Fluoride is used to prevent cavities. The small F- ions partially replace the OH- ions in the mineral components of tooth enamel. This corresponds to the partial conversion of the mineral hydroxyapatite to hydrolysis-resistant fluoroapatite.
Radioactive fluorine compounds (18F) are used in nuclear medical diagnostics (positron emission tomography, PET). Fluorodesoxyglucose, in which one OH- group of the glucose molecule has been replaced with 18F-, plays a special role here. It is able to elucidate metabolic processes.
Iodine is an essential element, which in the form of the thyroid hormones thyroxin (tetraiodothyronin) and triiodothyronin is very important for the regulation of energetic metabolism (ATPase biosynthesis). The function of these polyiodinated hormone molecules is steered by a structure-based receptor mechanism.
The radioactive iodine isotopes 123I and 131I are important in nuclear medicine, both in diagnostics and therapy. Iodinated organic molecules are used.