False-colour scintigram (gamma camera scan) of a normal human thryoid gland. The image is a record of the distribution of a radioactive tracer, Iodine-123, following its intravenous injection into the subject. Hormones containing normal (that is, non-radioactive) iodine are among those secreted by the thryoid: tri-iodothyronine (T3) and thyroxine (T4): T4 is converted to T3 in the blood circulation. T3 is involved in the regulation of basal metabolic growth.