<qa>  What is the delta-4-isomer?Tibolone is a tissue-specific compound used for treatment of climacteric complaints and prevention of osteoporosis and does not stimulate the endometrium like estrogens 6. It is converted by steroid metabolizing enzymes in the liver and intestine into three active metabolites: two hydroxy-metabolites: 3a-hydroxy-tibolone and 3b-hydroxy-tibolone exerting estrogenic effects and in vivo present in significant amounts and the delta-4-isomer exerting progestagenic and androgenic effects, and in vivo present in circulation for a relatively short period of time 7. The estrogenic metabolites are responsible for the effects on hot flushes, vagina and bone. 