Catalyzes the last step in the transsulfuration pathway from methionine to cysteine. Has broad substrate specificity. Converts cystathionine to cysteine, ammonia and 2-oxobutanoate. Converts two cysteine molecules to lanthionine and hydrogen sulfide. Can also accept homocysteine as substrate. Specificity depends on the levels of the endogenous substrates. Generates the endogenous signaling molecule hydrogen sulfide (H2S), and so contributes to the regulation of blood pressure.

Defects in CTH are the cause of cystathioninuria (CSTNU). It is an autosomal recessive phenotype characterized by abnormal accumulation of plasma cystathionine, leading to increased urinary excretion.