Current knowledge of the effect of tibolone on the breast and uterus: an extract from the guidelines for the use of tibolone in South Africa

BackgroundTibolone is an analogue of the progestin, norethynodrel. After ingestion, it is converted to three metabolites, namely 3 alpha and 3 beta hydroxytibolone which have oestrogenic effects, and delta 4 isomerase, which has progestogenic and androgenic properties. Both the oestrogenic metabolites bind to the alpha oestrogen receptor, but not the beta oestrogen receptor, whilst the delta 4 isomer binds to the alpha and beta oestrogen, the progestogen and the androgen receptors. Tibolone also is a sulphatase inhibiter, blocking conversion of oestrone sulphate to oestrone, as well as stimulating local sulphotransferase activity. In contrast to other forms of postmenopausal hormonal therapy, it decreases sex hormone binding globulin and hence increases circulating free testosterone, and thereby further adding to its androgenicity. Tibolone significantly decreases vasomotor symptoms, mood disorders, insomnia, bone loss, vaginal atrophy. It has a favourable impact on the cardiovascular system and minimal i...