Puberty is a period of physical and behavioral changes [1]. In this period, reproductive systems are maturated by hypothalamic-pituitary-gonadal axis activation. The onset of puberty is affected by many hormones and genes and even the environment [2]. In healthy condition, genetic features tend to determine pubertal changes. Leptin and ghrelin are hormones that are supposed to affect to the beginning of puberty [1,2]. Leptin, a hormone produced by adipocyte, is mainly thought to influence the pubertal onset. It has been found to have direct stimulatory effects on the secretion of gonadotropin-releasing hormone (GnRH) and gonadotropin [3].

"Precocious puberty" is defined as the development of pubertal changes before the age of 8 in girls and 9 in boys and increasing numbers of children are being diagnosed with precocious puberty as childhood obesity is also increasing [4]. There have been many investigations about the factors that promote these early changes such as races, maternal menarche, child's body weight, and body mass index (BMI) including environmental influences [5,6]. There are clear associations between childhood obesity and early pubertal changes [3]. Although the cause of this relationship is still not fully understood, as excessive leptin level precedes the pubertal LH elevation and the timing of menarche is related to serum leptin level, it is thought that leptin plays a key role in pubertal onset [3,7].

Obesity is a metabolic disease that affects the endocrinological system and a range of researches about the relationship between obesity and hormones has been conducted. In particular, there is much research about the relationship between obesity and thyroid function since thyroid hormones modulate metabolic pathways which is relevant to basal metabolic rate [8]. Most papers have reached the conclusion that thyroid function is usually normal in obese patients [9], although thyroid stimulation hormone (TSH) and BMI are positively correlated [10]. The causal relationship between TSH and BMI is still unclear, however TSH levels are usually elevated and free thyroxine (fT4) levels are in the normal range in obese people [9,11].

Central precocious puberty (CPP) is strongly affected by obesity and so it is reasonable to consider the relationship between CPP and thyroid function. However, few data regarding the relationship between CPP and thyroid function are available. Therefore, the aim of this study was to evaluate thyroid function in girls with CPP and the relationship between CPP and serum TSH concentration.

Comparing characteristics between the CPP and non-CPP groups, there were significant differences in serum TSH and serum fT4 levels. TSH concentration in the CPP group was significantly higher than that of the non-CPP group (3.19±1.55 IU/L vs. 2.58±1.34 IU/L, P<0.001). Serum fT4 concentration in the CPP group was significantly lower than that of the non-CPP group (1.38±0.14 ng/dL vs. 1.44±0.18 ng/dL, P<0.001). Most importantly, the proportion of hyperthyrotropinemia was notably higher in the CPP group than in the non-CPP group (15.7% vs. 8.9%, P<0.001).

Bone age, bone age advances, IGF-1, IGF-1 SDS, LH and basal and peak FSH were also significantly higher in the CPP than in the non-CPP group. Weight, weight- SDS, BMI and BMI SDS were higher in the non-CPP than the CPP group.

However, age, height, height SDS, IGFBP-3 and IGFBD-3 SDS showed no differences between the 2 groups. (Table 1)

We analyzed the correlation of these characteristics with TSH concentration. TSH concentration had a positive correlation with age, height, weight, BMI, bone age, IGF-1, IGF-1 SDS, basal LH, peak LH and basal FSH. Additionally, TSH concentration showed a negative correlation with fT4. Contrastingly, height SDS, weight SDS, BMI SDS, IGFBP-3, IGFBP-3 SDS and peak FSH showed no correlation with TSH level (Table 2).

To investigate the independent predictor factor of serum TSH level, a multiple linear regression test was used. Age, height, weight, BMI, IGF-1, basal LH, peak LH, basal FSH, and peak FSH were analyzed. Age (β=0.548, P<0.001) and peak LH (β=0.019; P=0.008) were proven as independent factors to influence serum TSH concentration (Table 3).

To the best of our knowledge, this study is the first research to investigate the relationship between TSH levels and CPP.

In this study, the prevalence of hyperthyrotropinemia was higher in the CPP group compared to the non-CPP group. Most importantly, age and peak LH were proven as independent predictor factors of serum TSH concentration. This means that pubertal onset may be associated with thyroid function.

Puberty occurs when there is reactivation of the hypothalamic-pituitary-gonadal axis [2]. Several factors are considered to influence the onset of puberty such as race, maternal menarche, low birth weight, excessive weight gain or obesity and, even environmental factors [14]. There have been many investigations into obesity's effect on the onset of puberty but the exact mechanism is still unknown [15]. Some studies suggest that leptin appears to be a main promoting factor in pubertal development. As leptin increases proportionally according to the amount of adipocyte, obese children have high levels of leptin. Leptin seems to accelerate GnRH pulse frequency [3,4].

Obesity also influences thyroid hormone levels [16]. Although thyroid function is usually normal in obese subjects, it is known that TSH and BMI are positively correlated [11,17]. Several studies have revealed that TSH, T3 is higher in obese children than in control groups and, contrastingly, that T4 has no significant difference between the 2 groups in the absence of thyroid antibodies [18-20]. The causes of thyroid hormonal changes in obese patient are various; autoimmunity, mutation of the TSH receptor gene, leptin, thyroid hormone resistance, mitochondrial dysfunction, and an adaptation process to increase energy expenditure are considered to influence thyroid function [9]. So far, there are many conflicting views as whether elevated TSH induces obesity or obesity itself induces TSH elevation [9,11,16]. Winter et al. [21] proved that derangement in the hypothalamic-pituitary axis is a cause of increased TSH levels in obesity. Conversely, Reinehr et al. [22] suggest that increased TSH levels in obesity are an adaptation mechanism to increased resting energy expenditure.

Considering the relationship between CPP, obesity, and TSH levels, it may be thought that because obese patients tend to have CPP, a CPP group may have high levels of TSH. It is easily thought that a tendency to obesity in a CPP group is relevant to TSH elevation. However, in this study, the CPP group had lower average of BMI SDS than the non-CPP group. This result is different to most of the related researches [4]. This difference may be related to the subject’s characteristics. As the subjects in this study were girls who were suspected of precocious puberty as showing clinical symptoms of puberty such as breast budding.

In this study, age, height, weight, bone age, IGF-1, basal LH, peak LH, basal FSH and also BMI had positive relationships with TSH. However, only age and peak LH showed as independent factors in the multiple regression test, hence the older the patient and the higher the peak LH level, the higher the TSH level that was found. Therefore, it is not obesity but puberty that is related with an elevation in TSH.

This may be explained by the molecular similarity of these hormones. FSH, LH, human chorionic gonadotropin and TSH are glycoprotein hormones which are similarly composed with a common α subunit and specific β subunits [23]. Thyrotropin releasing hormone (TRH) and GnRH have several common proteins that involve the regulation of TRH and GnRH receptors. This interaction may induce high levels of both LH and TSH [24,25].

There are limitations to this study. Firstly, no sex and age-matched healthy control group was included; only data from girls who were suspected of CPP were compared. In addition, the thyroid function test was only conducted once when the subjects first underwent the GnRH stimulation test. No follow-up data was taken. There is also a lack of data about thyroid hormone antibodies.

In conclusion, we suggest that hyperthyrotropinemia is relevant to pubertal onset. Elevated TSH in girls with CPP should be associated with pubertal LH elevation. Still, no association has been found between elevated TSH in CPP and obesity. Further longitudinal studies with normal control groups are needed to investigate the causal relationship between CPP and hyperthyrotropinemia.