Testosterone in the laboratory

What total and free testosterone measure

Total testosterone is the diagnostic baseline value. However, a diagnosis is not made based on laboratory values ​​alone, but rather requires appropriate symptoms or signs. SHBG, the sex hormone binding globulin, is particularly relevant if its concentration is changed and the classification of the overall value therefore becomes more uncertain.

Free testosterone is not a universally comparable single value. Its determination depends on the measurement or calculation method, the assay used and the laboratory reference interval.

How a low finding is secured

For diagnostic clarification, the guidelines recommend a fasting morning measurement of total testosterone using a reliable assay. If the result is low, it should be repeated on a second fasting morning.

A small clinical cross-over experiment examined ten men on experimental gonadotropin suppression using hourly blood samples over 24 hours. There was no diurnal variation in testosterone in the treated group. The model cannot be equated with every replacement therapy and therefore does not allow a general transfer of the natural daily rhythm to exogenous testosterone.

When free testosterone and SHBG provide more information

Free testosterone is particularly useful when total testosterone is close to the lower limit of normal or there is a condition that alters SHBG. The guideline mentions equilibrium dialysis or a precise calculation from total testosterone, SHBG and albumin as suitable methods.

Equilibrium dialysis is considered a reference method, but is technically complex for routine laboratories. Assays are not harmonized, common reference intervals are missing, and calculated values ​​may be inaccurate in populations for which the calculation model was not developed.

Why reference ranges are not universal

A harmonized analysis of 9054 community-dwelling men from four cohorts in the United States and Europe reported a range of 264 to 916 ng/dL for the 2.5. up to 97.5. percentiles. For the 5th to 95th percentiles, the range was 303 to 852 ng/dL. These values ​​applied to healthy, non-obese men aged 19 to 39 years and standardized measurement methods.

The range describes total testosterone in a narrowly defined reference population. It does not automatically apply to other age groups, to obese or highly trained men, to every laboratory, or to free testosterone and SHBG. A substantial portion of the differences between the studied cohorts were attributed to assay differences.

Which nutrition and training data change the classification

A meta-analysis of seven randomized trials showed no consistent direction under calorie restriction. In overweight or obese men, total testosterone increased significantly in three of four studies. In healthy men of normal weight, it fell significantly in two of three studies. A calorie deficit cannot therefore be generally described as having a lowering effect.

In the four studies that measured SHBG, SHBG increased significantly under calorie restriction compared to control, regardless of body composition. However, it does not automatically follow from this how free testosterone or symptoms develop in an individual person.

A meta-analysis of eleven randomized studies with 421 apparently healthy, insufficiently active men aged 19 to 75 years found a negligible overall effect for resting total testosterone. The standardized mean difference was 0.00 with a 95% confidence interval of -0.20 to 0.20. Endurance, strength and combined training over at least four weeks, with a median of twelve weeks, were examined.

This training data does not come from highly trained bodybuilders and does not examine clearly established overtraining. The null finding in insufficiently active men therefore does not directly answer this specific question.

What will still be clarified after a confirmed defect

After an androgen deficiency has been identified, the guidelines recommend additional diagnostic clarification of the cause. These include gonadotropins such as LH and FSH. However, the available material does not support a fixed combination of directions for each individual finding and no blanket obligation to determine all markers at the same time during each screening.

What can be practically applied to strength training

For follow-up checks, comparability is more important than an isolated number. Assay, laboratory reference, acceptance conditions and, in the case of therapy, the formulation used should be documented because measurement methods are not harmonized and assay differences can shift laboratory values.

With injectable testosterone, the natural daily rhythm must not be transferred across the board to the therapy. The timing in the injection interval must be specified along with the formulation; There is no evidence of a generally valid valley value.

Nutrition and training should also not be evaluated based on a simple if-then rule. Body composition, population studied, training status, and the difference between a rest marker and a performance endpoint limit transferability.

What limits remain for performance statements?

The examinations described here primarily record testosterone concentrations and diagnostic laboratory parameters. The training meta-analysis examined resting total and free testosterone, not performance, muscle building or health, in 421 insufficiently active, eugonadal men.

Whether a single total or free testosterone value reliably predicts performance, muscle building or health in strength athletes remains an open question.

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