The Tanner progression

The visible course of puberty, traditionally staged in five steps. Tanner stages are observational categories — what an examiner sees on physical exam — and they map onto the underlying endocrine progression but are not defined by it.

The staging system

Tanner described two parallel scales in males: G (genital development) and PH (pubic hair). The G stages track testicular and penile growth driven by the HPG axis; the PH stages track pubic-hair development driven mostly by adrenal androgens. They usually advance in parallel but can dissociate — a boy with isolated HPG failure can have PH4 from adrenarche alone while remaining G1.

The G and PH systems each go from stage 1 (prepubertal) to stage 5 (adult).

G1 / PH1 — prepubertal

The starting state. Testes 1–3 mL, no scrotal thinning, no pubic hair beyond perhaps fine vellus.

G2 / PH2 — the first true sign

The defining event of pubertal onset:

Testicular volume ≥ 4 mL — measured with a Prader orchidometer, this is the official threshold for the start of puberty.
Scrotum — thins and reddens; skin texture changes.
Pubic hair (PH2) — sparse, fine, straight hairs at the base of the penis.
Growth rate — subtle increase, often not yet noticed.

Hormones: testosterone in the low pubertal range (perhaps 50–150 ng/dL).

This stage is often missed clinically because the changes are subtle. Many boys and parents do not realize puberty has begun until G3.

G3 / PH3 — accelerating

Now things become unmistakable:

Penile lengthening begins (girth lags).
Testes continue growing (typically 6–12 mL).
Pubic hair (PH3) — coarsens, darkens, spreads upward and outward.
Growth spurt — accelerating; height velocity rises toward peak.
Voice — begins to change subtly as the larynx grows.
Body odor — more pronounced.
Acne — may appear.
Gynecomastia — mild transient breast tissue in 50–60% of boys, reflecting a temporary estradiol-to-testosterone ratio favoring estrogen.

Hormones: testosterone rises substantially (200–400 ng/dL).

Nocturnal LH pulses extend into the day as the diurnal pattern matures.

G4 / PH4 — peak growth and spermarche

The most dramatic stage in terms of growth and reproductive function:

Peak height velocity (PHV) — typically 9–10 cm/year, occurring around bone age 13–14.
Penile girth — develops prominently.
Testes — continue growing (12–17 mL).
Pubic hair (PH4) — adult-textured but still confined to the pubic area.
Voice — deepens noticeably; the characteristic cracking and dropping of pitch.
Axillary hair — appears.
Spermarche — the onset of sperm production; typically occurs during G4. First ejaculations are possible.
Body composition — shifts toward more muscle mass.
Acne — often peaks.
Bone age — advances rapidly under estradiol influence (estradiol, made by peripheral aromatization of testosterone, is the actual driver of growth-plate closure in both sexes).

Hormones: testosterone approaches adult levels (300–600 ng/dL).

G5 / PH5 — adult configuration

The end of Tanner-stage progression, but not the end of male maturation (see post-pubertal maturation).

Adult genital size and configuration.
Pubic hair (PH5) — extends to the medial thighs.
Adult body habitus — shoulder/chest broadening, jaw widening beginning.
Facial hair — begins to fill in (often continues developing well into the 20s).
Body hair — continues developing on chest, abdomen, back (often into the 20s).
Final adult height — reached as epiphyseal plates fuse (driven primarily by estradiol).
Hormones — adult testosterone (300–1000 ng/dL), adult LH and FSH with established pulsatile patterns.

Timing summary

Stage	Typical age range	Key marker
G1	< 9	Prepubertal
G2	9–14	Testicular enlargement ≥ 4 mL
G3	11–15	Penile lengthening, growth spurt accelerates
G4	12–16	Peak height velocity, spermarche, voice drops
G5	14–18	Adult genital configuration, growth plates close

The age ranges are wide because the onset age varies (genetics, body composition, ethnicity); the internal pacing (how long between stages) is more uniform — usually 0.5–1 year per stage.

Key idea

Tanner stages are an observational map of what the underlying HPG activation produces on the outside. The most informative single measurement at any point in this progression is testicular volume — it captures the FSH-driven Sertoli-cell proliferation that is the most reliable index of where the axis is in its activation curve.