomic number 53, it shares some chemical properties with the metallic elements of Group 7, which includes technetium. However, technetium's chemical properties are more similar to those of manganese and rhenium than to iodine.

Because of its low affinity for thyroglobulin, technetium is not incorporated into the thyroid hormones. Instead, it is rapidly taken up by the thyroid gland but not bound to thyroglobulin. As a result, technetium is not effectively organified by the thyroid and is quickly cleared from the body.

Therefore, technetium cannot replace iodine in the synthesis of thyroid hormones, and its use as a radioisotope for thyroid imaging is based on its ability to concentrate in the thyroid gland, not its incorporation into thyroid hormones.