Explaining Low Free Testosterone to Your Patient
Tell your patient that free testosterone is the "active" form of testosterone in their blood that's actually available for their body to use, and when it's low, it can cause sexual problems, low energy, and mood changes even when their "total" testosterone level appears normal.
What Free Testosterone Actually Means
Free testosterone represents the small fraction (typically 1-3%) of testosterone that circulates unbound to proteins in the blood. The majority of testosterone is bound to sex hormone-binding globulin (SHBG) or albumin and cannot enter cells to exert its effects 1. Think of it like money: total testosterone is all the money you have, but free testosterone is the cash in your wallet that you can actually spend right now.
Why Free Testosterone Matters More Than Total
The key clinical point is that free testosterone often correlates better with actual symptoms of testosterone deficiency than total testosterone alone 1, 2. This is particularly important because:
- Age increases SHBG levels, which binds more testosterone and reduces the free fraction available to tissues 3
- Obesity decreases SHBG, which can make total testosterone appear low even when free testosterone is adequate 1
- Men with normal total testosterone but low free testosterone commonly experience sexual dysfunction, reduced energy, and other hypogonadal symptoms 1
- Conversely, men with low total testosterone but normal free testosterone (typically younger, obese men) often lack symptoms of androgen deficiency 1
Clinical Presentation to Explain
When discussing with your patient, focus on these symptom patterns:
Symptoms Associated with Low Free Testosterone:
- Sexual dysfunction (erectile problems, reduced desire, fewer orgasms) 2
- Decreased energy and vitality
- Reduced muscle mass and strength
- Mood changes or depressive symptoms 4
- Lower hemoglobin levels 1
A critical threshold appears to exist around 163 pmol/L (approximately 47 pg/mL) for erectile function and 138 pmol/L (approximately 40 pg/mL) for orgasm frequency, below which symptoms markedly worsen 2.
Diagnostic Approach
When to Measure Free Testosterone:
The 2025 European Association of Urology guidelines emphasize that diagnosis requires both persistent symptoms AND confirmed biochemical testosterone deficiency 3. However, the guidelines primarily reference total testosterone thresholds.
Measure calculated free testosterone (cFT) when:
- Total testosterone is borderline (10.5-12 nmol/L or 300-350 ng/dL) 1, 5
- Patient has clear hypogonadal symptoms but normal total testosterone 1
- Patient is older (>60 years) with high SHBG 3
- Patient is obese with low total testosterone but unclear symptomatology 1
Important Caveats:
- Avoid testing during acute illness, as this can artificially lower results 3
- Morning samples are preferred for total testosterone; free testosterone is less affected by diurnal variation
- Only 24.7% of men with borderline total testosterone actually have confirmed hypogonadism when free testosterone is measured 6, preventing overdiagnosis and unnecessary treatment
Treatment Implications
When Free Testosterone Guides Management:
According to the 2020 ACP guidelines, testosterone treatment should only be considered for men with age-related low testosterone who have sexual dysfunction and want to improve sexual function 7. The guidelines recommend:
- Discussing potential benefits, harms, costs, and patient preferences before initiating treatment 7
- Reevaluating symptoms within 12 months and discontinuing if no improvement 7
- NOT treating to improve energy, vitality, physical function, or cognition, as evidence is insufficient 7
Critical Treatment Considerations:
Testosterone therapy is absolutely contraindicated in men seeking fertility, as it suppresses the hypothalamic-pituitary-gonadal axis 3. For men with secondary hypogonadism who want children, gonadotropin therapy is the appropriate choice 3.
First-Line Interventions:
Before considering testosterone therapy, address modifiable factors:
- Weight loss through low-calorie diets can reverse obesity-associated hypogonadism and normalize both testosterone and gonadotropins 3
- Physical activity provides similar benefits, though testosterone increases are modest (1-2 nmol/L) 3
- Eliminate medications or substances interfering with testosterone production 3
Common Pitfalls to Avoid
Don't rely solely on total testosterone in older men or those with obesity, as SHBG alterations can mislead diagnosis 1, 6
Don't overtreate based on numbers alone - the 2020 ACP guidelines emphasize that treatment should be reserved for sexual dysfunction specifically, not general symptoms 7
Don't use screening questionnaires alone - they lack specificity for systematic screening 3
Don't forget to assess fertility concerns before initiating testosterone therapy 3
Be aware that calculated free testosterone is an approximation - while strongly correlated with clinical outcomes, it's not as accurate as equilibrium dialysis measurement 8, 9
Patient Communication Framework
Tell your patient: "Your free testosterone is low, which means the active form of testosterone your body can use is insufficient. This likely explains your [specific symptoms]. Before considering medication, we should address [weight/lifestyle factors]. If we do start treatment, we'll focus specifically on improving your sexual function and reassess in one year to see if it's actually helping."