Summarized & reviewed by The Peptide Dispatch Editorial Team · Last reviewed July 27, 2026
There is a specific and very common clinical situation that goes something like this. A man in his late forties reports low energy, flat mood, poor recovery from training, and a body composition that has been drifting the wrong way for two years. His physician orders a testosterone level. It comes back at 480 ng/dL. The reference range printed next to it runs roughly 264 to 916. The result is…
This dispatch covers Your Testosterone Result Is a Number About a Protein You Have Never Heard Of in the metabolic research category, authored by The Peptide Dispatch Editorial Team. Estimated reading time: 8 minutes. The Peptide Dispatch curates peer-reviewed peptide research for self-directed learners. All summaries are presented for Research Use Only and do not constitute medical advice.
There is a specific and very common clinical situation that goes something like this. A man in his late forties reports low energy, flat mood, poor recovery from training, and a body composition that has been drifting the wrong way for two years. His physician orders a testosterone level. It comes back at 480 ng/dL. The reference range printed next to it runs roughly 264 to 916. The result is inside the range, so the conversation ends there.
The problem is that 480 ng/dL is not a measurement of how much testosterone is available to his tissues. It is a measurement of how much testosterone is present in his bloodstream, the overwhelming majority of which is chemically unavailable to do anything at all. Whether that 480 represents a comfortable androgen status or a functionally deficient one depends almost entirely on a liver protein called sex hormone binding globulin, or SHBG, which most panels do not measure.
Testosterone circulates in three states. A small fraction, typically 1 to 3 percent, floats free in plasma. A larger fraction is loosely bound to albumin, held weakly enough that it dissociates readily at the capillary and is generally considered available to tissue. The remainder, often 40 to 60 percent or more, is locked to SHBG with high binding affinity.
The free hormone hypothesis, which underpins most of endocrine physiology, holds that only the unbound and weakly bound fractions can enter cells and act on the androgen receptor. The SHBG-bound portion is essentially in storage. It is real testosterone, it shows up on the assay, and it is doing nothing for muscle, mood, libido, bone, or metabolic function while it stays bound.
A total testosterone result adds all three pools together and reports one number. Two men can both return 480 ng/dL and have meaningfully different androgen exposure, because one has SHBG of 25 nmol/L and the other has SHBG of 70 nmol/L. The second man has far more of his testosterone tied up and considerably less circulating in usable form. He may be symptomatic. His lab result will still read "normal."
SHBG is produced by the liver, and its production is regulated by a fairly specific set of inputs.
Insulin suppresses it. Higher circulating insulin, which is the hallmark of insulin resistance, drives hepatic SHBG production down. Hepatic fat accumulation does the same. This is why low SHBG clusters tightly with visceral adiposity, fatty liver, and metabolic syndrome.
Thyroid hormone raises it. Hyperthyroidism drives SHBG up, hypothyroidism drives it down.
Estrogen raises it, and the route matters. Oral estrogen passes through the liver first and produces a large increase. This is the mechanism by which combined oral contraceptives commonly double or more a woman's SHBG, with a corresponding fall in her free testosterone. Transdermal estrogen bypasses first-pass hepatic metabolism and has a much smaller effect.
Age raises it in men. SHBG climbs steadily across the adult male lifespan, roughly 1 percent per year in many cohorts. This produces one of the more consequential patterns in men's health: total testosterone declines modestly with age, but free testosterone declines considerably faster, because the rising SHBG is capturing a larger share of a shrinking pool. A man's total testosterone can look stable across a decade of annual physicals while his usable androgen quietly falls by a third.
Androgens and growth hormone lower it. Certain medications, including some anticonvulsants, raise it. Advanced liver disease raises it. Nephrotic syndrome and glucocorticoids lower it.
The practical consequence is that SHBG is not a fixed personal constant. It moves with metabolic status, hepatic health, thyroid function, medication, and age, which is precisely why interpreting a total testosterone without it is guesswork.
The most interesting development in this area over the past fifteen years is that SHBG stopped being purely a transport protein of interest to andrologists and became a subject of metabolic research.
According to PubMed, a nested case-control analysis published in the New England Journal of Medicine examined SHBG levels and subsequent type 2 diabetes in postmenopausal women in the Women's Health Study, with replication in men from the Physicians' Health Study II (DOI). The gradient was steep. Compared with the lowest quartile of SHBG, the multivariable odds ratio for developing type 2 diabetes was 0.09 in the highest quartile among women, with a comparable figure of 0.10 in men.
The authors then went further and ran a Mendelian randomization analysis using two SHBG gene polymorphisms known to influence circulating levels. Because genetic variants are assigned at conception and are not subject to reverse causation or lifestyle confounding, this design tests whether the relationship is likely causal rather than merely correlated. The predicted odds ratio per standard deviation increase in SHBG was 0.28 in women and 0.29 in men, which the authors interpreted as suggesting SHBG may play a causal role in diabetes risk rather than simply serving as a passive readout of insulin resistance.
That distinction matters for how you read a low SHBG value. It is not only telling you that free testosterone may be higher than the total suggests. It is flagging a metabolic state that carries independent forward risk.
The same causal-inference approach has been applied in women's reproductive endocrinology. A review in the Journal of Clinical Endocrinology and Metabolism summarizing Mendelian randomization work in polycystic ovary syndrome reported that serum SHBG concentration appears among the factors with evidence for a causal contribution to PCOS, alongside obesity, testosterone, and fasting insulin (DOI).
If SHBG is the missing variable, the obvious response is to measure free testosterone directly. Here the picture gets genuinely messy, and it is worth being honest about it.
A review of free testosterone assessment in the Journal of Steroid Biochemistry and Molecular Biology lays out the state of play (DOI). Equilibrium dialysis is the reference method, but the authors describe it as too complex for routine clinical laboratories. Assays are not harmonized across labs, so there are no common reference intervals to anchor interpretation.
Calculated free testosterone, derived from total testosterone, SHBG, and albumin using published equations, is the practical alternative. The same review is blunt that these algorithms are inaccurate, because they were built on binding models that turned out to be incomplete, while still concluding that they can give clinically useful results. That is a fair summary: imperfect, directionally informative, and far better than a total testosterone alone.
The free androgen index, which is simply testosterone divided by SHBG, gets a harder verdict. The authors state it is not recommended in men because it becomes inaccurate at the extremes of SHBG concentration, and that it can also produce inaccurate results in women when SHBG is low. Those extremes are exactly the patients whose results are hardest to interpret, so a tool that fails there is failing where it is needed most.
There is also a widely used direct analog free testosterone immunoassay that appears on many commercial panels. It is inexpensive and it is not well regarded, particularly at the low end. If a panel reports free testosterone without also reporting SHBG and albumin, it is worth knowing which method produced the number.
The reasonable position, given all of this, is that total testosterone is an input rather than an answer.
Interpreting androgen status sensibly requires total testosterone, SHBG, and albumin together, ideally from a morning draw, and ideally repeated, since testosterone has meaningful day-to-day variability and single values get over-read routinely. From those three, calculated free and bioavailable testosterone can be derived with the acknowledged limitations above.
SHBG then deserves a second reading on its own terms. A value at the low end alongside elevated fasting insulin, elevated triglycerides, and a raised GGT or ALT is describing hepatic and metabolic strain, and it will not be fixed by looking at hormones alone. A high value in an aging man with symptoms explains why his total testosterone looks reassuring while his free testosterone does not.
None of this is exotic. SHBG is an inexpensive, widely available test that has been on lab menus for decades. It is simply not part of the standard workup, which means the number most people are handed is a sum that hides its own components.
The Peptide Dispatch publishes research summaries for educational purposes. This material describes what the published literature reports about laboratory markers and physiology. It is not medical advice, it is not a diagnosis, and it does not recommend or endorse any treatment, product, or provider. Laboratory results require interpretation by a licensed clinician in the context of your individual history. Always discuss testing and treatment decisions with a qualified healthcare professional.
Educational content — not medical advice. Effects described are drawn from cited research in study subjects.