 Seaborg remained at the University of California , Berkeley , for post @-@ doctoral research . He followed Frederick Soddy 's work investigating isotopes and contributed to the discovery of more than 100 isotopes of elements . Using one of Lawrence 's advanced cyclotrons , John Livingood , Fred Fairbrother , and Seaborg created a new isotope of iron , iron @-@ 59 ( Fe @-@ 59 ) in 1937 . Iron @-@ 59 was useful in the studies of the hemoglobin in human blood . In 1938 , Livingood and Seaborg collaborated ( as they did for five years ) to create an important isotope of iodine , iodine @-@ 131 ( I @-@ 131 ) , which is still used to treat thyroid disease . ( Many years later , it was credited with prolonging the life of Seaborg 's mother . ) As a result of these and other contributions , Seaborg is regarded as a pioneer in nuclear medicine and is one of its most prolific discoverers of isotopes . 
