Is a decrease in testosterone levels from 39.9 nmol (nanomoles per liter) to 34.6 nmol (nanomoles per liter) over 5 months a meaningful decrease?

Is a 5-Month Testosterone Decrease from 39.9 to 34.6 nmol/L Clinically Meaningful?

This decrease is not clinically meaningful—both values remain well within the normal range for a 30-year-old male, and the variation falls within expected biological and assay variability.

Understanding Normal Testosterone Ranges for Your Age

Your testosterone levels need to be interpreted in the context of age-specific reference ranges, not the outdated 300 ng/dL (10.4 nmol/L) cutoff that was derived from older men:

• For men aged 20-24 years: Normal range is 14.2-19.4 nmol/L 1
• For men aged 25-29 years: Normal range is 14.3-19.9 nmol/L 1
• For men aged 30-34 years: Normal range is 12.5-17.3 nmol/L 1

Both your July measurement (39.9 nmol/L) and December measurement (34.6 nmol/L) are substantially above the normal range for your age group—in fact, they're more than double the upper limit of normal 1.

Why This Variation Is Expected

Biological Variability

• Testosterone levels exhibit significant intra-individual variability between measurements, even when taken under identical conditions 2
• When testosterone levels are in the equivocal range (7.0-12.5 nmol/L), at least one additional measurement is recommended to account for this variability 2
• Your levels are far above this equivocal range, making the 5.3 nmol/L difference between measurements well within normal biological fluctuation 2

Measurement Considerations

• Testosterone should be measured in the morning before 11:00 AM due to marked diurnal variation 2
• Different assay methods can produce varying results 2
• If your samples were taken at different times of day or using different laboratory methods, this could easily account for the observed difference

What Would Constitute a Meaningful Decrease?

A clinically meaningful decrease would require:

1. Symptoms of hypogonadism including 3:

   • Decreased libido (reduced sexual desire)
   • Erectile dysfunction
   • Decreased spontaneous or morning erections
   • Reduced physical function or vigorous activity
   • Mood changes or decreased motivation

2. Biochemical confirmation with 4, 3:

   • Two separate morning testosterone measurements below age-specific thresholds
   • For your age (30 years), levels would need to drop below approximately 12.5 nmol/L to be considered low 1
   • Repeat testing to confirm persistent hypogonadism 4

3. Additional workup if truly low 4:

   • Measurement of LH and FSH to distinguish primary from secondary hypogonadism
   • Free testosterone by equilibrium dialysis
   • Sex hormone-binding globulin (SHBG) levels

Important Context About Testosterone Trends

While population-level studies show declining testosterone levels in young men over recent decades 5, your individual measurements remain robustly normal. The population decline is associated with factors like elevated BMI, but even men with normal BMI show this trend 5. However, this population-level observation doesn't make your normal levels abnormal.

Bottom Line

Your testosterone levels remain excellent for your age. The 13% decrease from July to December represents normal biological variation and does not approach the threshold for hypogonadism 2, 1. Unless you're experiencing specific symptoms of testosterone deficiency (particularly sexual symptoms like decreased libido or erectile dysfunction), no further evaluation or intervention is warranted 4, 3.