Other physical changes show sexual dimorphism at puberty. In boys both the membranous and cartilaginous portions of the vocal cords lengthen much more than they do in girls, accounting for deepening of the voice. Comedones, acne, and seborrhea of the scalp begin because of increased secretion of adrenal and gonadal steroids at puberty. In general, early-onset acne correlates with the development of severe acne later in puberty. The appearance of comedones in the nasal creases and behind the pinna may be the first indications of impending pubertal development.
Hormonal Changes By 10 weeks of gestation, gonadotropin-releasing hormone (GnRH) is present in the hypothalamus, and luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are present in the pituitary gland (14). Gonadotropin levels are elevated in both female and male fetuses before birth; the levels of FSH are higher in females. At birth, gonadotropin and sex steroid concentrations are still high, but the levels decline during the first several weeks of life and remain low during the prepubertal years. The hypothalamic–pituitary unit appears to be suppressed by the extremely low levels of gonadal steroids present in childhood. Gonadal suppression of gonadotropin secretion is demonstrated by higher gonadotropin levels in children with gonadal dysgenesis and those who undergo gonadectomy before puberty (15).
Several of the hormonal changes associated with pubertal development begin before any of the physical changes are obvious. Early in puberty, there is increased sensitivity of LH to GnRH. Sleep-entrained increases in both LH and FSH can be documented early in puberty (16). In boys, the nocturnal increases in gonadotropin levels are accompanied by simultaneous increases in circulating testosterone levels (17). In contrast, in girls, the nighttime increases in circulating gonadotropin levels are followed by increased secretion of estradiol the next day (18) (Fig. 29.5). This delay in estradiol secretion is believed to result from the additional synthetic
Figure 29.5 Patterns of circulating luteinizing hormone (LH), follicle-stimulating hormone (FSH), and estradiol in a stage 3 pubertal girl over a 24-hour period with the encephalographic stage of sleep indicated. (From Boyar RM, Wu RHK, Roffwarg H, et al. Human puberty: 24hour estradiol patterns in pubertal girls. J Clin Endocrinol Metab 1976;43:1418-421, with permission.) steps required in the aromatization of estrogens from androgens. Basal levels of both FSH and LH increase through puberty. The patterns differ in boys and girls, with LH levels (measured in mIU/mL) eventually becoming greater than FSH levels (19) (Fig. 29.6). Although it now appears that gonadotropins are always secreted in an episodic or pulsatile fashion, even before puberty, the pulsatile secretion of gonadotropins is more easily documented as puberty progresses and basal levels increase (20).
Increased adrenal androgen secretion is important in stimulating adrenarche, the appearance of pubic and axillary hair, in both boys and girls. Pubarche specifically refers to the appearance of pubic hair. Progressive increases in circulating levels of the major adrenal androgens, dehydroepiandrosterone (DHEA) and its sulfate (DHEAS), begin as early as 2 years of age, accelerate at 7 to 8 years of age, and continue until 13 to 15 years of age (21–23). The accelerated increases in adrenal androgens begin about 2 years before the increases in gonadotropin and gonadal sex steroid secretion when the hypothalamic–pituitary–gonadal unit is still functioning at a low prepubertal level.