Sex hormone-binding globulin (SHBG) is thought to mainly function as a transporter and reservoir for the estradiol and testosterone sex hormones. However it has also been demonstrated that SHBG can bind to a cell surface receptor (SHBG-R). The SHBG-R has not been completely characterized. A subset of steroids are able to bind to the SHBG/SHBG-R complex resulting in an activation of adenylyl cyclase and synthesis of the cAMP second messenger.  Hence the SHBG/SHBG-R complex appears to act as a transmembrane steroid receptor that is capable of transmitting signals to the interior of cells.
We have also noted that 24-hour urinary estrogens can be a sensitive monitor of liver detoxification capability. Elevated urinary estrogens in normally-cycling women may indicate a history of exposure to liver stresses such as excessive environmental organic chemicals. Interventions intended to improve liver function result in a gradual normalization of the abnormal estrogen levels. Thus, measurement of urinary estrogens can give insight into other aspects of physiology. This phenomenon is also noted in peri- or post-menopausal women who have previously taken Premarin, and have switched to triple-estrogen replacement with less-than-optimal symptom relief.
Some of the approved drugs are synthetic versions of the natural hormones, such as trenbolone acetate and zeranol. Just like the natural hormone implants, before FDA approved these drugs, FDA required information and/or toxicological testing in laboratory animals to determine safe levels in the animal products that we eat (edible tissues). Furthermore, FDA required that the manufacturers demonstrate that the amount of hormone left in each edible tissue after treatment is below the appropriate safe level. As described above, a safe level is a level which would be expected to have no harmful effect in humans.