The age of onset of pubertal development in healthy Thai boys in Khon Kaen, Thailand

Between May and July 2011, a cross-sectional prospective study of the sexual maturation of healthy Thai boys was conducted in primary and secondary schools in Khon Kaen, northeast Thailand. All boys were Thai aged 8 to 16 years. Genital (G) and pubic hair (PH) maturity were assessed by well-trained male pediatricians and graded according to the 5-stage scales described by Marshall and Tanner (Table 1). Testicular size or volume was assessed using a Prader orchidometer and reported in milliliters (mL). The testicular volume of 1–3 mL was genital stage G1, 4–9 mL was G2, 10–14 mL (G3), 15–20 mL (G4), and greater than 20 mL was G5. At the examinations, measurements of weight and height were also recorded. The exclusion criteria encompassed underlying chronic systemic diseases, which effect sex development, history of steroid usage or any drugs that have effects on growth and pubertal development. The study was approved by the Ethics Committee of the Faculty of Medicine, Khon Kaen University. Written informed consent was obtained from all parents and assent from the boys.

Table 1

Tanner stage in boys (adapted from [2])

The distribution of the onset of puberty at various stages by Probit analysis was estimated. Data analyses were performed by using STATA version 7.0 and reported in maximum, minimum, median, mean, 95% confidence interval (CI) and standard deviations (SD). A Student t test was used to compare potential difference of ages at onset of puberty with the previous studies and P < 0.05 was considered significant.

Nine hundred school boys were invited to join the study and 320 boys participated. Of the 320 boys, 4 boys were excluded; 2 were foreigners and 2 regularly used inhaled corticosteroids. There were 316 boys enrolled in the study, all had good physical and mental health. The data of the mean weight, height, and BMI were categorized by age groups in Table 2. Tables 3 and 4 shows the median and mean ages at various stages of puberty. The median and the mean ± SD ages of the onset of genital stages (gonadarche, G2) were 11.3 and 11.3 ± 1.3 years respectively. Five (7%) boys had testicular volumes of 4 mL or greater before the age of 9 years. The median and the mean ± SD ages of the onset of pubic hair stages (pubarche, PH2) were 12.8 and 12.5 ± 1.2 years. Not one of the boys developed pubic hair before the age of 9 years. Comparison of the ages of onset of puberty between the present and the previous studies in Thai boys are shown in Table 5. There was a statistically significant difference of the mean age of PH2 between the previous studies and this study. Figure 1 shows the range and mean ages for each pubertal stage.

Figure 1

Table 2

Mean ± SD of height, weight and BMI of 316 boys

Table 3

Ages at 5 genital (G) stages in 316 school boys

Table 4

Age at pubic hair (PH) development in 316 school boys

Table 5

Ages (years) of onset of genital (G) and pubic hair (PH) development in Thai boys, comparison between the present and previous studies

The age of onset of puberty is determined by genetic factors, geographic locations, socioeconomic factors and nutritional status. The secular trend to earlier pubertal maturation was observed especially in girls [5-8], possibly because of improved nutrition and increased socioeconomic status that corresponded with the markedly increased prevalence of obesity in Thai school children in the recent years [9]. In girls, earlier age at menarche was reported after exposure to many endocrine disrupters such as polychlorinated biphenyls (PCBs), polybrominated biphenyls, pesticides-dichlorodiphenyltrichloroethane (DDT), and phthalate esters. In boys, exposure to PCBs and pesticides was associated with delayed puberty or decreased penile length [10]. There are few studies of the timing of puberty in boys [3, 4, 7, 11, 12]. This is probably because boys may refuse examination of their genitalia or it may be because of less interest in male puberty reflecting less cultural awareness of male pubertal development compared with girls [4, 7]. There was little change in the timing of the onset of puberty in boys both in gonadarche and pubarche [7, 12]. Data for genital and pubic hair development in US during 1940 to 1994 are insufficient to suggest a trend towards an earlier puberty in boys because orchidometry was not performed in any of the US population-based studies [7]. In general, no marked secular trend towards earlier age at pubertal onset in boys was evident from the mid-1960s to the late 1990s [12, 13]. The largest pubertal study in boys (n = 18,807) was conducted in an Asian country, China between 2003 and 2005 [14]. Using a cut-off of testicular volume of 4 mL, they found that the onset of puberty is at a median age of 10.55 years, which was earlier than most of European boys. Moreover 12.99% of urban Chinese boys had testicular volume of 4 mL or greater even before the age of 9 years. The previous study in Thai boys who lived in Bangkok during 1997 to 1999 suggested that the mean age of onset of puberty and pubarche were 11.3 and 12.2 years [3, 4]. However, some data were obtained from children by self-assessment using photographs of Tanner stages and early pubic hair and exact testicular volumes would not have been detectable. Moreover boys have a tendency to underestimate their genital and pubic hair stage in the advanced stage compared with those in early stage of puberty [4, 15]. The present study showed that the onset of gonadarche is at a mean age of 11.3 years with the range of 8.0–14.9 years and only 5 (7%) Thai boys had testicular volume of 4 mL or greater even before the age of 9 years. The onset of pubarche was found to be at 12.5 years with the range of 9.1–14.9 years and no boy in this study developed pubic hair before 9 years. The secular trend for the age of onset of gonadarche of Khon Kaen boys was not changed, but the age at pubarche was slightly later. The Lawson Wilkins Pediatric Endocrine Society issued the recommendations for the age at which puberty should be considered precocious at before 9 years in Caucasian boys [16]. The Thai Society of Pediatric Endocrinology also use the cut-off at 9 years for precocious puberty in Thai boys, therefore recommendations for the age at which puberty should be considered precocious remained unchanged for boys from Khon Kaen.

The age of onset of pubertal development in healthy Thai boys in Khon Kaen was not changed from the previous studies for gonadarche, but pubarche seems to be later.