Puberty — Normal Development and Tanner Staging

• Girls typically begin at 8–13 years. The sequence is thelarche → pubarche → growth spurt → menarche, with menarche arriving late, roughly 2–2.5 years after breast budding. The growth spurt occurs earlier in girls, before menarche.
• Boys typically begin at 9–14 years. The first change is testicular enlargement to ≥4 mL or ≥2.5 cm, then pubarche, then penile growth, with the growth spurt occurring later than in girls.
• Adrenarche — adrenal androgen production driving pubic and axillary hair and body odour — is a separate axis from gonadarche and can occur independently.
• Tanner (sexual maturity rating) stages 1–5 describe breast and genital development and pubic hair separately, because they can be discordant.

The GnRH pulse generator and its brake

• Fetal/neonatal activity then quiescence: the axis is active in utero and during the mini-puberty of infancy, then is restrained through childhood by central inhibition of GnRH pulsatility — not by lack of pituitary or gonadal capacity.
• KNDy neurons of the arcuate nucleus (co-expressing kisspeptin, neurokinin B, dynorphin) are the pacemaker. Kisspeptin acting on KISS1R (GPR54) on GnRH neurons is the proximate trigger; loss-of-function causes hypogonadotropic hypogonadism, activating mutations cause central precocious puberty.
• MKRN3, a paternally expressed imprinted gene, is an inhibitory brake whose expression falls before puberty; leptin from adipose tissue is permissive (a metabolic signal, not an initiator), which is why severe energy deficit stalls the axis.

Gonadal response

• Boys: LH → Leydig cells → testosterone; FSH → Sertoli cells → inhibin B and spermatogenesis. Because seminiferous tubules constitute most testicular mass, FSH-driven tubular growth makes testicular enlargement the earliest sign.
• Girls: the two-cell, two-gonadotropin model — LH drives thecal androgen synthesis, FSH induces granulosa aromatase converting androgen to estradiol. Estradiol drives thelarche and uterine/endometrial growth; maturation of estrogen positive feedback on LH permits ovulation, so early post-menarchal cycles are commonly anovulatory.

Premature activation (central, GnRH-dependent)

• Idiopathic central precocious puberty: most common cause in girls; normal sequence, just early, with pubertal-range LH on GnRH-stimulation testing. Pediatric Endocrine Society/ESPE consensus favours GnRH agonist therapy (e.g., leuprolide) to preserve adult height.
• CNS lesions: hypothalamic hamartoma (classically with gelastic seizures), glioma, hydrocephalus, cranial irradiation. Central precocity in a boy is pathologic until proven otherwise — brain MRI is expected.
• MKRN3 loss-of-function: familial central precocious puberty inherited from the father (imprinting).

Peripheral (GnRH-independent, LH suppressed)

• McCune–Albright syndrome: GNAS activating mutation — café-au-lait macules with irregular "coast of Maine" borders, polyostotic fibrous dysplasia, autonomous ovarian cysts.
• Familial male-limited precocious puberty ("testotoxicosis"): activating LHCGR mutation — virilization with prepubertal-sized testes.
• Congenital adrenal hyperplasia (21-hydroxylase deficiency): virilization plus small testes; hCG-secreting tumors (hepatoblastoma, germ cell) in boys; granulosa cell tumor (isosexual precocity in girls) or unilateral Leydig cell tumor (asymmetric testicular enlargement).
• Benign variants: isolated premature thelarche or premature adrenarche without growth acceleration or bone-age advance.

Failure to initiate

• Constitutional delay of growth and puberty: the most common cause — delayed bone age, positive family history, normal eventual puberty.
• Hypogonadotropic (low LH/FSH): Kallmann syndrome (failed GnRH neuron migration, anosmia), CHARGE, Prader–Willi, hyperprolactinemia, and functional suppression from anorexia nervosa, endurance training, celiac disease or IBD.
• Hypergonadotropic (high LH/FSH): Turner syndrome (45,X, streak gonads, short stature) and Klinefelter syndrome (47,XXY, small firm testes, tall, low inhibin B), gonadal dysgenesis, chemotherapy/radiation.
• End-organ defects: complete androgen insensitivity (breasts, scant pubic hair, blind vaginal pouch); 5α-reductase deficiency (virilization at puberty); aromatase or estrogen-receptor defects (unfused epiphyses, continued linear growth, osteoporosis).

• First sign, by sex: thelarche (Tanner 2 breast bud) in girls; testicular volume ≥4 mL in boys. A stem describing pubic hair as the "first" sign is testing adrenarche, a separate axis — do not call it gonadarche.
• Definitions to memorize: precocious = secondary sexual characteristics before 8 in girls, 9 in boys. Delayed = no breast development by 13, no testicular enlargement by 14, or no menarche by 15 (or >3 years after thelarche).
• Best next step in any precocity or delay: a left hand/wrist radiograph for bone age. Advanced bone age suggests true sex-steroid exposure; delayed bone age with delayed puberty points to constitutional delay.
• GnRH-stimulation (leuprolide) test separates the two mechanisms: a pubertal LH rise = central; suppressed LH with elevated sex steroids = peripheral. Then measure hCG, DHEAS, 17-hydroxyprogesterone, and image gonads/adrenals.
• The classic association: hypothalamic hamartoma → gelastic (laughing) seizures + central precocious puberty. In boys, central precocity should prompt brain imaging far more readily than in girls.
• Growth spurt timing is the favourite discriminator: it is early in girls (around Tanner 2–3, before menarche) and late in boys (around Tanner 3–4). Menarche therefore occurs after peak height velocity, leaving only limited residual growth.
• Estrogen closes epiphyses — in both sexes. Aromatase deficiency or estrogen-receptor mutation produces a tall patient with open epiphyses; untreated precocious puberty produces the opposite, a tall child who becomes a short adult.
• Common distractors: benign premature thelarche/adrenarche have normal growth velocity and bone age — no GnRH agonist needed; and anovulatory, irregular cycles in the first 1–2 years after menarche are physiologic, not PCOS.