Hormonal Changes of Puberty

Pubertal onset is initiated by reactivation of hypothalamic gonadotropin-releasing hormone (GnRH) secretion (Figure 67-1). The hypothalamic–pituitary–gonadal (HPG) axis is active during fetal development and the first few months of life but is suppressed in infancy by neural input to the arcuate nucleus and remains so throughout childhood (juvenile pause). The transition from a quiescent state toward puberty involves gradually increasing, pulsatile release of GnRH by the hypothalamus, which leads to pituitary release of gonadotropins, first luteinizing hormone (LH) and then follicle-stimulating hormone (FSH). An increased amplitude and frequency of LH pulses during the night are the first detectable hormonal changes at the onset of puberty. Gonadotropins induce gonadal growth and maturation (gonadarche), marked by increasing production of sex steroids, mainly testosterone and estradiol. Apart from the increase in size of the testes, nearly all of the physical changes of puberty result from increasing levels of androgens and estrogens (Table 67-1).

Figure 67-1 Hormonal events of puberty.

Table 67-1 Physical Effects of Sex Hormones

Early androgenic physical changes commonly considered part of puberty in both boys and girls (pubic and axillary hair, body odor, acne) result from gradual mid-childhood maturation of the adrenal glands (adrenarche) and increasing secretion of adrenal androgens (DHEA [dehydroepiandrosterone], DHEA-S [dehydroepiandrosterone sulfate], and androstenedione). Adrenarche overlaps in timing with gonadarche, but the processes are separate and can occur independently.

Normal Pubertal Timing

The timing of the onset of puberty is determined by a variety of genetic factors and environmental influences in addition to gender. The heritability of pubertal timing has been estimated at 50% to 80% based on correlations within ethnic groups and families and between monozygotic twins. Recent genome-wide association studies have identified some of the genetic markers, particularly a height-related gene, LIN28B (6q21), that may affect the timing of menarche.

Much public attention has been paid recently to environmental factors, especially in the context of reports that children are entering puberty at earlier ages. Although several studies suggest that the age of pubertal onset has declined in recent decades, the decline is modest (menarche perhaps 3 months earlier than 30 years ago). Although many environmental factors have been identified as associated with earlier or later puberty, especially in girls, the magnitudes of effects are small and socially charged and can rarely be determined for an individual child. Much of the scientific attention has been directed at increased body mass and at environmental chemicals. Adiposity affects initiation of puberty, especially in girls, with larger body mass associated with earlier puberty and slender build associated with later puberty. Environmental hormones and endocrine-disrupting chemicals, such as polychlorinated biphenyls, organochlorine pesticides, and phthalates, can bind with sex steroid receptors and either mimic or inhibit the hormone effects. No assays for such agents are commercially available in clinical practice.

Although the age of onset may vary, the stages of puberty and their durations tend to be fairly constant (Figure 67-2). The most widely used system for describing the stages of puberty is the modified Tanner system (Figure 67-3), by which girls are assessed using the stage of breast development and pubic hair and boys by genitalia, pubic hair, and testicular volumes. The temptation to describe children with “unitary Tanner stages” should be resisted because it discourages recognition of important discordances that may indicate disease.

Figure 67-2 Relationship of pubertal milestones.

Figure 67-3 Tanner staging system.

Girls tend to begin puberty earlier than boys. The first physical sign of female puberty is usually the appearance of breast buds (thelarche) at a mean age of 10.5 to 11 years. Pubic hair (pubarche) generally follows gonadarche by a few months but may precede it in 10% of girls. Menarche occurs approximately 2 years after thelarche. Pubertal growth acceleration coincides with thelarche for most girls and slows by menarche. Most girls are approaching adult height by 4 years after thelarche and 2 years after menarche.

In boys, the first sign of pubertal development is an increase in testicular volume to 4 mL, which occurs at a mean age of about 11.5 years. As in girls, pubarche usually follows but may precede gonadarche. Pubertal growth in boys accelerates more gradually, becoming noticeable in the second year, and reaches adult height about 6 years from the onset of puberty.

Premature Sex Hormone Effects

Traditionally, early sex hormone effects have been classified as central, peripheral, or incomplete (Box 67-1). Central puberty reflects activation of the entire HPG axis, and the physical changes are typically those of normal puberty for a child of that sex. In contrast, peripheral sex hormone sources include adrenal and gonadal disorders, abdominal or pelvic tumors, or exogenous sex steroids. Physical changes reflect the predominant excess hormones (androgenic or estrogenic) and are often markedly discordant from normal pubertal development. Incomplete puberty describes early physical changes that do not have a pathologic source but do not progress to full development at the usual tempo.

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