When injected as a mononegative ion into experimental animals, astatine concentrates in the thyroid. The alpha-emitting astatine isotopes offer an advantage over radioiodine in the treatment of hyperthyroidism and thyroid cancer, since the alpha deposit more energy in a limited area than the beta particles of the radioiodine. However, astatine has shown a tendency to induce tumors and this obviously presents a severe obstacle to its clinical use.