Metastable radionuclides, of which Tc-99m is an example, are still excitable after nuclear disintegration of the parent and continue to emit gamma radiation until they reach the ground state. Mo-99, for example, decays by beta minus emission to Tc-99 (12% of disintegrations) and Tc-99m (88%). Tc-99m decays to Tc-99 with a half-life of 6 h. A Mo-99 generator has a half-life of 2.75 days, which gives it a useful life span of 1 week and from which Tc-99m, as pertechnetate (Tc-99mO4) can be drawn daily. Tc-99mO4 accumulates in salivary glands, thyroid gland and gastric mucosa. It is most widely used to image the thyroid in adult patients with thyrotoxicosis and infants with hypothyroidism. The three common causes of adult thyrotoxicosis are thyroiditis, toxic autonomous nodules and Graves' disease. In infants, hypothyroidism is generally congenital and the thyroid shows reduced uptake, except in dyshormonogenesis when it is increased. The other clinical application for Tc-99mO4 is location of ectopic gastric mucosa.