At what total testosterone level should a free testosterone level be checked in an adult male undergoing Testosterone Replacement Therapy (TRT) for hypogonadism?

When to Check Free Testosterone During TRT

Free testosterone should be checked when total testosterone levels are between 280-400 ng/dL, as total testosterone in this range lacks sufficient sensitivity and specificity to reliably determine true androgen status. 1

The Critical "Gray Zone" for Free Testosterone Assessment

The evidence clearly demonstrates that total testosterone measurement alone has significant limitations in specific ranges:

• Below 150 ng/dL: Total testosterone reliably predicts low free testosterone (specificity 98.9%), making additional free testosterone testing unnecessary 1

• Between 280-350 ng/dL: This range is problematic—total testosterone has only 91% sensitivity for detecting low free testosterone, meaning it will miss hypogonadism in approximately 1 in 10 men 1

• Between 350-400 ng/dL: Total testosterone must exceed this threshold to reliably predict normal free testosterone (sensitivity increases to 96.8-98.2% above 350 ng/dL) 1

• Above 400 ng/dL: Free testosterone measurement becomes less critical as total testosterone reliably indicates adequate androgen status 1

Why This Matters in Clinical Practice

The American College of Physicians and Endocrine Society recommend measuring free testosterone when total testosterone is near the lower limit of normal or when SHBG abnormalities are suspected. 2, 3 This recommendation is based on the understanding that:

• Men with obesity commonly have low SHBG, which artificially lowers total testosterone while free testosterone may remain normal 2, 3

• Free testosterone by equilibrium dialysis (the gold standard method) or calculated free testosterone using total testosterone, SHBG, and albumin provides more accurate assessment of bioavailable androgen status 2, 3

Practical Algorithm for TRT Monitoring

When monitoring patients on TRT:

1. If total testosterone is 280-400 ng/dL: Always measure free testosterone, SHBG, and albumin to calculate or directly measure free testosterone 2, 1

2. If total testosterone is below 280 ng/dL: Free testosterone will almost certainly be low (91% sensitivity), but measuring it confirms the diagnosis and guides dosing 1

3. If total testosterone exceeds 400 ng/dL: Free testosterone measurement adds limited value unless specific clinical concerns exist (obesity, suspected SHBG abnormalities) 1

Common Pitfalls to Avoid

• Don't rely solely on total testosterone between 280-400 ng/dL—this range has poor predictive value and will lead to both over-treatment and under-treatment 1

• Always obtain fasting morning samples—testosterone exhibits significant diurnal variation, and the American College of Physicians recommends two separate morning measurements to confirm diagnosis 2

• Recheck levels 2-3 months after initiating or adjusting therapy to allow steady-state levels, measuring both total and free testosterone if total testosterone falls in the 280-400 ng/dL range 2, 3