Figure 1.

The different metabolic pathways in radioisotope imaging of thyroid nodules. ¹⁸F-FDG is transported into the follicular cell by the trans-membrane glucose transporter GLUT1. The intracellular ¹⁸F-FDG is then phosphorylated by HKII to ¹⁸F-FDG6P, which traps the compound and prevents its efflux. In malignant cells, both the expression of GLUT1 transporters and the activity of hexokinase may be increased. Thyroid follicular cells trap ¹²³I using the sodium-iodine-symporter, which concentrates iodine intra-cellularly and incorporate the ¹²³I into Tg. The ¹²³I is oxidized by thyroid peroxidase at the follicular cell colloid interface to neutral iodine, which binds to tyrosine residues on Tg to form thyroid hormones (T3 and T4) stored in the colloid follicular lumen. ^99mTcO₄- is trapped by the thyroid follicular cells in an identical manner as ¹²³I, but is not organified nor incorporated into thyroid hormones, and then is not retained in the thyroid. [^99mTc]MIBI is a lipophilic monovalent cationic agent which cross the cell membrane, and concentrates in the mitochondria due to its positive electric potential.