The chemical reactions and pathways resulting in the formation of trypanothione (N1,N6,-bis(glutathionyl)spermidine) in two steps from glutathione and spermidine via an N1- or N8-glutathionylspermidine intermediate. Trypanothione appears to be an essential redox intermediate in intracellular thiol redox regulation. It also plays a role in protecting against oxidative stress.