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Testosterone
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Testosterone is the primary male hormone secreted by the testes, ovaries, and adrenal glands found in men and women. Without testosterone, male sexual development would not occur. The effects of testosterone begin in the fetus, as early as 40 days after conception.
In the blood stream testosterone exists in 3 general phases:
- Free, not bound to a protein. This is the most active form of testosterone.
- Loosely bound to albumin.
- Bound tightly to sex hormone-binding globulin, SHBG.
Only about 4% of the total testosterone exists as free T, able to enter the cell. The rest of the testosterone is tied to albumin or SHBG. As the total amount of SHBG rises, the free T decreases. Free testosterone is like money in your pocket, available to spend, while bound testosterone is money in a savings account, not as available. As men age, the SHBG increases while the total testosterone declines, but not to the extent that estrogen does in the aging woman. The result is that while the total testosterone declines gradually as men age, the free or active form of testosterone decreases markedly.
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