In one line
Puberty is the GnRH-driven reactivation of the hypothalamic–pituitary–gonadal axis, and almost every disorder of timing is read off two axes — whether the abnormal signal sits above the gonad (central/gonadotrophin-dependent) or at the gonad and beyond (peripheral/gonadotrophin-independent) — so the single decision that organises management is whether gonadotrophins are inappropriately high, low, or absent for the clinical stage in front of you.
Mechanism & pathophysiology
The pubertal cascade is the third act of a system that has run before. GnRH neurones, migrating from the olfactory placode into the hypothalamus in fetal life, fire in a pulsatile minipuberty in the first months after birth and then fall silent through childhood under central restraint. Puberty is the re-emergence of pulsatile GnRH secretion, not its first appearance — which is why a failure of GnRH neurone migration (Kallmann syndrome) and a failure of pubertal reactivation share the same final phenotype.
The brake on the prepubertal axis is now understood at the level of the kisspeptin–neurokinin B–dynorphin (KNDy) neurones of the arcuate nucleus, which sit upstream of the GnRH pulse generator. The permissive signal to release that brake integrates energy stores (leptin from adipose tissue), genetic timing and central maturation. Two mutational classes map directly onto the two clinical poles: loss-of-function mutations in KISS1/KISS1R or TAC3/TACR3 cause hypogonadotropic hypogonadism, while gain-of-function signalling — most importantly loss-of-function of MKRN3, an imprinted, paternally-expressed gene that normally restrains the pulse generator — releases the brake early and is now the commonest identified monogenic cause of familial central precocious puberty. MKRN3 is the molecular reason a precocious-puberty history can run down the paternal line.
Once GnRH pulses resume, the gonadotrophs translate frequency into the right mix of LH and FSH, and FSH drives the ovarian two-cell machinery — theca cells make androgen under LH, granulosa cells aromatise it to oestradiol under FSH — so that rising oestradiol produces the visible sequence. The order is itself diagnostic information: thelarche (breast budding, the first sign, oestrogen-driven) → pubarche (pubic then axillary hair, which is adrenal androgen-driven and can dissociate from the gonadal axis) → the growth spurt → menarche, typically about 2–2.5 years after thelarche at Tanner breast stage 4. Adrenarche — the rise in DHEAS from adrenal zona reticularis maturation — is a separate clock from gonadarche, and the dissociation matters: isolated premature pubarche reflects early adrenarche, not central puberty, and usually needs reassurance and exclusion of non-classic congenital adrenal hyperplasia rather than an MRI.
The growth spurt is the part most often mishandled in reasoning, because oestrogen is paradoxical at the growth plate. Low oestradiol concentrations promote chondrocyte proliferation and the pubertal height velocity peak; high concentrations accelerate epiphyseal fusion and close the plate. This single fact reconciles two clinical observations that otherwise look contradictory — that early sex-steroid exposure both makes a child taller than peers in the short term and shorter as an adult, because the same hormone that drives the spurt also spends the remaining growth potential prematurely. It is why bone age, not chronological age, is the currency of every height decision in this topic, and why a GnRH agonist preserves final height by protecting the growth plate from premature closure, buying back years of growth at a normal tempo.
Tanner staging quantifies this sequence (breast B1–B5, pubic hair PH1–PH5) and is the clinical currency of the assessment. The normal window matters as much as the sequence. The conventional thresholds — thelarche before 8 years as precocious, absent thelarche by 13 years or absent menarche by 15 years (or within 3 years of thelarche, or by 14 years with hirsutism/suspected outflow obstruction) as delayed/primary amenorrhoea — are not arbitrary cut-points but the tails of a population distribution, and a secular trend toward earlier thelarche (particularly in higher-BMI girls) means the lower boundary is genuinely contested rather than fixed.
The pathophysiology of the disorders falls out of this architecture. Precocious puberty is either central (premature reactivation of the whole GnRH axis — so gonadotrophins rise and the sequence is concordant) or peripheral (autonomous sex-steroid production downstream of a suppressed axis — so gonadotrophins are low and the sequence is often discordant, e.g. vaginal bleeding without progressive breast development, or virilisation). Within central precocious puberty the great majority of girls are idiopathic, but the differential that the imaging is designed to catch is the CNS lesion that has removed the central brake — a hypothalamic hamartoma (the classic ectopic GnRH-secreting lesion, often with gelastic seizures), optic-pathway glioma (consider neurofibromatosis type 1), other tumours, hydrocephalus, cranial irradiation, or post-inflammatory damage. The peripheral causes group by their steroid source: an autonomous ovarian follicular cyst or granulosa-cell tumour, an adrenal adenoma/carcinoma, congenital adrenal hyperplasia, McCune–Albright syndrome, exogenous oestrogen exposure, and — a frequently-missed mechanism — severe primary hypothyroidism, where very high TSH cross-reacts at the FSH receptor (the Van Wyk–Grumbach syndrome) to produce ovarian stimulation and even multicystic ovaries, the one peripheral precocity that comes with slow growth and a delayed bone age rather than acceleration, and that reverses completely on thyroxine. Delayed puberty and primary amenorrhoea are read the same way inverted: low gonadotrophins point above the gonad (constitutional delay, or hypogonadotropic hypogonadism), high gonadotrophins point to primary gonadal failure (hypergonadotropic hypogonadism — Turner syndrome, primary ovarian insufficiency), and a normally oestrogenised girl who does not menstruate points instead to an anatomical end-organ problem (outflow obstruction, or absent uterus in Müllerian agenesis and androgen insensitivity).
Assessment
The history and examination are designed to place the child on those two axes before any blood is drawn.
- History: age and sequence of each pubertal event (a documented timeline beats a single snapshot); growth pattern and parental heights (a target-height calculation and the child's own growth-chart trajectory); rate of progression (rapidly advancing signs are the ones that threaten final height); neurological symptoms (headache, visual change, seizures — a CNS driver of central precocious puberty); a careful drug/exposure history (oestrogen creams, anabolic supplements, endocrine-active products); and family history of pubertal timing (early menarche in mother/sisters; the paternal line for MKRN3-linked precocity). In a girl with absent or arrested puberty: anosmia or hyposmia (Kallmann), chronic illness, intense exercise or low weight (functional hypogonadotropism), and cyclical pelvic pain with primary amenorrhoea (obstructed outflow).
- Examination: accurate Tanner staging of breast and pubic hair separately (their dissociation is the clue); height, weight, BMI and height velocity plotted; signs of an underlying syndrome (Turner stigmata — short stature, webbed neck, widely-spaced nipples, cubitus valgus, lymphoedema; midline defects; visual fields and fundi); signs of androgen excess (clitoromegaly, acne, hirsutism → peripheral/adrenal cause); abdominal/pelvic mass; and, where indicated and consented, external genital inspection for a blind or absent vagina and an imperforate or bulging hymen.
- Hormonal first line: basal LH and FSH are the pivot. A basal LH already in the pubertal range (by an ultrasensitive assay) confirms central activation; a high FSH/LH signals primary gonadal failure; a low LH with low oestradiol is the hypogonadotropic pattern shared by constitutional delay and permanent GnRH deficiency. Add oestradiol, and — where androgen excess or peripheral precocity is in play — testosterone, DHEAS, 17-hydroxyprogesterone (non-classic CAH), and βhCG/AFP (a hormone-secreting germ-cell tumour).
- GnRH (or GnRH-agonist) stimulation test is the confirmatory step for central precocious puberty when the basal LH is equivocal but suspicion is high: a brisk LH rise (an LH-predominant response) confirms a primed pituitary; a flat, FSH-predominant response argues against central puberty. A low basal LH does not exclude central precocious puberty, which is exactly why the stimulation test still earns its place.
- Bone age (left wrist radiograph) is indispensable in both directions: advanced for chronological age in precocious puberty (and the substrate for predicting compromised final height), delayed in constitutional delay and in untreated hypogonadism. The gap between bone age and chronological age, read against the pace of progression, is what distinguishes a benign variant from a height-threatening one.
- Imaging: pelvic ultrasound to assess uterine size and endometrial thickness (an oestrogenised, enlarging uterus supports central activation) and ovarian morphology (a unilateral cyst/mass suggests a peripheral oestrogen source). Brain MRI is directed, not reflexive. It is mandatory in any child with neurological signs and in younger children (girls under 6 years and boys under 8 years, where the rate of intracranial pathology is higher). It is not routine in a neurologically well girl of 6 to 8 years or boy of 8 to 9 years, where the yield of pathology is low; imaging in that band is individualised on the clinical features.
- Karyotype is obtained in any hypergonadotropic picture (to diagnose Turner syndrome and its variants/mosaicism, and to detect Y-chromosome material that mandates gonadectomy), and where the uterus is absent (to separate 46,XX Müllerian agenesis from 46,XY androgen insensitivity).