In our cohort of girls spanning the range of pubertal development, obesity is associated with marked HA and hyperinsulinemia, and this was especially evident in pre- and early pubertal girls. Total T was elevated in obese peripubertal girls: thus, while reduced SHBG contributes to the elevation of free T in these subjects, T secretion was also increased. Additionally, DHEAS was elevated in prepubertal obese girls, which is consistent with earlier reports (6
). These data augment our earlier observations (7
) and provide detailed assessment of hormonal values across pubertal maturation. They also emphasize that obesity is commonly associated with HA, with 66–94% of obese Tanner 1–3 girls having an elevated free T.
In keeping with the known association between childhood obesity and both insulin resistance and hyperinsulinemia (17
), fasting insulin was elevated in obese peripubertal girls, especially in Tanner 1–3 girls. This finding was supported by elevations in HOMA in obese girls, though it is unclear why both parameters appear disproportionately elevated before and during early puberty. The concomitant elevation of insulin and T in this and other studies (2
) suggest a possible mechanism for HA in obese girls. Namely, insulin may act as a co-gonadotropin on the pubertal ovarian theca cell compartment to promote androgen production (1
). Insulin also decreases hepatic SHBG production, contributing to elevations in free T (1
). Additionally, extensive data supports an important role of hyperinsulinemia in the pathogenesis of adult PCOS, primarily via mechanisms discussed above. However, partial correlation analysis in our earlier report (7
) revealed a strong association between obesity and free T even after adjusting for differences in fasting insulin, suggesting that insulin may not be the only mediator of HA in obese girls. We recognize that our assessment of insulin resistance and hyperinsulinemia is limited, as insulin values were not available for all subjects, and a single fasting insulin (or HOMA) is an imprecise marker of hyperinsulinemia. Regardless, available data support a role of insulin in the HA associated with peripubertal obesity.
Our results suggest that early morning LH is relatively low in pre- and early pubertal obese girls, but normalizes as pubertal development progresses. This observation in Tanner 1–3 girls is reminiscent of an inverse relationship between BMI and both mean LH and LH amplitude in adults with PCOS (19
). However, the association of obesity with advanced physical exam characteristics of puberty has been well-described (21
). Thus, the lower LH in the Tanner 2 obese girls may in part reflect their younger chronological age and relatively immature GnRH-gonadotrope axis compared to their normal weight counterparts. Taken together, the patterns of testosterone, LH, and insulin changes across puberty suggest that insulin plays a particularly important role in obesity-associated HA in pre- and early pubertal girls, with progressive maturation of the hypothalamic-pituitary unit (concomitant with Tanner stage 3) and associated rise in gonadotropins further promoting ovarian T production. Indeed, our earlier partial correlation analysis (7
) disclosed a positive correlation between LH and free T after adjusting for differences in age, pubertal stage, BMI, DHEAS, and insulin. We recognize that early morning LH values do not fully characterize LH secretion during puberty, and further study is required to assess LH secretory patterns in obese peripubertal girls. Overall, however, these data suggest that hyperinsulinemia plays a major role in obesity-associated HA in pre- and early puberty, with both LH and insulin contributing to HA as puberty progresses.
Our group of subjects included a number of girls with clinical evidence of hyperandrogenism. We previously argued (7
) that systematic exclusion of such girls may inappropriately lessen the apparent relationship between adiposity and androgen excess. Regardless, analysis after exclusion of such girls was not materially different (data not shown) and did not alter interpretation. Any recruitment bias was especially unlikely to have influenced results in the Tanner 1–3 group: very few of these girls had clinical hyperandrogenism, as the manifestations of androgen excess develop slowly.
Previous reports described diurnal variation in T (9
) and E2
), with peaks occurring between 0600 and 1000 h. Progesterone was not measured in these studies, and our results show that in normal weight girls, mean P concentrations rose 2.3-fold from 2300 h to 0600 h, similar to the increase observed for T. Although P and T appear to increase overnight in obese Tanner 1–3 girls, these differences did not quite reach statistical significance, possibly reflecting the higher evening levels and the limited numbers of obese subjects studied. Overnight changes of E2
were not observed in our study; this may reflect the timing of surveillance, as E2
tends to peak later in the morning (8
The origin of pubertal P and T secretion—and overnight increases in plasma concentrations—remain unclear: ovarian P secretion may be stimulated by the overnight increases in LH observed in pubertal girls (22
), and adrenal P secretion may also increase under ACTH control. The importance of overnight increases of sex steroids during puberty is also unclear, but may influence LH pulsatility in pubertal girls. For instance, acute E2
infusion mitigates the overnight increase of LH secretion in pubertal girls (23
). In addition, whereas normal Tanner 1–3 girls demonstrate nocturnal increases in LH (and by inference GnRH) pulse frequency, age-matched girls with gonadal dysgenesis (i.e., absent ovarian steroid secretion) do not (22
). Of parallel interest is that, when expressed in mass or molar terms, P concentrations exceeded those of both T and E2
in normal weight girls, and P is the major regulator or GnRH pulse frequency in adult women (25
). Taken together, these findings highlight the importance of delineating the potential regulatory role of P and E2
in directing diurnal changes of GnRH and LH secretion during female puberty.
In adults with PCOS, relative resistance of the GnRH pulse generator to negative feedback by P plays a role in the persistently rapid GnRH pulse frequency, elevated LH concentrations, and relative FSH deficiency characteristic of this syndrome (26
). Some adolescents with hyperandrogenemia (HA) demonstrate a similar feedback defect (29
). In adult PCOS, this relative insensitivity is reversed by androgen-receptor blockade (flutamide), suggesting that it is a consequence of hyperandrogenemia per se
). We have proposed (31
) a hypothetical paradigm in which early morning increases in sex steroids (especially P)—either directly by actions on the GnRH pulse generator or via facilitation of higher CNS mechanisms—contribute to the reduction of GnRH and LH pulsatility during the following day. Diurnal slowing of GnRH pulsatility would favor FSH synthesis, secretion, and subsequent follicular development. In girls with HA-induced decreases in hypothalamic sensitivity to feedback inhibition, overnight increases of sex steroids would not slow GnRH pulsatility during the subsequent day. A persistently (24 h) rapid GnRH pulse stimulus would enhance LH and ovarian androgen secretion, and lead to relative FSH deficiency with impaired follicular development. We seek to explore the viability of this paradigm in ongoing and future studies.
In conclusion, peripubertal obesity is associated with hyperinsulinemia and hyperandrogenemia, which is particularly marked in pre- and early pubertal girls. Additionally, progesterone and testosterone concentrations in normal Tanner 1–3 girls increase from 2300 to 0600 h. Elucidation of the origin and consequences of the normal overnight increase in sex steroids and of obesity-associated hyperandrogenemia requires additional study.