For approximately 4 billion years after the birth of life, iron, the fourth most abundant metal on earth, has played a critical role in redox reactions. Iron is a trace metal that is required for life activity, and no organism can survive without it. However, it has recently been discovered that iron causes life-threatening toxicity as organisms live longer. Ten years have passed since the discovery of iron-dependent cell death (ferroptosis), and the interest in iron is growing rapidly in various fields. Many diseases, such as neurodegenerative diseases and cancers, have been linked to abnormalities in iron metabolism, and completely new treatment methods based on iron regulation have been developed. In this article, we will assess the secretory mechanism ferritin, an iron storage protein that we recently discovered, as well as the role of secreted ferritin in diseases.