4 Serum bioavailability of sex steroid hormones

This chapter has reviewed the factors underlying the transport of testosterone and oestradiol into tissues in vivo. The following points have been emphasized. (1) Albumin-bound testosterone is nearly freely available for transport into brain and liver and is partially available for transport into salivary gland and lymph node; testosterone transport into hair follicles has not been measured thus far.

(2) SHBG-bound testosterone is not available for transport into tissues; SHBG-bound oestradiol is available for transport into liver, salivary gland, and lymph node, but not into brain under normal conditions.

(3) The transport of hormone from the circulating plasma protein-bound pool involves tissue-mediated enhanced dissociation of the hormone from the protein without significant exodus of the plasma protein from the microcirculation compartment. The tissue-mediated enhanced dissociation mechanism varies in activity between different organs and is a much more important factor than organ differences in capillary transit times in regulating the amplification of hormone delivery to different tissues.

(4) The concentration of free testosterone inside cells in the absence of significant cellular metabolism of the hormone is nearly ten times greater than the concentration of free testosterone in vitro, but is nearly equal to the concentration of free plus albumin-bound hormone.

(5) In the presence of active tissue metabolism of hormone, the concentration of cellular free testosterone may be much less than the albumin-bound hormone and may fortuitously approximate the concentration of free testosterone in vitro. This is the situation in salivary gland; the low concentration of testosterone in saliva appears to be due to active salivary metabolism of the hormone, since both free and albumin-bound testosterone are available for transport into salivary gland.