The present invention concerns methods and kits for diagnosing a disease condition characterized by non-physiological levels of hepcidin protein, including prohepcidin and fragments thereof, comprising obtaining a tissue or fluid sample from a subject; contacting the sample with an antibody or fragment thereof that specifically binds to a polypeptide corresponding to the mid-portion or C terminus of a hepcidin protein, and quantifying the hepcidin level using an assay based on binding of the antibody and the polypeptide; wherein the non-physiological level of hepcidin is indicative of the disease condition. The present invention also concerns diagnostic methods and kits for applications in genetic technological approaches, such as for overexpressing or down-regulating hepcidin. The present invention further concerns therapeutic treatment of certain diseases by treatment of subjects with hepcidin and agonists or antagonists of hepcidin.