Study analysis · Reproductive Biology and Endocrinology : RB&E · 2020
Men's testosterone levels are dropping – and it's not just because they're getting older or fatter.
A large Israeli study found that testosterone levels in men have declined by about 10% in just over a decade, independent of age and weight.
Overview
What the study found
The study in plain English — the bottom line, every takeaway we extracted, and what to do with them.
In simple terms
This study looked at a big group of men who got their testosterone checked at the doctor over many years. They saw that the numbers went down over time. But because they only looked at those numbers and didn't follow the same men over time, we can't say that something caused the drop, only that it happened.
What’s the bottom line?
Scientists looked at thousands of men's testosterone tests over 13 years and found that on average, testosterone levels have been going down, even when accounting for age.
How strong is this study?
This study is like a snapshot of a lot of men at different times. It's big and uses the same lab, which is good. But it only includes men who already went to the doctor for a testosterone test, and they didn't ask about other things like weight or habits, so we can't be sure the drop is real for all men.
40 / 100
- COI disclosure+40/40
- Data availabilitydata not shared
- Code availabilitycode not shared
25 / 100
- Randomizationnot randomized
- Blindingblinding unclear
- Control groupno control group
- Sample size (n=102334)+20/20
- Follow-upno follow-up reported
100 / 100
23 / 100
- P-values+15/15
- Effect sizeno effect size reported
- Confidence intervalsno confidence intervals
- Pre-registrationnot pre-registered
Each component is scored out of 100 and then capped by the study design — a case series cannot reach the ceiling a randomised trial can, however well it is reported.
Where it sits
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 543 / 100
Probability of being correct
Snapshots of a population at a single point in time, or descriptions of small groups. Can identify correlations and prevalence, but cannot determine cause and effect.
This design cannot establish causation — the findings describe an association, not a cause. Cross-sectional design cannot establish temporality or rule out confounding; it only shows associations at a single point in time, so causal claims are not supported.
No Conflicts
No conflicts of interest identified
No conflicts of interest declared; no external funding was provided for the study.
The authors are from Maccabi Healthcare Services, a state-mandated care provider, but no conflicts were declared. The study appears to be independently conducted with no external financial support.
Key takeaways
- 01
For example, a 21-year-old man in 2006 had about 19.68 nmol/L of testosterone, but a 21-year-old in 2016 had only 17.76 nmol/L.
- 02
This drop was seen across all age groups and happened even though men's weights didn't change.
- 03
This means that if you're a young man today, your testosterone might be lower than it was for men of the same age 10-15 years ago.
- 04
It's a noticeable change that could affect health.
Surprising findings
- The drop in testosterone was independent of age, meaning even men in their 20s saw significant declines.Many assume testosterone decline is just a normal part of aging, but this shows a period effect over and above aging.
- Obesity didn't explain the decline – BMI remained stable over the study period.Obesity is often cited as a major cause of low testosterone, but this study suggests other factors are at play.
Practical takeaways
Men should be aware of their testosterone levels, especially if they're experiencing symptoms like fatigue or low libido, and get tested as part of routine health checks.
This study doesn't tell us how to prevent the decline; it only documents it. Individual lifestyle factors may still be important.
Medium confidenceFor clinicians, consider monitoring testosterone trends in patients, especially younger men, and discuss potential environmental factors.
The decline might be due to unmeasured factors, so clinical decisions should be individualized.
Medium confidenceWhy this study matters
The 10% Cliff: Young Men's Testosterone Drop
At age 21, the peak of testosterone, levels fell from 19.68 nmol/L in 2006–2009 to 17.76 nmol/L in 2016–2019 – a drop of about 10%. This decline was consistent across all age groups, not just older men.
It's counterintuitive – we expect older men to have lower testosterone, but this decline affects even the youngest and healthiest men, potentially impacting fertility and vitality.
Obesity Isn't to Blame
The study found no meaningful change in BMI over the same period (η²=0.001), ruling out weight gain as a cause. This challenges the common assumption that rising obesity explains falling testosterone.
It points to other environmental or lifestyle factors, which is a burning question for many.
A Global Trend? From Israel to the World
This Israeli study mirrors earlier declines in the US and Europe from the 1970s to 2000s. The consistency suggests a broader, multi-decade trend in developed nations.
It suggests something systematic is happening to men's hormonal health, not just a local phenomenon.
But Wait – Is It Real or Just Who Gets Tested?
The study included only men referred for testosterone tests, which might introduce selection bias. However, the decline remained significant even when restricting to normal-range results, suggesting it's not just a testing artifact.
This adds nuance to the findings and sparks debate about the true population-level decline.
Want the whole report?
Detailed mode opens the full scientific breakdown — every score component, the methodology, conflicts of interest, the evidence analysis behind each claim, and the raw study data.
Overview
What the study found
The study in plain English — the bottom line, every takeaway we extracted, and what to do with them.
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
Scientists looked at thousands of men's testosterone tests over 13 years and found that on average, testosterone levels have been going down, even when accounting for age.
Research results
For example, a 21-year-old man in 2006 had about 19.68 nmol/L of testosterone, but a 21-year-old in 2016 had only 17.76 nmol/L. This drop was seen across all age groups and happened even though men's weights didn't change.
What this means - more context
This means that if you're a young man today, your testosterone might be lower than it was for men of the same age 10-15 years ago. It's a noticeable change that could affect health.
To examine secular trends in testosterone levels in Israeli men in the first and second decades of the 21st century.
The study found a significant age-independent decline in total testosterone levels among Israeli men from 2006 to 2019, with levels falling by about 10% at peak age 21. The decline was not explained by obesity trends.
Methods Used
Cross-sectional analysis of 102,334 first-time total testosterone tests from men aged 13-80 in a large Israeli health provider, Maccabi Healthcare Services, from 2006 to 2019. Age and year effects were modeled using regression.
Main Finding
Total testosterone levels declined significantly (p<0.001) across most age groups, with an age-independent trend. At age 21, levels fell from 19.68 nmol/L in 2006-2009 to 17.76 nmol/L in 2016-2019, a ~10% drop. BMI did not change meaningfully (η²=0.001).
Confidence Level
Moderate. Large sample size but cross-sectional design with potential selection bias (referred patients).
Study Flags
Red Flags
- •Selection bias - participants were referred for testing
- •No adjustment for BMI
- •Single laboratory measures
No biological mechanisms were identified in this study. This may be an epidemiological, observational, or survey-based study that reports associations rather than proposing causal biological pathways.
Surprising Findings
The drop in testosterone was independent of age, meaning even men in their 20s saw significant declines.
Many assume testosterone decline is just a normal part of aging, but this shows a period effect over and above aging.
Practical Takeaways
Men should be aware of their testosterone levels, especially if they're experiencing symptoms like fatigue or low libido, and get tested as part of routine health checks.
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 543 / 100
Probability of being correct
Snapshots of a population at a single point in time, or descriptions of small groups. Can identify correlations and prevalence, but cannot determine cause and effect.
Human Cross-Sectional
Subject
Moderate probability
on the GRADE evidence scale
This study looked at a big group of men who got their testosterone checked at the doctor over many years. They saw that the numbers went down over time. But because they only looked at those numbers and didn't follow the same men over time, we can't say that something caused the drop, only that it happened.
No conflicts of interest were detected in this study. No score impact.
Strengths
- Large sample size
- Single central laboratory
- Long study period
Weaknesses
- Cross-sectional design
- Selection bias
- Lack of individual-level confounder data
Methodology
Evidence Keywords
Statistical Reporting
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
Scientists looked at thousands of men's testosterone tests over 13 years and found that on average, testosterone levels have been going down, even when accounting for age.
Research results
For example, a 21-year-old man in 2006 had about 19.68 nmol/L of testosterone, but a 21-year-old in 2016 had only 17.76 nmol/L. This drop was seen across all age groups and happened even though men's weights didn't change.
What this means - more context
This means that if you're a young man today, your testosterone might be lower than it was for men of the same age 10-15 years ago. It's a noticeable change that could affect health.
To examine secular trends in testosterone levels in Israeli men in the first and second decades of the 21st century.
The study found a significant age-independent decline in total testosterone levels among Israeli men from 2006 to 2019, with levels falling by about 10% at peak age 21. The decline was not explained by obesity trends.
Methods Used
Cross-sectional analysis of 102,334 first-time total testosterone tests from men aged 13-80 in a large Israeli health provider, Maccabi Healthcare Services, from 2006 to 2019. Age and year effects were modeled using regression.
Main Finding
Total testosterone levels declined significantly (p<0.001) across most age groups, with an age-independent trend. At age 21, levels fell from 19.68 nmol/L in 2006-2009 to 17.76 nmol/L in 2016-2019, a ~10% drop. BMI did not change meaningfully (η²=0.001).
Confidence Level
Moderate. Large sample size but cross-sectional design with potential selection bias (referred patients).
Study Flags
Red Flags
- •Selection bias - participants were referred for testing
- •No adjustment for BMI
- •Single laboratory measures
No biological mechanisms were identified in this study. This may be an epidemiological, observational, or survey-based study that reports associations rather than proposing causal biological pathways.
Surprising Findings
The drop in testosterone was independent of age, meaning even men in their 20s saw significant declines.
Many assume testosterone decline is just a normal part of aging, but this shows a period effect over and above aging.
Practical Takeaways
Men should be aware of their testosterone levels, especially if they're experiencing symptoms like fatigue or low libido, and get tested as part of routine health checks.
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 543 / 100
Probability of being correct
Snapshots of a population at a single point in time, or descriptions of small groups. Can identify correlations and prevalence, but cannot determine cause and effect.
Human Cross-Sectional
Subject
Moderate probability
on the GRADE evidence scale
This study looked at a big group of men who got their testosterone checked at the doctor over many years. They saw that the numbers went down over time. But because they only looked at those numbers and didn't follow the same men over time, we can't say that something caused the drop, only that it happened.
No conflicts of interest were detected in this study. No score impact.
Strengths
- Large sample size
- Single central laboratory
- Long study period
Weaknesses
- Cross-sectional design
- Selection bias
- Lack of individual-level confounder data
Methodology
Evidence Keywords
Statistical Reporting
Scoring
How strong is this study?
This study is like a snapshot of a lot of men at different times. It's big and uses the same lab, which is good. But it only includes men who already went to the doctor for a testosterone test, and they didn't ask about other things like weight or habits, so we can't be sure the drop is real for all men.
40 / 100
- COI disclosure+40/40
- Data availabilitydata not shared
- Code availabilitycode not shared
25 / 100
- Randomizationnot randomized
- Blindingblinding unclear
- Control groupno control group
- Sample size (n=102334)+20/20
- Follow-upno follow-up reported
100 / 100
23 / 100
- P-values+15/15
- Effect sizeno effect size reported
- Confidence intervalsno confidence intervals
- Pre-registrationnot pre-registered
Each component is scored out of 100 and then capped by the study design — a case series cannot reach the ceiling a randomised trial can, however well it is reported.
Where it sits
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 543 / 100
Probability of being correct
Snapshots of a population at a single point in time, or descriptions of small groups. Can identify correlations and prevalence, but cannot determine cause and effect.
This design cannot establish causation — the findings describe an association, not a cause. Cross-sectional design cannot establish temporality or rule out confounding; it only shows associations at a single point in time, so causal claims are not supported.
No Conflicts
No conflicts of interest identified
No conflicts of interest declared; no external funding was provided for the study.
The authors are from Maccabi Healthcare Services, a state-mandated care provider, but no conflicts were declared. The study appears to be independently conducted with no external financial support.