Chemical structures of bufadienolide (a) and cardenolide (b) CTS. Displacement of binding of 3E9 (c) and 4G4 (d) anti-MBG mAbs to MBG–thyroglobulin conjugates by MBG (○), cinobufotalin (●), bufalin (▲), cinobufagin (△), ouabain (■), and digoxin (□) in DELFIA competitive fluoroimmunoassay. CTS, cardiotonic steroids; DELFIA, dissociation-enhanced fluoroimmunoassay; mAb, monoclonal anti-marinobufagenin antibody; MBG, marinobufagenin.