Sex hormone-binding globulin (SHBG) is a glycoprotein that binds at the plasma level (in the blood) to sex hormones, specifically testosterone and estradiol based hormones. Other steroid hormones such as progesterone, cortisol, and other corticosteroids are bound by transcortin (a plasma marker of overtraining).
Transport of Sex hormones
These sex hormones circulate in the bloodstream, bound mostly to SHBG and to some degree bound to serum albumin. Only a small fraction is unbound, or "free," and thus biologically active and able to enter a cell and activate its receptor. The SHBG inhibits the function of these hormones. The binding globulin then releases the hormonal compound at a controlled rate (the decay of the SHBG protein).Thus bioavailability of sex hormones is influenced by the level of SHBG.
SHBG is produced by the liver cells and is released into the bloodstream. Other sites that produce SHBG are the brain, uterus, and placenta and vagina. In addition SHBG is produced by the testes; testes-produced SHBG is also called androgen-binding protein.
SHBG levels appear to be controlled by a delicate balance of up and down regulating mechanisms. Its level is decreased by high levels of insulin and insulin-like growth factor 1 (IGF-1) and possibly Growth Hormone. Also, high androgen levels decrease SHBG, while high estrogen and thyroxine levels
When determining levels of circulating estradiol or testosterone, either a total measurement could be done that includes the "free" and the bound fractions, or only the "free" hormone could be measured. A free androgen index expresses the ratio of testosterone to the sex hormone binding globulin and can be used to summarize the activity of free testosterone. Sex hormone binding globulin can be measured separate from the total fraction of testosterone, however the total testosterone is likely the most accurate measurement of plasma androgen levels.
Androgen-binding protein (ABP) is a glycoprotein (beta-globulin) produced by the Sertoli cells in the seminiferous tubules of the testis that binds specifically to testosterone (T), dihydrotestosterone (DHT), and 17-beta-estradiol.By binding to T and DHT these hormones are made less lipophilic and become concentrated within the luminal fluid of the seminiferous tubules. The higher levels of these hormones enable spermatogenesis in the seminiferous tubules and sperm maturation in the epididymis.ABP has the same amino acid sequence as sex hormone-binding globulin (SHBG); the difference is the site of production and the addition of different sugar moieties. ABP’s production is regulated under influence of FSH on Sertoli cell, enhanced by insulin, retinol, and testosterone.
Binding proteins are likely simultaneously the most important, misunderstood and ignored aspect of hormonal application/manipulation. Binding globulins like SHBG (and others) act as a buffer or time release mechanism to “prevent” a large (sudden) influx of hormone to its target tissue.
With a little thought and imagination a perspective “endocrinologist” can begin to see the pointlessness of many of the common practices in sports performance. Tapering, Front-Loading and many other misguided practices can be “explained away” once binding proteins are fully considered.
Maintaining elevated plasma Insulin levels (hard for the low carb contingent!) and minimizing catecholamine production (overtraining) are simple non-pharmacological means of maintaining a positive bound of unbound sex hormone ratio.
B.”EvilGenius”Chavez EvilGenius Sports Performance