Your total testosterone can be normal while you still do not feel right. The difference often sits in your free testosterone and your SHBG. Free testosterone is the active fraction your tissues use, SHBG is the protein that binds the rest. Below you will read how they connect and which values are best measured together.
This is exactly why a single number can mislead you.
What is the difference between total and free testosterone?
Total testosterone is the sum of all the testosterone in your blood: the bound part and the free part. Free testosterone is the small fraction, roughly 1 to 3 percent, that is not bound and that your body can use directly. On top of that, a part is loosely bound to albumin and is also reasonably available.
It comes down to this: your tissues do not feel your total, but mostly your free and loosely bound testosterone.
That is why someone with a tidy total testosterone can still keep complaints. You only see what is going on once you place the free fraction next to your total testosterone. Your free testosterone tells the part of the story you actually feel.
What does SHBG do to your testosterone?
SHBG, in full sex hormone-binding globulin, is the protein that binds testosterone tightly. The more SHBG, the less testosterone is freely available. So a high SHBG can make a normal total testosterone work out low for your body.
SHBG shifts along with other things. A few examples:
- High SHBG: can relate to ageing, a fast thyroid or a low body weight.
- Low SHBG: can relate to excess weight, insulin resistance or a slow thyroid.
That makes SHBG not a side note, but the key to understanding your total and free testosterone.
Which values do you measure together?
For a usable picture you measure total testosterone, free testosterone and SHBG in one go. Many labs calculate free testosterone from your total, SHBG and albumin. With those three together you see not just how much testosterone you have, but how much of it is available.
You can have this combination measured with the General Hormones panel. The practical side of the draw is in testing testosterone, and which complaints fit is in low testosterone in men.
My advice: never look at your free testosterone without your SHBG next to it. Apart, they tell only half.
References
- Bhasin S, Brito JP, Cunningham GR, et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. Journal of Clinical Endocrinology and Metabolism. 2018;103(5):1715-1744. PMID: 29562364.
- Wu FCW, Tajar A, Beynon JM, et al. Identification of late-onset hypogonadism in middle-aged and elderly men. New England Journal of Medicine. 2010;363(2):123-135. PMID: 20554979.
- Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. Journal of Clinical Endocrinology and Metabolism. 1999;84(10):3666-3672. PMID: 10523012.
Disclaimer
Every blood test result includes a professional assessment by a BIG-registered doctor. This article gives general information and is not a substitute for medical advice. A blood test is a tool to enter the conversation with your GP better informed, not a diagnosis in itself. For treatment decisions, discuss your results with your GP.
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