The Science Behind Testosterone Biomarkers

Delve into the world of testosterone biomarkers and uncover the scientific explanations behind their role in health and wellness.

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By Staff Writer

Published May 30, 2024.

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Testosterone is a steroid hormone that is essential for muscle development and strength, bone health, sexual function, overall energy, and athletic performance. Although both men and women produce testosterone, women normally have very little of this hormone - a fraction of the amount that men typically have. However, having too much or too little testosterone can cause problems. Excess testosterone decreases the levels of HDL (good) cholesterol, affects heart health, and impairs sexual and reproductive function. In contrast, low testosterone can make you feel tired, uninterested in sex, and less competitive, as well as diminishing your athletic performance. Possible causes of low testosterone include overtraining and low levels of zinc and magnesium.

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While it is helpful to know your total testosterone level, testosterone metabolism is complex. Therefore, this review will focus on four testosterone-related markers that, taken collectively, provide a more complete understanding of how testosterone is working in your body:

  • Total Testosterone - all the testosterone in the body.
  • Free Testosterone - the amount of testosterone that is bioactive, that is, ready for the body to use.
  • Sex-Hormone Binding Globulin (SHBG) - a protein that binds to, transports and inhibits the function of testosterone.
  • Albumin - the most abundant protein in your blood, albumin binds to and transports hormones, including testosterone. When bound to albumin, testosterone is not as available for the body to use as when it is free, thus albumin inhibits the function of testosterone.

Testosterone and free testosterone

Testosterone is produced predominantly in the testes of men, and to a lesser extent, in the ovaries of women. Testosterone’s androgenic effects include the maturation of male sex organs and the appearance of secondary characteristics such as body hair and a deepening voice. This hormone also has important anabolic effects, including the increase of muscle mass and bone strength. Maintaining muscle mass becomes more difficult as men age, because testosterone levels decline each year (by about 2% per year). Within the body, serum testosterone spends most of its time bound to other biomolecules, including albumin and sex hormone-binding globulin (SHBG). Binding inhibits the function of the hormone. Only about 2 to 3% of testosterone is unbound, and this “free” testosterone is biologically active (meaning it is being used by the body).

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Sex hormone-binding globulin (SHBG) is a glycoprotein produced mainly by the liver and released into the bloodstream. It binds and transports sex hormones, such as testosterone, throughout the bloodstream. Approximately 45% of the testosterone within the body is bound to SHBG. SHBG levels tend to increase in parallel with testosterone levels, as it is the protein responsible for regulating sex hormone levels. Additionally, SHBG prevents sex hormones from being cleared from the bloodstream, acting as a pool of reserved hormones that can be tapped into when levels become too low. Since SHBG does in fact bind testosterone, high levels may create a false pretense of low testosterone levels, which may affect energy levels, muscle development, sex drive, and bone health.


Produced by the liver, albumin is the most abundant protein in human blood. This protein carries free fatty acids to the liver, transports medication, binds with calcium, and helps maintain blood acidity in a narrow range. In addition, albumin plays a key role in maintaining osmotic pressure, preventing plasma inside the blood vessels from leaking out into surrounding tissues. Importantly, albumin also acts as a carrier protein for steroids, including testosterone. About 50% of the body’s testosterone is bound to albumin. Although albumin-bound testosterone is biologically inactive, this binding is much weaker than SHBG binding. Thus in the interaction between these two molecules, testosterone bound to albumin spends less time in the inactive (bound) state and more time in the active (free state). Low serum albumin levels can be a sign of kidney or liver disease, or an indication that the body is not getting enough nutrients. High serum albumin levels can indicate dehydration or be caused by a high protein diet.

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Why are the testosterone ratios important?

As mentioned above, testosterone is an anabolic hormone (build muscle). Anabolism is the set of metabolic pathways that utilize energy to construct molecules from smaller units. Catabolism is the opposite (break muscle); it is the set of metabolic pathways that breaks down molecules into smaller units to release energy. Cortisol is a catabolic hormone that increases protein breakdown, inhibits glucose uptake, and promotes the breakdown of lipids (fats). Therefore, the ratio of total testosterone to cortisol (T/C) can indicate the body’s anabolic/catabolic state. A high ratio suggests the body is in a state conducive to building muscle, and a low ratio indicates a state more conducive to breaking down muscle. The ratio of free testosterone/cortisol (FT/C) is similar. A low FT/C score can be an indicator of over-training, high stress levels, or poor quality sleep, any of which can result in muscle breakdown and fatigue. A high FT/C score can show that your body is getting enough sleep and recovery time to increase muscle mass and strength.

While it is useful and important to measure your total testosterone level, knowing your levels of free testosterone, SHBG, albumin and testosterone/cortisol ratio gives you a much more detailed understanding of how much testosterone is active in your body, as well as your anabolism/catabolism status. For example, a high level of SHBG effectively reduces your free testosterone and may cause symptoms similar to those of low total testosterone. If you are seeking to achieve your peak performance, consider getting an InsideTracker Ultimate test to learn about your testosterone levels and how to optimize them.

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