The invention provides 5-deuterium-enriched 2,4-thiazolidinediones (e.g., 5-[4-[2-(5-ethyl-2-pyridyl)-2-oxoethoxy]benzyl]-5-deutero-thiazolidine-2,4-dione), deuterated derivatives thereof, stereoisomers thereof, pharmaceutically acceptable salt forms thereof, and methods of treatment using the same.