Low Testosterone in Men Under 40: Causes, Diagnosis, and Treatment in Australia

Low Testosterone in Men Under 40: Causes, Diagnosis, and Treatment in Australia

The narrative around low testosterone often centres on men in their 50s and 60s. But testosterone deficiency is not an older man's condition. An increasing number of Australian men in their 20s and 30s are presenting with confirmed low testosterone — and many are living with symptoms for years before they're taken seriously.

Is Low Testosterone Common in Young Men?

It's more prevalent than most people realise. While classical hypogonadism (primary testicular failure) is present in roughly 1–2% of young men, a broader category — functional or secondary hypogonadism caused by obesity, sleep disruption, metabolic dysfunction, and lifestyle factors — is substantially more common.

The Endocrine Society estimates that hypogonadism affects approximately 30–50% of men with obesity or type 2 diabetes, conditions no longer exclusive to older age groups. With obesity rates in young Australian men rising, the downstream effects on testosterone are significant.

Why Low Testosterone Presents Differently in Younger Men

The classical signs in older men — decreased libido, erectile dysfunction, reduced muscle mass — are often less pronounced in younger men. In men under 40, lack of energy is the most predictive symptom of testosterone deficiency. Brain fog, difficulty concentrating, emotional flatness, and unexplained fatigue are frequently the presenting complaints.

This means many young men are assessed for depression, burnout, or thyroid dysfunction before testosterone is even considered — often resulting in years of mismanagement.

The Main Causes of Low Testosterone in Men Under 40

1. Obesity and Metabolic Dysfunction

Excess adipose tissue — particularly visceral abdominal fat — increases aromatase activity, converting testosterone to oestradiol. Elevated oestradiol then suppresses the HPG axis, reducing LH output and testosterone production. Men with a BMI over 35 can have testosterone levels up to 50% lower than age-matched men at a healthy weight.

2. Obstructive Sleep Apnoea (OSA)

Testosterone is primarily synthesised during sleep, particularly during REM cycles. OSA disrupts this process, suppressing nocturnal testosterone production. OSA severity is significantly correlated with lower testosterone, independent of obesity and BMI. It is underdiagnosed in young men.

3. Anabolic Steroid Use

The most common preventable cause of low testosterone in men under 35 in Australia. Exogenous anabolic steroids suppress the HPG axis. Men who have used steroids recreationally often present with secondary hypogonadism that persists long after cessation. Recovery of the HPG axis can take months to years.

4. Primary Hypogonadism (Testicular Causes)

Conditions directly damaging the testes include Klinefelter syndrome (XXY karyotype — the most common genetic cause, affecting ~1 in 600 men), mumps orchitis, testicular torsion, testicular cancer treatment, and trauma. Primary hypogonadism is characterised by high LH and FSH with low testosterone and always warrants endocrinologist investigation.

5. Hyperprolactinaemia

A pituitary adenoma (prolactinoma) causes elevated prolactin, which suppresses GnRH and results in secondary hypogonadism. This is typically identified on a standard pre-TRT prolactin blood test and is a reversible cause — dopamine agonist treatment resolves the prolactinoma and restores testosterone in most cases.

6. Chronic Stress, Poor Sleep, and Overtraining

Chronically elevated cortisol suppresses the HPG axis. While rarely the sole cause of clinically significant deficiency, these factors are common contributors that warrant attention alongside medical management.

Diagnosis in Men Under 40: Specific Considerations

Two morning testosterone measurements, with a full panel including LH, FSH, prolactin, and SHBG.

• High LH and FSH + low testosterone: Primary hypogonadism. Endocrinologist referral appropriate.
• Low/normal LH and FSH + low testosterone: Secondary hypogonadism. Investigate reversible causes before committing to TRT.

Treatment Considerations for Men Under 40

For men with reversible secondary hypogonadism: Lifestyle intervention — weight loss, OSA treatment, reduced steroid use, improved sleep — should always be attempted first.

For men requiring pharmacological treatment:

• HCG monotherapy or SERMs (clomiphene): Preferred in men who want to preserve fertility
• Standard TRT: Appropriate when fertility is not a current concern and reversible causes have been addressed
• Fertility counselling and sperm banking: Should be discussed with all men under 40 before commencing TRT

Frequently Asked Questions

Can a 25-year-old have low testosterone? Yes. While uncommon, testosterone deficiency can occur at any age. Primary causes and secondary causes all occur in young men.

Will I be on TRT forever if I start in my 30s? Not necessarily. If the cause is reversible, addressing it may restore normal testosterone without lifelong TRT.

Do I need a referral to get tested? No. A GP can arrange the full testosterone pathology panel directly.

TRT GP Australia

TRT GP Australia provides telehealth GP consultations for men of all ages, with no referral required. We arrange pathology, review results, and discuss treatment options — including fertility-preserving alternatives where appropriate. Join our waitlist when we launch in your state.

References

1. Bhasin S, et al. Testosterone Therapy in Men with Hypogonadism. J Clin Endocrinol Metab. 2018;103(5):1715–1744.
2. Endocrine Society of Australia. Position statement on male hypogonadism. MJA. 2016;205(4).
3. Wu FCW, et al. Identification of Late-Onset Hypogonadism. N Engl J Med. 2010;363:123–135.