Estrone is bound approximately 16% to sex hormone-binding globulin (SHBG) and 80% to albumin in the circulation,[1] with the remainder (2.0 to 4.0%) circulating freely or unbound.[16] It has about 24% of the relative binding affinity of estradiol for SHBG.[1] As such, estrone is relatively poorly bound to SHBG.[17]