Story
Testosterone & Healthy Aging in Men
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In one pass How much testosterone you make is not the testes' own decision.
Educational content, not medical advice — consult a clinician.
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Chapter 1
How testosterone is controlled
How much testosterone you make is not the testes' own decision. It is governed by a self-balancing loop between the brain and the testes.
The hypothalamus, at the base of the brain, releases gonadotropin-releasing hormone (GnRH) in pulses, prompting the pituitary to send two orders into the blood: (LH) and (FSH). LH tells the testes to make testosterone; FSH works on sperm production. Once there is enough testosterone in the blood, the upstream eases off. This is called negative feedback.
This loop runs through everything else in this story. Its most important consequence: testosterone added from outside makes the upstream think there is already enough, so your own testosterone production and sperm development are both shut down. How testosterone falls with age, how to test it, and whether supplements and treatment are worth it — all of it comes back to this loop.
The hypothalamus, at the base of the brain, releases gonadotropin-releasing hormone (GnRH) in pulses, prompting the pituitary to send two orders into the blood: (LH) and (FSH). LH tells the testes to make testosterone; FSH works on sperm production. Once there is enough testosterone in the blood, the upstream eases off. This is called negative feedback.
This loop runs through everything else in this story. Its most important consequence: testosterone added from outside makes the upstream think there is already enough, so your own testosterone production and sperm development are both shut down. How testosterone falls with age, how to test it, and whether supplements and treatment are worth it — all of it comes back to this loop.
Mechanism · How the command line runs
Once every station on this line has a name, you will not need to memorize why testosterone from outside shuts down the testes and sperm production.The hypothalamus does not signal continuously; it releases gonadotropin-releasing hormone (GnRH) in pulses. The pituitary responds by sending out two orders: (LH) and (FSH). LH travels in the blood to the testes and acts on a type of cell called Leydig cells, the workshop where testosterone is made. FSH acts on Sertoli cells, where sperm develop.
Once testosterone rises, it turns back and holds down the hypothalamus and pituitary, turning the upstream signal down. That is negative feedback: a loop that finds its own balance, not testes that make as much as they like.
When testosterone from outside enters the blood, the upstream reads there is already enough, and GnRH, LH and FSH are all held down together. With no orders coming in, the Leydig cells stop work, and the sperm line in the Sertoli cells stops with them. This is the mechanism behind the fertility cost described in the chapter Pros and cons of testosterone therapy: the drug does not poison the testes; an outside signal switches this loop's lights off.
This story has a companion (see Andropause). That story explains in more detail why true late-onset hypogonadism affects only a minority of men, and the five kinds of cause that can be reversed; this one lays out the mechanism, the testing and the marketing claims one by one.
Mechanism · What testosterone actually does
What testosterone is responsible for in a man's body is specific. It is not a fuel where more means more manly.The cluster most firmly tied to it is libido, erectile function and morning erections (EMAS, Wu 2010).
Several other things are linked to it, but do not depend on it alone:
Muscle mass and strength: testosterone promotes muscle-protein synthesis, but training and protein intake matter just as much (see Exercise as medicine).: part of its effect comes from aromatase in bone converting testosterone into estrogen, and estrogen then protects bone mass.Red-cell production: testosterone drives blood production, which is also why testosterone replacement therapy raises hematocrit (the share of the blood made up of red cells).Mood and energy: there is an effect, but it is far weaker than marketing claims, and the causes are many — sleep, depression and chronic illness often matter more. In the large testosterone trials, vitality did not improve significantly.
So testosterone is one hormone inside a finely regulated loop, and what it does is specific. Brain fog, trouble losing weight and feeling drained all over usually have other causes. The later chapters go through which symptoms really track it.
Chapter 2
A slow decline, not a sudden drop
The fall in men's testosterone with age is a gentle slope over decades — not a cliff within a few years, and not a male version of menopause.
The most-cited figures come from the Massachusetts Male Aging Study (MMAS, Feldman 2002), which followed the same group of middle-aged men over time: total testosterone fell by about 1.6% a year on average, and bioavailable testosterone (the part not tightly held by a transport protein, which the body can use) by about 2–3% a year. If you simply compare men of different ages at a single point in time, the decline looks gentler. Most healthy men still have measurable testosterone at 70–80, often still in the normal range.
This matters because telling the slope as a cliff turns normal aging into a disease that everyone gets and everyone must treat.
The most-cited figures come from the Massachusetts Male Aging Study (MMAS, Feldman 2002), which followed the same group of middle-aged men over time: total testosterone fell by about 1.6% a year on average, and bioavailable testosterone (the part not tightly held by a transport protein, which the body can use) by about 2–3% a year. If you simply compare men of different ages at a single point in time, the decline looks gentler. Most healthy men still have measurable testosterone at 70–80, often still in the normal range.
This matters because telling the slope as a cliff turns normal aging into a disease that everyone gets and everyone must treat.
Numbers · Two ways to measure, two rates of decline
Male menopause is a popular phrase, but it frames male hormonal aging as a copy of female menopause. That is wrong, and the error is used to sell a lot of things.The real data come from the Massachusetts Male Aging Study (MMAS, Feldman 2002, *JCEM*), a randomly sampled population cohort in the Boston area of the US that measured the same men twice. It gave rates by two methods at once:
Following the same man over time (longitudinal): total testosterone fell by about 1.6% a year on average, and bioavailable testosterone by about 2–3% a year.Comparing men of different ages at one point in time (cross-sectional): in the follow-up round, total testosterone fell by about 0.8% per year of age, and free testosterone by about 1.7–2%.
The cross-section compares different men of different ages, not how fast one man falls, so the two sets of figures should not be mixed. Free testosterone falls faster than total because, with age, (SHBG) in the blood rises by about 1.6% a year and holds on to more testosterone.
The same study made two more findings worth remembering. Men in apparent good health (no chronic illness, no prescription medication, no obesity, no heavy drinking) had noticeably higher levels of several androgens. And the decline within the same man was steeper than the cross-section; the authors suggest that illness appearing during those years speeds the decline.
The popular line testosterone falls 1–2% a year after 30 is a rough summary of data of this kind; MMAS itself followed men from middle age onward. The key word is slow: a slide over decades, not a cliff within a few years. Most healthy men still have measurable testosterone at 70–80, often still in the normal range — completely unlike the clear endpoint in women of 12 months in a row without a period. The contrast on the female side is estrogen swinging wildly for a few years and then falling off a cliff (see Perimenopause).
Myth · Do men have a menopause-style cliff?
The cliff narrative is dangerous because it invents a moment of disease that everyone reaches and everyone must treat, dressing normal aging up as a defect that needs intervention.The reality: compared with the same man at 35, a healthy 65-year-old has on average 25–50% lower testosterone. That in itself is a physiological change, not necessarily disease. Whether it counts as disease depends on the two conditions set out in the chapter What counts as truly low testosterone: matching symptoms, plus repeatedly confirmed low levels.
There is one more layer: a decline across generations at the population level. Data from the same Massachusetts Male Aging Study (Travison 2007, an observational cohort) found that at the same age and in the same area, men measured in later years had lower testosterone, and that this fall was larger than the decline with age. The cause is not clear: the changes the researchers measured, such as in smoking and obesity, did not explain it; it may relate to differences between birth cohorts, or to health or environmental factors that were not measured.
This is not a reason to take testosterone. It describes a shift in a generation's average, and says nothing about whether any one man is deficient. For an individual, the levers that really move things are still weight, sleep and metabolic health, not pills.
So the fall of testosterone with age in men is a real, slow slide, but male menopause is the wrong name for it. Presenting a slow physiological change as a cliff-edge disease is exactly the opening move of the testosterone-clinic business.
travison-2007-population-testosterone-decline
Chapter 3
How to test testosterone properly
Most of the testosterone in blood is held by transport proteins; only about 1–2% is free and able to enter cells and act directly. Drawing a conclusion from a single number is the most common source of a mistaken low testosterone verdict.
Testosterone is highest in the morning and can be 30–40% lower by the afternoon; the same man tested two days apart might read 600 once and 350 the next time. Total testosterone is also pulled around by a transport protein, (SHBG): when SHBG is low, the total looks low while the usable share may not be short at all. That is why the Endocrine Society guideline asks for a fasting morning blood draw, at least two tests, and both low before it counts; a borderline value should also be combined with SHBG to calculate free testosterone, with an eye on the error of the lab method itself.
Testosterone is highest in the morning and can be 30–40% lower by the afternoon; the same man tested two days apart might read 600 once and 350 the next time. Total testosterone is also pulled around by a transport protein, (SHBG): when SHBG is low, the total looks low while the usable share may not be short at all. That is why the Endocrine Society guideline asks for a fasting morning blood draw, at least two tests, and both low before it counts; a borderline value should also be combined with SHBG to calculate free testosterone, with an eye on the error of the lab method itself.
In practice · When to test, and how many times
The daily rhythm: testosterone is highest in the morning, around 7–10 am, and can be 30–40% lower in the afternoon or evening. A low afternoon reading may just be the day's natural low point. That is why the clinical guideline (Endocrine Society, Bhasin 2018) asks for a fasting morning blood draw.One test is not enough: testosterone itself varies a great deal, and the same man might read 600 one day and 350 the next. The guideline asks for at least 2 morning draws, a few weeks apart, both low before it counts. Do not test during an acute illness, soon after major surgery, or on a day of extreme training.
What to measure: a reasonable first workup includes morning total testosterone, (SHBG), calculated free testosterone, and (LH) plus (FSH). The last two tell apart a problem in the testes themselves from a problem upstream in the brain. Measuring one total testosterone and then prescribing is not what the guideline does.
Mechanism · Why total testosterone can mislead
Most of the testosterone in blood is tightly held by (SHBG), some hangs loosely on albumin, and only about 1–2% is fully free and able to enter cells and act. Free testosterone is the biologically active share.SHBG is shifted by many factors, which can make the total testosterone figure misleading:
Low SHBG (obesity, insulin resistance, type 2 diabetes, fatty liver, an underactive thyroid): total testosterone looks low, while free testosterone may be normal. This explains why men with obesity or diabetes can have low total testosterone without a true deficiency (see Type 2 Diabetes & Prediabetes).High SHBG (older age, an overactive thyroid, cirrhosis of the liver, HIV infection): total testosterone looks normal or even high, while free testosterone may be too low.
So when total testosterone is borderline, measure SHBG at the same time and calculate free testosterone, rather than reading one total figure.
The lab method is a trap too: cheap direct immunoassays are inaccurate in the low range. Liquid chromatography–tandem mass spectrometry (LC-MS/MS) is the reference method, and its results agree better between labs. Do not diagnose from a single low immunoassay result.
So the answer to what is my testosterone depends on when it was measured, how many times, whether it was total or free testosterone, and what SHBG was doing. A single isolated number tells you almost nothing.
Chapter 4
What counts as truly low testosterone
A shortage of testosterone that appears with age is called late-onset hypogonadism (LOH) in medicine. For it to count, two things must hold at the same time: matching symptoms, and testosterone that is low on repeated testing. A low number alone does not count, and symptoms alone do not count either.
The European Male Ageing Study (EMAS, Wu 2010) used a stricter research definition: low testosterone together with three sexual symptoms. By that standard, only about 2% of men aged 40–79 truly qualify. Many men have low numbers and no symptoms; many others feel tired, foggy and unable to lose weight while their testosterone is completely normal.
In older men whose symptoms match and whose testosterone really is low, the most certain gain from testosterone treatment is better sexual function. Vitality and energy, the things people most want, did not improve significantly in the large trials.
The European Male Ageing Study (EMAS, Wu 2010) used a stricter research definition: low testosterone together with three sexual symptoms. By that standard, only about 2% of men aged 40–79 truly qualify. Many men have low numbers and no symptoms; many others feel tired, foggy and unable to lose weight while their testosterone is completely normal.
In older men whose symptoms match and whose testosterone really is low, the most certain gain from testosterone treatment is better sexual function. Vitality and energy, the things people most want, did not improve significantly in the large trials.
Evidence · How EMAS defined true testosterone deficiency
Judging late-onset hypogonadism (LOH) has two parts that must both be met: testosterone that stays low, plus symptoms that match it. Only both together count. This is the single most important sentence in this story.The Endocrine Society's 2018 guideline (Bhasin 2018) states it this way: diagnose only when there are symptoms and signs of testosterone deficiency and total or free testosterone in the blood is clearly and repeatedly low.
The European Male Ageing Study (EMAS, Wu 2010, *NEJM*) is a population study of middle-aged and older men in Europe. In nearly 3,400 men it screened a long list of candidate symptoms and found that the ones that clustered with low testosterone were 3 sexual symptoms: reduced libido, fewer morning erections and weaker erectile function. It used low testosterone plus these 3 as its research definition, and only about 2% of men (aged 40–79) met it. Note that the 3 symptoms are EMAS's research definition; the guideline does not require all 3.
Many men sit in the low part of the normal range with no symptoms at all and need no treatment; many others have a pile of symptoms (tiredness, brain fog, trouble losing weight) with normal testosterone, and those symptoms have other causes.
That is why a low reading means you should supplement is wrong: low testosterone by itself is not a reason to treat. Symptomatic, repeatedly confirmed low testosterone is.
Evidence · What the testosterone trials showed
The Testosterone Trials (TTrials) are the most rigorous set of to date, and Snyder 2016 is their main paper in the *NEJM*. It enrolled about 790 men over 65 who had symptoms and genuinely low testosterone (< 275 ng/dL), and treated them with testosterone gel or placebo for 1 year. Its value is in marking out the real limits of what testosterone replacement therapy does, and the list below covers only the outcomes measured in this paper.Sexual function (libido, erections, satisfaction): statistically significant improvement — the most certain benefit of testosterone replacement therapy.Mood and depressive symptoms: slightly better than placebo.Vitality: no significant benefit. Do not describe this paper as if vitality also improved a little.Walking and physical function: small improvements overall, and inconsistent results.
This main paper reported neither cognition nor ; those appear in separate TTrials papers published later. Do not use Snyder 2016 to claim testosterone cures brain fog, or that it builds bone.
The key limits: the participants were specially selected older men with symptoms and confirmed low testosterone, so the findings cannot be extended to men with normal or borderline testosterone who simply want more energy. The trial was also neither long nor large enough to judge long-term heart and prostate safety — the question the later TRAVERSE trial set out to answer. What it showed is it helps sexual function in the right men, not testosterone is an elixir of youth.
In practice: if you suspect late-onset hypogonadism, first test by the rules in the chapter How to test testosterone properly (morning, repeated, total plus free testosterone plus , and ), and at the same time look for causes that can be reversed (obesity, sleep apnea, chronic stress, long-term opioid painkillers, depression). If you do not meet persistently low plus sexual symptoms, it is not late-onset hypogonadism, and there is no need to rush into medication. The full list of the 5 reversible causes: see Andropause.
Chapter 5
Lifestyle beats testosterone boosters
The things that really move testosterone work through the body's own loop: fat, sleep, breathing, training and stress. Most testosterone boosters on the shelf have no human evidence that they raise testosterone.
Fat around the organs contains an enzyme called aromatase that converts testosterone into estrogen; testosterone production peaks during night-time sleep; and in population studies, men with moderate to severe sleep apnea have lower testosterone. These are real switches on the loop.
Now look at the supplements: a study that examined 50 testosterone-booster products sold online found that 90% claimed to raise testosterone, yet among their ingredients only about a quarter had any research data showing a rise, and more than three in five had no data at all. No food or supplement tops testosterone up by magic.
Fat around the organs contains an enzyme called aromatase that converts testosterone into estrogen; testosterone production peaks during night-time sleep; and in population studies, men with moderate to severe sleep apnea have lower testosterone. These are real switches on the loop.
Now look at the supplements: a study that examined 50 testosterone-booster products sold online found that 90% claimed to raise testosterone, yet among their ingredients only about a quarter had any research data showing a rise, and more than three in five had no data at all. No food or supplement tops testosterone up by magic.
Myth · Do testosterone-booster ingredients work?
Low testosterone, always tired, want to feel more manly — this story is so common that over-the-counter testosterone boosters became marketing's most fertile ground. First, what does not work.At the product level: Clemesha 2020 (*World Journal of Men's Health*) found 50 testosterone-booster supplements through a Google search, checked each one's ingredients and claims, and then searched PubMed for studies of every ingredient. 90% of the products claimed to raise testosterone. Of 109 different ingredients, only 24.8% had research data showing a rise in testosterone, 10.1% actually had data showing a fall, 18.3% had data showing no change, and 61.5% had no data at all. Many products contained B vitamins and zinc far above the recommended daily amount, and 13 products had ingredients above the US tolerable upper intake level (zinc, niacin, magnesium).
Single ingredients:
D-aspartic acid: a double-blind (Melville 2017, *PLOS One*) gave 22 young, trained men 6 g a day for 12 weeks alongside structured strength training. Neither total nor free testosterone changed, and actually fell in the supplement group. Earlier positive results in untrained men did not repeat in trained men.Tribulus, dehydroepiandrosterone (DHEA), zinc-magnesium blends (ZMA): several trials were negative or inconsistent; not recommended as testosterone boosters.Ashwagandha and fenugreek: a few small trials report a mild signal, but the samples are small and the results inconsistent — nowhere near a testosterone miracle. Treat them at most as a limited add-on, not the core (the same restraint applies to ashwagandha's anti-anxiety effect; see Chronic Stress).Zinc and vitamin D: only if you are actually deficient can filling the gap help testosterone; taking more when you are not deficient does not raise it further. Do not misread correcting a deficiency as the more you take, the higher it goes.
In practice · Five levers that actually move testosterone
What really supports a healthy testosterone level is the handful of things that can move this loop at the level of mechanism. Their effects on testosterone come mostly from observational studies and small trials, but each one's benefit to health stands on its own.Managing weight, especially belly fat: aromatase in the fat around the organs converts testosterone into estrogen, and estrogen in turn holds down the upstream, pulling testosterone lower still. In population follow-up data, the more weight men lost, the more their testosterone rose; losing 5–10% of body weight usually brings testosterone back up somewhat, and it is the highest-return move (for why losing weight is hard but worth it, see The Genetics of Weight — It Isn't Just Willpower).Sleep: testosterone production peaks during night-time sleep, and chronic short sleep pushes it down. Getting back to 7–9 hours a night is the baseline (see Sleep Architecture & Sleep Debt).Treating sleep apnea: in observational studies, men with moderate to severe obstructive sleep apnea () have lower testosterone; but after treatment with a breathing machine (), changes in testosterone have been inconsistent across studies. Treating OSA is well worth it for its own sake (for how common it is, see the global estimate in Benjafield 2019); just do not treat it as a way to raise testosterone.Strength training: regular strength training preserves muscle and improves metabolic health — an underrated habit that is friendly to testosterone (see Exercise as medicine).Managing chronic stress: persistently high cortisol holds down the hypothalamic-pituitary-gonadal axis (HPG axis) (see Chronic Stress).
No food or supplement tops testosterone up by magic. What really moves it is weight, sleep, sleep apnea, training and stress; here, lifestyle carries far more weight than pills. Spending money on testosterone boosters is paying for a marketing story.
Chapter 6
Pros and cons of testosterone therapy
Testosterone replacement therapy () is a prescription drug, not an anti-aging supplement. Testosterone added from outside travels the negative-feedback loop described at the start of this story and switches off and together; your own testes stop producing, and your sperm count falls with them.
Its genuine indications are narrow. The largest safety trial to date, TRAVERSE, enrolled 5,246 men aged 45–80 with testosterone < 300 ng/dL and raised cardiovascular risk. Major adverse cardiovascular events (: heart attack, stroke and cardiovascular death) were no more common than on placebo, but atrial fibrillation, pulmonary embolism and acute kidney injury were more common. So it can be used in the right men, but it is not safe to use casually. Men who want children should be especially careful not to start it directly.
Its genuine indications are narrow. The largest safety trial to date, TRAVERSE, enrolled 5,246 men aged 45–80 with testosterone < 300 ng/dL and raised cardiovascular risk. Major adverse cardiovascular events (: heart attack, stroke and cardiovascular death) were no more common than on placebo, but atrial fibrillation, pulmonary embolism and acute kidney injury were more common. So it can be used in the right men, but it is not safe to use casually. Men who want children should be especially careful not to start it directly.
Clinical · Who actually qualifies for therapy
Testosterone replacement therapy does work when there is a genuine indication, but it is a prescription drug, not an anti-aging supplement. This chapter lays out its benefits, its risks and its conflict with fertility plainly — neither selling it nor scaring you.The genuine indications (Endocrine Society, Bhasin 2018):
Classical hypogonadism: for example Klinefelter syndrome, structural disease of the pituitary or testes, or damage after chemotherapy. This is a clear indication.Late-onset hypogonadism (LOH): repeatedly confirmed low testosterone, plus matching sexual symptoms, with reversible causes already dealt with — only then is it considered, under specialist evaluation.Just wanting more energy or to feel young again, without meeting the criteria: the evidence-based answer is to get the lifestyle basics solid first (see the chapter Lifestyle beats testosterone boosters), not to get a shot at a testosterone clinic.
Evidence · What the largest safety trial found
For heart safety, look to the largest safety to date: TRAVERSE (Lincoff 2023, *NEJM*). It enrolled 5,246 men aged 45–80 with symptoms, testosterone < 300 ng/dL and raised cardiovascular risk, and compared testosterone gel absorbed through the skin with placebo.The primary outcome was major adverse cardiovascular events (: heart attack, stroke, cardiovascular death), and the two groups did not differ significantly. In other words, when testosterone is used with a proper indication, the earlier worry that testosterone replacement therapy raises cardiovascular risk was not confirmed by this trial.
But it is not entirely harmless: the testosterone group had slightly more atrial fibrillation and other heart-rhythm problems, pulmonary embolism and acute kidney injury. What this trial shows is no increase in major heart events in the right men, not safe to use casually.
Safety · The costs that get played down
Several costs that marketing tends to play down all hang on the loop described at the start of this story.Suppressed fertility, the most important one: testosterone from outside switches off and through negative feedback, the testes stop producing, and the sperm count drops sharply — often very low, and in some men to zero. Men who want children should not start testosterone replacement therapy directly; they should first discuss options that preserve sperm production with a specialist, such as human chorionic gonadotropin (hCG) or clomiphene.Too many red blood cells: testosterone drives blood production, and a hematocrit that is too high raises the risk of blood clots, so it needs regular monitoring, and blood may need to be taken off.Prostate: known prostate cancer is a contraindication, and (PSA) should be monitored during treatment.It often becomes long-term treatment: stopping the outside testosterone is easy, but getting an upstream that has been idle for a long time to run on its own again takes months or longer, and a return to the old level is not guaranteed. So once they start, many men stay on it long term.
In practice · Suspect low testosterone? Go in order
If you suspect low testosterone, go in this order1. Sexual symptoms and suspected low testosterone: first test by the rules (morning, repeated, total plus free testosterone plus , and and ).
2. At the same time, deal with causes that can be reversed, for 6-12 months: lose belly fat, sleep well, get sleep apnea checked and treated, lower stress, and have your doctor review the medicines you take (the list of five reversible causes: see Andropause).
3. Still meeting persistently low plus sexual symptoms: see an endocrinologist or urologist for a full evaluation and long-term follow-up; do not go to an online testosterone clinic, and do not buy the drug and use it yourself.
4. If you want children, tell your doctor from the start and take the path that preserves sperm production.
5. Not meeting the criteria: get the lifestyle basics solid — that is the real lever for healthy aging.
This is not a diagnosis and cannot replace your doctor. Whether to take medication, and whether to use testosterone replacement therapy, should be decided together with your doctor. See a doctor promptly for a breast lump, loss of part of your field of vision together with headache (the pituitary needs checking), an unexplained rise in , or infertility together with low testosterone.
Stories connected to this one:
see Andropause: the companion story, with more detail on measurement traps, the five reversible causes and a 6-month lifestyle plansee Perimenopause: the contrast on the female side, a true cliff, where men have a slow slidesee Sleep Architecture & Sleep Debt, see Exercise as medicine, see Chronic Stress: the lifestyle levers that genuinely move testosteronesee The Genetics of Weight — It Isn't Just Willpower: why losing fat is hard, yet the highest-return move for testosteronesee Type 2 Diabetes & Prediabetes: how insulin resistance and obesity make total testosterone look low through SHBG
A slow fall in testosterone is a real physiological change; but male menopause is the wrong name for it, testosterone boosters are mostly empty talk, and testosterone replacement therapy is a prescription drug with narrow indications, not an anti-aging elixir. Know the mechanism, and you will neither panic nor get fleeced.
References · 8
- Wu, F. C. W., Tajar, A., Beynon, J. M., Pye, S. R., Silman, A. J., Finn, J. D., et al. (2010). Identification of late-onset hypogonadism in middle-aged and elderly men. The New England Journal of Medicine, 363(2), 123-135. 10.1056/NEJMoa0911101
- Feldman, H. A., Longcope, C., Derby, C. A., Johannes, C. B., Araujo, A. B., Coviello, A. D., Bremner, W. J., & McKinlay, J. B. (2002). Age trends in the level of serum testosterone and other hormones in middle-aged men: longitudinal results from the Massachusetts Male Aging Study. The Journal of Clinical Endocrinology & Metabolism, 87(2), 589-598. Follow-up cross-section: total T ~0.8%/yr, free T ~1.7%/yr. Longitudinal (same men): total T 1.6%/yr, free T 2.8%/yr, bioavailable T 2-3%/yr. 10.1210/jcem.87.2.8201
- Bhasin, S., Brito, J. P., Cunningham, G. R., Hayes, F. J., Hodis, H. N., Matsumoto, A. M., Snyder, P. J., Swerdloff, R. S., Wu, F. C., & Yialamas, M. A. (2018). Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. The Journal of Clinical Endocrinology & Metabolism, 103(5), 1715-1744. 10.1210/jc.2018-00229
- Snyder, P. J., Bhasin, S., Cunningham, G. R., Matsumoto, A. M., Stephens-Shields, A. J., Cauley, J. A., et al. (2016). Effects of testosterone treatment in older men. The New England Journal of Medicine, 374(7), 611-624. Main TTrials paper: sexual function improved; mood and depressive symptoms slightly better than placebo; no significant benefit for vitality. Bone density and cognition are reported in separate TTrials papers, not this one. 10.1056/NEJMoa1506119
- Clemesha, C. G., Thaker, H., & Samplaski, M. K. (2020). 'Testosterone boosting' supplements composition and claims are not supported by the academic literature. The World Journal of Men's Health, 38(1), 115-122. Most OTC testosterone-booster ingredients lack evidence of raising testosterone. Product audit plus literature review, not a trial: 50 'T booster' products found by Google search; 90% claimed to boost testosterone, 50% to improve libido, 48% to make users 'feel stronger'. 109 unique ingredients (mean 8.3 per product). PubMed data: 24.8% showed a rise in T, 10.1% a fall, 18.3% no change, 61.5% no data - the abstract says 'supplements', but the four shares sum to 114.7%, so they read as shares of ingredients. Median doses: vitamin B12 1,291% of the RDA, B6 807.6%, zinc 272%; 13 products exceeded the upper limit (zinc, vitamin B3, magnesium) (abstract, PMID 31385468). 10.5534/wjmh.190043
- Melville, G. W., Siegler, J. C., & Marshall, P. W. M. (2017). The effects of d-aspartic acid supplementation in resistance-trained men over a three month training period: a randomised controlled trial. PLOS ONE, 12(8), e0182630. 6 g/day D-aspartic acid for 12 weeks produced no change in total or free testosterone. 10.1371/journal.pone.0182630
- Benjafield, A. V., Ayas, N. T., Eastwood, P. R., Heinzer, R., Ip, M. S. M., Morrell, M. J., et al. (2019). Estimation of the global prevalence and burden of obstructive sleep apnoea: a literature-based analysis. The Lancet Respiratory Medicine, 7(8), 687-698. 10.1016/S2213-2600(19)30198-5
- Lincoff, A. M., Bhasin, S., Flevaris, P., Mitchell, L. M., Basaria, S., Boden, W. E., et al. (2023). Cardiovascular safety of testosterone-replacement therapy (TRAVERSE). The New England Journal of Medicine, 389(2), 107-117. In 5,246 men 45-80 with low T and CV risk, TRT was non-inferior for MACE but showed more atrial fibrillation, pulmonary embolism, and acute kidney injury. 10.1056/NEJMoa2215025