Testosterone Test: Understanding Levels, Timing and Free Testosterone
A testosterone value is no simple truth on a lab sheet. When it was measured, what is bound and what is free, and how the lab calculates all play a part. Once you understand that, you read your own result with different eyes.
Many men bring me a lab sheet and a sentence to go with it: "The value is normal, so it can't be the testosterone." I understand that. A number feels clear-cut. But a testosterone value is a snapshot, not a verdict. It depends on when it was measured, whether you were fasting, how much of it is freely available and which lab did the calculation. This article shows you how to place your own result in context, without trusting it blindly and without dismissing it too quickly.
Perhaps you know this situation. You have felt low on drive for months, your desire is flatter, your sleep less restful. You have your testosterone measured, the value is "within the normal range," and you go home with the feeling that the answer is still missing. Or the other way around: a single low value unsettles you, even though you actually feel quite well. Both are common. And both often have to do with the test itself, not just with your body.
In this spoke we look closely at the testosterone test. Why the timing counts. Why fasting matters. What total testosterone, free testosterone and SHBG mean. Why an abnormal value should be confirmed a second time. What a level chart actually tells you and where the difference lies between the reference range and the functional optimum. And finally, why LH and FSH belong in the picture once the value is confirmed low. By the end you read your result with more calm.
The timing decides: why morning and fasting
Testosterone is not a constant value that stays the same all day. It follows a daily rhythm. A large part is produced during sleep, and in the early morning the level is highest. Over the course of the day it falls again. Anyone measured in the afternoon therefore often gets a lower value than in the morning, without anything having changed in the body. That is exactly why guidelines recommend measuring in the morning.
Testosterone rises during sleep and falls while awake
Experiment, n=7, 24-hour profile John Axelsson and colleagues measured testosterone hourly over 24 hours in healthy young men in 2005 in the Journal of Clinical Endocrinology and Metabolism, even when sleep was artificially shifted into the day. Testosterone rose markedly during each sleep period and fell again while awake. The pure clock-time effect, by contrast, was weak. This suggests that the morning peak is mainly tied to sleep. For the test this means: the morning after a normal night is the most informative time.
Axelsson J, Ingre M, Akerstedt T, Holmbäck U. J Clin Endocrinol Metab. 2005;90(8):4530-4535. doi:10.1210/jc.2005-0520 · PMID: 15914523
How large the difference is over the day also depends on age. In younger men the value swings more across the day, in older men the rhythm is often flatter. A study of men with symptoms of testosterone deficiency found that the largest differences between morning and afternoon occurred in those under thirty, while many older men showed barely any difference (doi:10.1093/jsxmed/qdae026, PMID: 38481019). This does not change the basic rule, because to place a low value reliably, the morning measurement remains the most dependable setting.
The second big factor is food. Here many people underestimate how strongly a single meal can move the value.
A sugar load lowered testosterone by about a quarter
Cross-sectional, n=74, oral glucose test Lisa Caronia and colleagues from the group around Frances Hayes studied 74 men in 2013 in Clinical Endocrinology who were given a standard sugar drink. Total testosterone fell by around 25 percent on average and stayed low over two hours, regardless of how good glucose tolerance was. Of the men with a normal baseline, 15 percent slipped temporarily into the low range. SHBG and LH barely changed. The authors conclude that a non-fasting low value should be repeated fasting.
Caronia LM, Dwyer AA, Hayden D, et al. Clin Endocrinol (Oxf). 2013;78(2):291-296. doi:10.1111/j.1365-2265.2012.04486.x · PMID: 22804876
The same effect shows up in the control hormones. A controlled crossover study by Ali Iranmanesh and colleagues in 2012 in the American Journal of Physiology found that glucose intake acutely dampened the pulsatile release of LH and testosterone production (doi:10.1152/ajpendo.00520.2011, PMID: 22252939). And now you know why the seemingly trivial question "Were you fasting?" helps decide how much your test can tell you.
A low testosterone value in the afternoon or after breakfast is not yet a diagnosis. It is a hint that should be checked under the right conditions. Before you worry or consider treatment, the simple question pays off: was the value drawn in the morning and fasting? If not, the most honest next step is often not worry, but a clean repeat.
Total, free and SHBG: three angles on the same hormone
When people talk about the testosterone value, they usually mean total testosterone. It measures the entire amount in the blood. Yet most of it is not freely available at all. It is bound to transport proteins, mainly to sex hormone-binding globulin, SHBG for short, and more loosely to albumin. Only a small unbound fraction counts as immediately active. Free testosterone describes this fraction.
As long as SHBG sits in the normal range, total testosterone usually reflects the picture well. It gets interesting when SHBG is shifted. Then total and free testosterone can point in different directions, and the total number alone can deceive.
Total testosterone
The sum of bound and free testosterone. At the cellular level, though, only the unbound part easily reaches the receptors. Total testosterone is the first step and is often enough, but it can mislead when the binding protein is shifted.
SHBG, the transport protein
SHBG binds testosterone tightly and helps determine how much stays free. It rises with an overactive thyroid, liver disease and with age, and falls with obesity and insulin resistance. At the cellular level it regulates the hormone's availability long before it reaches the cell.
Free testosterone
The small unbound fraction that can dock directly onto the cell's receptors. It becomes important when SHBG is abnormal or when symptoms and total value do not match. Measured or calculated, it describes what is actually available to the cell.
Albumin-bound fraction
Part of the testosterone clings loosely to albumin and can easily come off again. Together with the free fraction it is sometimes described as bioavailable testosterone. At the cellular level, what counts most is how quickly the hormone can release from its binding and act.
But how do you get to the free testosterone? There are two ways here, and both have their pitfalls.
Calculated free testosterone comes close to the reference method
Method comparison, n=56 Ravi Kacker and colleagues compared free testosterone from three methods in 56 men in 2013 in Aging Male: the demanding equilibrium dialysis as reference, a direct measurement, and the calculation from total testosterone and SHBG using the Vermeulen formula. Both the calculated and the directly measured variant showed very close agreement with the reference. However, the absolute numbers differed widely by method. The authors stress that each method needs its own reference values, because the numbers cannot simply be carried across.
Kacker R, Hornstein A, Morgentaler A. Aging Male. 2013;16(4):164-168. doi:10.3109/13685538.2013.835800 · PMID: 24090209
The calculation from total testosterone and SHBG is the most common route in everyday practice, because equilibrium dialysis is too demanding for routine labs. A review by Brian Keevil and Jo Adaway in 2019 in the Journal of Steroid Biochemistry and Molecular Biology points out, though, that the formulas rest on simplified binding models and can turn out differently accurate depending on the patient group (doi:10.1016/j.jsbmb.2019.04.008, PMID: 30970279). Calculated free testosterone is therefore a useful orientation, not an exact measurement. And now you know why your doctor asks for SHBG when the total value is borderline.
Once is not enough: why a value should be confirmed
A single testosterone value is surprisingly mobile. Daily rhythm, food, sleep, an infection, stress, even the lab's assay shift the number. This is no flaw of the test, but the nature of a living system. That is exactly why guidelines recommend not taking a low value at face value, but confirming it.
Treat low testosterone as a deficiency only after a repeat
Consensus Guideline The Endocrine Society guideline by Shalender Bhasin and colleagues in 2018 in the Journal of Clinical Endocrinology and Metabolism recommends making a diagnosis only in men with matching symptoms and unequivocally and repeatedly low testosterone. As a first step it names morning, fasting total testosterone with a reliable assay, which should be confirmed by a second morning measurement. With a borderline value or altered SHBG it additionally recommends free testosterone by dialysis or calculation. This is a clear call for care rather than a snap judgment.
Bhasin S, Brito JP, Cunningham GR, et al. J Clin Endocrinol Metab. 2018;103(5):1715-1744. doi:10.1210/jc.2018-00229 · PMID: 29562364
That a repeat makes sense also has to do with the lab itself. Different assays do not always give the same number for the same blood sample, especially in the low range that is decisive for the diagnosis.
Assays differ, especially at low values
Method comparison, multiple labs A study by Sung-Eun Cho and colleagues in 2022 in Annals of Laboratory Medicine compared accurate mass spectrometry with a common immunoassay. In the low testosterone range, exactly where the diagnosis of a deficiency falls, the immunoassay showed a systematic upward bias. This fits earlier comparisons in which immunoassays scattered more among each other than mass spectrometry. For you this means: which assay the lab uses can help decide whether a value counts as low or normal.
Cho SE, Han J, Park JH, et al. Ann Lab Med. 2022;43(1):19-28. doi:10.3343/alm.2023.43.1.19 · PMID: 36045053
The same issue was already described in 2009 by a comparison study from Hubert Vesper and colleagues in Steroids, which showed that the scatter between mass spectrometry methods was much smaller than that between immunoassays, and which argued for standardizing the methods (doi:10.1016/j.steroids.2009.01.004, PMID: 19428438). And now you know why the most honest answer to a single low value is often: measure again, same conditions, ideally the same lab.
The level chart: what the reference range reveals and what it does not
On every lab sheet, next to your value, there is a reference range. It describes the span in which most men of a comparison group fall. That is useful as orientation. But a reference range is a statistical comparison, not a biological ideal. And it depends more on the lab and the assay than the clear number suggests.
A harmonized normal range for young, healthy men
Cohorts, n=9054 Thomas Travison and colleagues pooled the data of more than 9000 men from four studies in the US and Europe in 2017 in the Journal of Clinical Endocrinology and Metabolism and calibrated the assays to a reference method. In healthy, non-obese men between 19 and 39 years, the harmonized normal range of total testosterone was about 264 to 916 nanograms per deciliter. A substantial part of the earlier differences between studies came simply from different assays, not from real biological differences.
Travison TG, Vesper HW, Orwoll E, et al. J Clin Endocrinol Metab. 2017;102(4):1161-1173. doi:10.1210/jc.2016-2935 · PMID: 28324103
A small overview serves for rough orientation. The following numbers are not rigid limits, but reference points that vary by lab. They do not replace a medical interpretation.
| Value | Rough orientation | What shifts it |
|---|---|---|
| Total testosterone (young, healthy) | about 264 to 916 ng/dl | time of day, food, lab, age |
| Diagnostic grey zone | often around 8 to 12 nmol/l (about 230 to 350 ng/dl) | SHBG, symptoms, repeat |
| SHBG | lab-dependent, varies by method | weight, thyroid, liver, age |
| Free testosterone | method-dependent, own reference needed | SHBG, calculation formula |
Here lies the most important thought of this article. The reference range describes what is common, not what is optimal for you. If the population average has fallen over the years, the lower bound of normal falls with it. A value can therefore be numerically normal and still be tight for your personal functional optimum.
"My value is within the normal range, so it can't be the testosterone." The normal range is a comparison with other men, not a measure of whether you feel well. Symptoms and value have to be looked at together. A value in the lower normal range can fit your symptoms, but it does not have to. Just as little does a value in the middle of the range prove that everything is fine. The number is one building block, not the whole answer.
LH and FSH: where the low value comes from
Suppose testosterone is low in the morning, fasting and on repeat, and there are matching symptoms. Then the really interesting question only begins: where does this come from? Here the control hormones LH and FSH come into play. They are produced in the pituitary gland and give the testes the signal to produce testosterone and sperm. Their pattern reveals at which level the system is stalling.
Picture the control like a heating system with a thermostat. If testosterone falls, the thermostat, that is LH, should normally call out more strongly. If it does not, the problem lies not in the radiator, but in the control.
Low T, high LH
The brain calls out loudly, but the testes respond weakly. That points to a disturbance of the testes themselves, a primary hypogonadism. Causes can be congenital or acquired, for example after injuries, inflammation or certain diseases. Here the bottleneck sits at the source.
Low T, low or normal LH
Testosterone is low, but the brain does not call out louder. That points to a disturbance of the higher-level control, a secondary hypogonadism. Often obesity, silent inflammation or another burden is behind it. This form is often the one where lifestyle can move a lot.
FSH and fertility
FSH mainly controls sperm production. It becomes important when fatherhood is the goal. A raised FSH with low testosterone can point to a testicular weakness. Anyone who wants to become a father should have this clarified medically before any therapy is started, because some treatments can impair fertility.
Prolactin as a player
A raised prolactin can dampen the control of LH and thereby lower testosterone. That is why prolactin belongs in the workup, especially with a secondary pattern. At the cellular level, a persistently high prolactin signal competes with the normal hormone control and can slow it down.
Why this step counts so much is shown by a large European study that gave the concept of late-onset hypogonadism a clearer shape. It linked low values to a meaningful diagnosis only when matching symptoms were also present.
Low value plus symptoms, not the value alone
Population study, n=3369 Frederick Wu and colleagues analyzed the European Male Aging Study with 3369 men between 40 and 79 years in 2010 in the New England Journal of Medicine. Testosterone was measured in the morning by mass spectrometry, free testosterone was calculated. Only three sexual symptoms showed a reliable link with low values. From this the authors concluded that late-onset hypogonadism is only sensibly defined when low values and matching symptoms come together, not by looking at a number alone.
Wu FCW, Tajar A, Beynon JM, et al. N Engl J Med. 2010;363(2):123-135. doi:10.1056/NEJMoa0911101 · PMID: 20554979
And now you know why a good workup does not stop at testosterone. Only LH, FSH and the whole picture turn a number into a meaningful story.
Three levers for a meaningful test
You can contribute a lot to making your testosterone test meaningful. These three levers are not a treatment plan, but an orientation on how to give your result reliability. The further path you find with medical guidance.
Have it measured in the morning and fasting
Because testosterone is highest in the morning and food can lower it for a short time, the blood draw belongs in the early hours and on an empty stomach. A non-fasting afternoon value can make a healthy man look falsely low. This simple setting alone can prevent a snapshot from becoming a premature diagnosis.
Confirm an abnormal value
A single low value is a reason for a repeat, not for worry. Since daily rhythm, food, infections and the lab itself fluctuate, a low morning value should be confirmed a second time, if possible at the same lab. That way a random outlier cannot distort the whole picture.
See the value in context
A testosterone value alone rarely tells the whole story. It can make sense to place it together with SHBG, LH, FSH and, depending on symptoms, with a blood count, thyroid, iron and blood sugar. That way treatable contributing causes can be found, rather than ascribing everything too quickly to one hormone. Which values make sense in an individual case is clarified by the medical examination.
A value is a beginning, not a verdict
Your testosterone value is a snapshot under certain conditions. When it was measured, whether you were fasting, how much is freely available and which lab did the calculation all belong to the story. When you read your result in this light, the single number loses its dread and gains its true meaning: a building block in the picture, not the picture itself.
Frequently asked questions about the testosterone test
When should you take a testosterone test?
The best time is the early morning, ideally between eight and eleven, and fasting. Testosterone follows a daily rhythm with the highest levels in the morning. On top of that, eating alone, especially a sugar load, can lower the value sharply for a short time. Guidelines therefore recommend a morning, fasting measurement as the first step. If blood is drawn at another time of day or after eating, an otherwise normal man can look falsely low. A single badly timed value tells you little. What matters is the right setting, not just the number.
Why does testosterone have to be measured in the morning and fasting?
Two things move the value in the short term. First, the daily rhythm: a large part of testosterone is produced during sleep and is highest in the morning, then falls. Second, food: in controlled studies a glucose load lowered testosterone by about a quarter on average, and some previously normal men slipped temporarily into the low range. That is why guidelines recommend a morning, fasting measurement. If a low value was not drawn fasting, it can pay to repeat it in the morning on an empty stomach before anyone speaks of a deficiency.
What is the difference between total testosterone and free testosterone?
Total testosterone measures the entire amount in the blood. Most of it, though, is bound to proteins, mainly to sex hormone-binding globulin, SHBG, and to albumin. Only a small, unbound fraction counts as freely available. Free testosterone describes that fraction. As long as SHBG is normal, total testosterone is usually enough. If SHBG shifts, for example with obesity, thyroid disorders or with age, total testosterone can mislead. Free testosterone then helps put the picture in context. It is not a better or worse value, just a different angle.
What does the SHBG value mean in a testosterone test?
SHBG is sex hormone-binding globulin, a transport protein that binds testosterone in the blood. It helps determine how much testosterone stays freely available. A high SHBG, for example with an overactive thyroid, liver disease or in older age, can make total testosterone look high even though the freely available fraction is tight. A low SHBG, often with obesity and insulin resistance, can make total testosterone look low even though the free fraction is still fine. That is why SHBG belongs in the picture once total testosterone is borderline or symptoms and value do not match.
Why should a low testosterone value be measured twice?
A single value fluctuates. Daily rhythm, food, sleep, illness, stress and even the laboratory itself influence the result. Guidelines therefore recommend confirming a low morning, fasting value with a second morning measurement before anyone speaks of a deficiency. That way a random low outlier does not become a premature diagnosis. This caution protects against misclassifying healthy men and treating them too soon. Only when low values and matching symptoms come together repeatedly does the word testosterone deficiency make any sense.
What does a testosterone level chart actually tell you?
A level chart shows a reference range, the span in which most men of a comparison group fall. In harmonized data of healthy, non-obese young men, total testosterone ran roughly from 264 to 916 nanograms per deciliter. The key point: a reference range is a statistical comparison, not a personal ideal. It depends heavily on the laboratory and the assay, which is why the same sample can come out differently in different labs. A chart is an orientation, not a verdict. What stays decisive is whether value, symptoms and the whole picture fit together.
What is the difference between the reference range and the functional optimum?
The reference range describes what is statistically common in a comparison group. The functional optimum, by contrast, means the range in which you personally feel well and capable. The two are not the same. A value can be numerically within the normal range and still be too low for you, especially if the population average has fallen over the years. The other way around, a value in the lower normal range does not automatically mean your symptoms come from it. That is exactly why the question is not only whether a value is normal, but whether value and well-being match.
Why are LH and FSH measured together with testosterone?
LH and FSH are the control hormones from the pituitary gland. They show where a low testosterone production comes from. If testosterone is low and LH is high, that points to a weakness of the testes themselves, a primary hypogonadism. If testosterone is low and LH is normal or low, the problem lies more in the higher-level control in the brain, a secondary hypogonadism, as is often seen with obesity. This distinction changes the further workup considerably. That is why LH and FSH belong to the diagnostics once testosterone is confirmed low, not only at the end.
Does eating affect the testosterone value?
Yes, and more than many people think. In controlled studies a sugar load lowered total testosterone significantly on average, and the value stayed low over two hours. Some previously normal men slipped temporarily into the low range. This happened across all levels of glucose tolerance. For the test this means: a non-fasting measurement can make a healthy man look falsely low. Anyone with a low, non-fasting value should have it repeated in the morning and fasting before any conclusions are drawn.
Which values besides testosterone belong in the workup?
A testosterone value alone rarely explains the whole picture. It can make sense to combine morning total testosterone with SHBG and, where needed, free testosterone, plus the control hormones LH and FSH. Because many symptoms have several causes, a blood count, thyroid values, iron status, blood sugar and prolactin often belong with them. That way treatable contributing causes can be found, from iron deficiency through the thyroid to prolactin. Which values make sense in an individual case is decided by the medical examination. This text does not replace it but supports you in understanding the findings better.
All topics in the cluster "Hormones in Men"
This spoke is part of a larger picture. From here you can go back to the pillar and into the related topics.
- Hormones in Men (overview/pillar)
- Testosterone is falling worldwide (each generation less)
- Testosterone deficiency: symptoms in men
- Raising testosterone naturally
- Testosterone test: understanding your levels
- TRT: testosterone replacement therapy
- Erectile dysfunction: causes
- Loss of libido in men
- Hypogonadism: forms and causes
- Gynecomastia: hormonal causes
- Sperm quality and fertility
- Testosterone boosters: what they do
- Andropause: the male change of life
- Micronutrients for testosterone
- DHT, hair loss and testosterone
- Estrogen in men and aromatase
- Cortisol, stress, sleep and testosterone
- Obesity, insulin and testosterone
- Xenoestrogens in men
- Exercise, strength training and testosterone
- Prolactin and the thyroid in men
Connections to other topics
The deeper context on when a low value really is a deficiency and which causes can lie behind it.
Why the stress axis can dampen testosterone production and how it is interwoven with the control via LH.
How the thyroid can shift SHBG and thereby free testosterone, and why it belongs in the workup.
Iron deficiency can create fatigue and reduced stamina that look like a pure hormone problem.
How hormones work as a connected system, with many parallels to the question of what a single value can tell.
The gut, via the immune system and silent inflammation, helps shape how well the hormone control stays in balance.
Sources and further reading
- Bhasin S, Brito JP, Cunningham GR, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2018;103(5):1715-1744. doi:10.1210/jc.2018-00229 · PMID: 29562364 [Consensus Guideline]
- Wu FCW, Tajar A, Beynon JM, et al. Identification of late-onset hypogonadism in middle-aged and elderly men. N Engl J Med. 2010;363(2):123-135. doi:10.1056/NEJMoa0911101 · PMID: 20554979 [Cohort]
- Travison TG, Vesper HW, Orwoll E, et al. Harmonized Reference Ranges for Circulating Testosterone Levels in Men of Four Cohort Studies in the United States and Europe. J Clin Endocrinol Metab. 2017;102(4):1161-1173. doi:10.1210/jc.2016-2935 · PMID: 28324103 [Cohort]
- Axelsson J, Ingre M, Akerstedt T, Holmbäck U. Effects of acutely displaced sleep on testosterone. J Clin Endocrinol Metab. 2005;90(8):4530-4535. doi:10.1210/jc.2005-0520 · PMID: 15914523 [Cohort]
- Caronia LM, Dwyer AA, Hayden D, Amati F, Pitteloud N, Hayes FJ. Abrupt decrease in serum testosterone levels after an oral glucose load in men: implications for screening for hypogonadism. Clin Endocrinol (Oxf). 2013;78(2):291-296. doi:10.1111/j.1365-2265.2012.04486.x · PMID: 22804876 [Cohort]
- Iranmanesh A, Lawson D, Veldhuis JD. Glucose ingestion acutely lowers pulsatile LH and basal testosterone secretion in men. Am J Physiol Endocrinol Metab. 2012;302(6):E724-E730. doi:10.1152/ajpendo.00520.2011 · PMID: 22252939 [RCT]
- Novaes LF, Flores JM, Benfante N, et al. Analysis of diurnal variation in serum testosterone levels in men with symptoms of testosterone deficiency. J Sex Med. 2024;21(5):408-413. doi:10.1093/jsxmed/qdae026 · PMID: 38481019 [Cohort]
- Kacker R, Hornstein A, Morgentaler A. Free testosterone by direct and calculated measurement versus equilibrium dialysis in a clinical population. Aging Male. 2013;16(4):164-168. doi:10.3109/13685538.2013.835800 · PMID: 24090209 [Cohort]
- Keevil BG, Adaway J. Assessment of free testosterone concentration. J Steroid Biochem Mol Biol. 2019;190:207-211. doi:10.1016/j.jsbmb.2019.04.008 · PMID: 30970279 [Review]
- Cho SE, Han J, Park JH, et al. Clinical Usefulness of Ultraperformance Liquid Chromatography-Tandem Mass Spectrometry Method for Low Serum Testosterone Measurement. Ann Lab Med. 2022;43(1):19-28. doi:10.3343/alm.2023.43.1.19 · PMID: 36045053 [Cohort]
- Vesper HW, Bhasin S, Wang C, et al. Interlaboratory comparison study of serum total testosterone measurements performed by mass spectrometry methods. Steroids. 2009;74(6):498-503. doi:10.1016/j.steroids.2009.01.004 · PMID: 19428438 [Cohort]
- Crawford ED, Poage W, Nyhuis A, et al. Measurement of testosterone: how important is a morning blood draw? Curr Med Res Opin. 2015;31(10):1911-1914. doi:10.1185/03007995.2015.1082994 · PMID: 26360789 [Cohort]