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Testosterone is made by specialized cells in the testes called Leydic cells. In an adult male, approximately 5 grams of testosterone are made per day. Testosterone is released in a pulsatile fashion (in bursts). There is a daily pattern to the secretion of testosterone with the peak occurring early in the morning and the low point in the late evening.
Only certain cells have receptors for testosterone and some of these cells later convert the testosterone into Dihydrotestosterone (DHT), which is three times as potent as the testosterone itself. Interestingly, the testosterone can also be converted into estrogens (the main female hormones). This occurs particularly in fat cells.
Most testosterone in the body is bound or "attached" to proteins. Thirty-percent is bound to a type of protein known as sex hormone-binding globulin (SHBG). The testosterone binds very tightly to SHBG, which has a tendency to increase as men age. The remaining testosterone is bound much less tightly to other proteins in the blood, the most prevalent of which is albumin. Two percent of the testosterone is unbound (not attached to any other protein) and is called "free testosterone." Free and albumin-bound portions of testosterone make up the measure known as "bioavailable testosterone." This is the testosterone that is seen in the tissue and that has the most effect on the body. Thus, any change that will affect either the total amount of testosterone, the amount of SHBG, or blood proteins will affect the amount of available testosterone and will have an affect on the body.
As men get older, their SHBG increases, leaving less available testosterone. Other hormones can also affect SHBG. Elevated female hormones and thyroid hormone will increase SHBG which will then, in turn, affect the "bioavailable testosterone."
Table 1
Relative Androgenic Activity of Adrenal Androgens |
Dihydrotestosterone |
300 |
Testosterone |
100 |
Androstenedione |
10 |
DHEA, DHEA-S |
5 |
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