Seaborg was the principal or co-discoverer of ten elements: plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium and element 106, which was named seaborgium in his honor while he was still living. He also developed more than 100 atomic isotopes. Early in his career, Seaborg was a pioneer in nuclear medicine and developed many isotopes of elements that helped to diagnose and treat diseases. For example, he studied iodine-131, which is used in the treatment of thyroid disease. He developed the actinide concept which placed the actinide series beneath the lanthanide series on the periodic table. Seaborg also proposed the placement of super-heavy elements in the transactinide and superactinide series.