A ketosteroid has been isolated from the oleo-resin of COMMIPHORA MUKUL and its structure (4,17(20)trans pregnandiene 3,16-dione; Z-guggulsterone) established from its physico-chemical properties, specially UV, IR, NMR and mass-spectra. The isolated ketosteroid showed a strong thyroid stimulatory action when administered to albino rats. Its administration (1 mg/100 g body weight) brought about an increase in iodine-uptake by thyroid and enhanced activities of thyroid peroxidase and protease as well as oxygen consumption by isolated slices of liver and biceps muscle.