Performs an essential role in the oxidative function of the citric acid cycle. D-threo-isocitrate + NAD(+) = 2-oxoglutarate + CO2 + NADH Binds 1 Mg(2+) or Mn(2+) ion per subunit. Octamer of two non-identical subunits IDH1 and IDH2. Belongs to the isocitrate and isopropylmalate dehydrogenases family.