What are the normal total and free serum testosterone concentrations for a 16‑year‑old male in mid‑ to late‑puberty?

Normal Testosterone Levels in a 16-Year-Old Male

For a 16-year-old male in mid-to-late puberty, normal total serum testosterone ranges from approximately 0.82 nmol/L (24 ng/dL) at the earliest stages of puberty to 16.5 nmol/L (476 ng/dL) by age 17, with most 16-year-olds falling in the mid-range of 8–15 nmol/L (230–430 ng/dL). 1

Age-Specific Reference Ranges

• Longitudinal data from 513 males aged 9–17 years demonstrate that average total testosterone rises progressively throughout adolescence, reaching 16.5 nmol/L (SD: 2.65) by age 17. 1

• At age 16, testosterone levels typically fall between 8–15 nmol/L (230–430 ng/dL), though individual variation is substantial depending on pubertal stage. 1

• Free testosterone should be measured by equilibrium dialysis or calculated using validated formulas (e.g., Vermeulen) when total testosterone is borderline or when clinical suspicion of hypogonadism exists, as direct immunoassays are unreliable. 2, 3

Measurement Considerations for Adolescents

• Morning measurements (8–10 AM) are mandatory because testosterone exhibits marked diurnal variation, with peak levels occurring in the morning; measurements taken later in the day risk false-low results. 2, 4, 3

• Two separate fasting morning measurements are required to confirm any abnormality, as testosterone concentrations fluctuate significantly both day-to-day and seasonally. 2, 4, 3

• When total testosterone is borderline (7.0–12.5 nmol/L or 200–360 ng/dL), at least one additional measurement plus assessment of free or bioavailable testosterone is necessary to account for intra-individual variability. 4, 5, 3

Pubertal Dynamics and Testosterone Trajectories

• Testosterone trajectories during adolescence are characterized by rapid acceleration during mid-puberty, with the steepest rise (Peak Testosterone Change) typically occurring between ages 13–15 years. 1

• Average exposure to testosterone over the entire pubertal period correlates with timing of the growth spurt and final testosterone levels at age 17, reflecting the integrated hormonal milieu driving physical and psychological development. 1

• Hemoglobin and hematocrit rise 15–20% during male puberty as testosterone stimulates erythropoiesis, a normal physiological response that should not be mistaken for pathology. 2

Clinical Context: When to Suspect Hypogonadism in Adolescents

• Prepubertal-onset hypogonadism presents with lack of virilization, eunuchoid body proportions, and undeveloped secondary sex characteristics, whereas post-pubertal onset causes loss of androgen-dependent functions. 5

• Total testosterone < 8 nmol/L (230 ng/dL) strongly supports hypogonadism in symptomatic adolescents, while levels > 12 nmol/L (350 ng/dL) are likely normal; the gray zone (8–12 nmol/L) requires free testosterone assessment. 5

• Measure LH and FSH after confirming low testosterone to distinguish primary (testicular) hypogonadism (elevated LH/FSH) from secondary (hypothalamic-pituitary) hypogonadism (low or low-normal LH/FSH), as this distinction is critical for treatment and fertility counseling. 2, 5

Laboratory Variability and Pitfalls

• Reference ranges vary widely across laboratories, with lower limits ranging from 130–450 ng/dL (350% difference) and upper limits from 486–1,593 ng/dL (325% difference), directly influencing clinical decision-making. 6

• Automated immunoassays used by most clinical laboratories provide satisfactory estimates of total testosterone in the eugonadal range (12.5–40 nmol/L or 360–1,150 ng/dL) but may be less reliable at borderline-low concentrations. 4, 3

• Do not diagnose hypogonadism on a single measurement or outside the 8–10 AM window, as this increases false-positive rates due to diurnal variation and assay variability. 2, 4, 3

Summary Algorithm for Adolescent Testosterone Assessment

1. Obtain two separate fasting morning (8–10 AM) total testosterone measurements if clinical suspicion exists. 2, 4, 3

2. If both values < 8 nmol/L (230 ng/dL): measure LH, FSH, and free testosterone by equilibrium dialysis to confirm hypogonadism and classify primary vs. secondary. 2, 5, 3

3. If values 8–12 nmol/L (230–350 ng/dL): calculate or measure free testosterone and assess clinical symptoms before considering further workup. 5, 3

4. If values > 12 nmol/L (350 ng/dL): testosterone levels are normal for age; investigate non-hormonal causes of symptoms. 5