Precocious puberty manifests as the progressive appearance of secondary sexual characteristics – breast development, pubic hair and menarche in girls, enlargement of testicular volume (testicular volume greater than 4 ml or testicular length greater than 25 mm) and the penis, and pubic hair development in boys [21, 22] – together with an acceleration of height velocity and bone maturation, which is frequently very advanced (by more than 2 years relative to chronological age). However, a single sign may remain the only sign for long periods, making diagnosis difficult, particularly in girls, in which isolated breast development may precede the appearance of pubic hair or the increase in growth velocity and bone maturation by several months. However, in some children, the increase in height velocity precedes the appearance of secondary sexual characteristics [23].

These tests include the assessment of bone age (which is usually advanced in patients with progressive precocious puberty), hormonal determinations, pelvic or testicular (if peripheral PP is suspected) ultrasound scans, and brain magnetic resonance imaging (MRI).

The biological diagnosis of precocious puberty is based on the evaluation of sex steroid secretion and its mechanisms. The diagnosis of central precocious puberty is based on pubertal serum gonadotrophin concentrations, with the demonstration of an activation of gonadotropin secretion [24].

The response to GnRH stimulation is considered the gold standard for the diagnosis of central precocious puberty. Stimulation tests involving a single injection of LHRH analogs can also be used [30, 31]. The major problem is defining the decision threshold. In both sexes, a central cause of precocious puberty is demonstrated an increase in pituitary gonadotropin levels. Indeed, the underlying mechanism of early central puberty is linked to premature activation of the hypothalamic-pituitary-gonadal axis, with the onset of pulsatile LH secretion and an increase in the secretion of pituitary gonadotropins both in basal conditions and after stimulation with LHRH. Before the onset of puberty, the FSH peak is greater than the LH surge. During and after puberty, the LH surge predominates. In cases of central precocious puberty, basal serum LH concentration usually is ≥0.3 IU/L and serum LH concentration after stimulation is ≥5 IU/L [1, 32]. FSH is less informative than LH, because FSH levels vary little during pubertal development. However, the stimulated LH/FSH ratio may make it easier to distinguish between progressive precocious puberty (with an LH/FSH ratio >0.66) and non-progressive variants not requiring GnRHa therapy.

Pelvic ultrasound scans can be used to assess the degree of estrogenic impregnation of the internal genitalia in girls, through measurements of size and morphological criteria. A uterine length ≥35 mm is the first sign of estrogen exposure. Morphological features are also important, as the prepubertal state is marked by a tubular uterus, which becomes more pearl-like in shape during the course of puberty, with a bulging fundus. Measurements of uterine volume increase the reliability of the examination (prepubertal ≤2 ml). Endometrial thickening on an endometrial ultrasound scan provides a second line of evidence. Ovary size and the number of follicles are not criteria for the assessment of pubertal development [25, 31, 33]. Testicular ultrasound should be performed if the testicles differ in volume or if peripheral precocious puberty is suspected, to facilitate the detection of Leydig cell tumors, which are generally not palpable.

Neuroimaging is essential in the etiological evaluation in progressive central precocious puberty. Magnetic resonance imaging (MRI) is the examination of choice in the study of the brain and of the hypothalamic-pituitary region, for the detection of hypothalamic lesions. The prevalence of such lesions is higher in boys (30–80 % of cases) than in girls (8–33 %) and is much lower when puberty starts after the age of 6 years in girls, this population accounting for the majority of cases. It has been suggested that an algorithm based on age and estradiol levels could replace MRI, but such an approach has not been clearly validated [34–36].

At the end of this analysis, the diagnostic approach should help to determine the progressive or non-progressive nature of pubertal precocity (Table 3.2) and to differentiate between the etiologies of central or peripheral precocious puberty.

Indeed, many girls with idiopathic precocious puberty display very slow progressive puberty, or even regressive puberty, with little change to predicted adult height and a normal final height close to their parental target height [5, 6]. Therapeutic abstention is the most appropriate approach in most of these cases, because puberty progresses slowly, with menarche occurring, on average, 5.5 years after the onset of clinical signs of puberty, and patient reaching a normal final height relative to parental target height. However, in some cases (about one third of subjects), predicted adult stature may decrease during the progression of puberty, in parallel with the emergence of evident biological signs of estrogenization and a highly progressive form of central PP. Thus, children for whom no treatment is justified at the initial assessment should undergo systematic clinical assessment, at least until the age of 9 years, to facilitate the identification of girls subsequently requiring treatment to block central precocious puberty.

The distinction between early puberty and normal puberty is not clear-cut. There are several variants of normal puberty, which may pose problems for differential diagnosis, particularly as they have a high prevalence [37–39].