Thyroid stimulating hormone (TSH) is associated with general and abdominal obesity in girls during puberty

Background: Childhood obesity is an important public health issue. Although both thyroid hormone and menarche are known to play a role in body metabolism and energy expenditure, evidence for these associations in girls around puberty was limited. This study was aimed to investigate the association of TSH with general and abdominal obesity in girls during puberty. Methods: A multi-stage cluster sampling method was used to select one junior middle school from each of 4 study areas: Minhang District in Shanghai, Haimen City in Jiangsu Province, Yuhuan City and Deqing County in Zhejiang Province. A total of 474 girls aged 11 to 14 years from 4 schools were enrolled. Information on demographic factors and puberty stage were collected, and anthropometric measurements and thyroid hormones were determined. Multivariate logistic regression models were used to assess the associations of Thyroid stimulating hormone (TSH) with the risk of obesity measured by body mess index (BMI) and waist circumference (WC). Results: Of the 474 girls, the prevalences of BMI-based general obesity and WC-based abdominal obesity were 19.8% (94/474) and 21.7% (103/474), respectively. Compared with normal weight girls, the mean serum TSH concentration was significantly higher in BMI-based general overweight or obese girls (P=0.037), but not in WC-based central overweight or obese girls (P=0.173). In the multiple logistic regression models, for girls with highest tertile of serum TSH concentration relative to those in the lowest tertile, the odds ratios were 2.63 (95% CI 1.34 to 5.14) and 2.53(95% CI 1.31 to 4.88) for overweight or obesity based on BMI and WC after adjustment for puberty stage and other covariates. Conclusions: Serum TSH concentration was positively associated with both general and abdominal obesity in school-age girls and the association was independent of puberty.


Background
Overweight and obesity are defined as abnormal and excess fat accumulation. The prevalence of overweight and obesity among children and adolescents aged 5-19 years has increased dramatically from 4% in 1975 to over 18% in 2016 globally. 1 Childhood obesity not only increases the risks of asthma and metabolic syndrome, 2 3 but also the risk of cardiovascular diseases (CVD) and premature death in adulthood. 4 5 Meanwhile, 30-50% of children with obesity tend to become obese adults. 6 Obesity is related to multiple endocrine alterations, where various hormones play a important role. {Beck, 1964 #344; Scacchi, 1999#342}{Scacchi, 1999Mayes, 2004 #343} 7 8 However, the pathophysiology of obesity is not fully understood. Identifying hormonal targets involved in this process may contribute to prevent and manage obesity.
Thyroid hormone has a strong effect on body metabolism and energy expenditure, 9 and is usually reactivated during puberty. 10 Upon the onset of puberty, the incidence of thyroid disease increases in females only, and decreases after menopause. 11 Evidence suggested that thyroid dysfunction contributes to obesity, and conversely, obesity induces thyroidal alterations. 12 In our previous study, we observed a positive association between thyroid nodules and obesity in Chinese children. 13 Women with a history of earlier menarche have a higher risk of obesity than those with later menarche. 14 Studies of children and adolescents drawn from pediatric outpatient clinics reported an association between thyroid stimulating hormone (TSH) and obesity. 15 16 In the current study, we assessed the associations of TSH with body mass index (BMI) and waist circumference (WC) in a girls aged 11-14 years in East China.

Study population
Four coastal cities in East China (Minhang District in Shanghai, Haimen City in Jiangsu Province, Yuhuan City and Deqing County in Zhejiang Province) were selected by purposive sampling. Previous studies have revealed an iodine-sufficient along with different iodized-salt consumption status among four sites. 17 18 19 20 One junior middle school, where students were mainly local residents, was selected from each city. All girls of six classes (grade 6 in Minhang and grade 7 in Haimen, Yuhuan, and Deqing) were enrolled into this study. Ones who had thyroid or pituitary abnormalities, and other disorders which affect thyroid hormone levels were excluded.
Informed written consents were obtained from all the participants and their parents or guardian, and the study was approved by the ethical review board of the School of Public Health of Fudan University. A total of 474 girls participated in the study, with a response rate of 98.54%.

Information collection
Information on demographic and lifestyle factors were collected by a self-administrated questionnaire. Self-reported pubertal maturation level was assessed by using the Pubertal Development Scale (PDS). 21 Anthropometric measurements, including standing height (cm), weight (kg), and circumferences of the waist, hip and chest (cm) were taken by local health professionals according to a standard protocol. Height and weight were respectively measured to the nearest 0.1 cm and 0.1 kg with the subjects standing without shoes and wearing light clothing only. Waist circumference (WC) was measured to 0.1 cm at the midpoint between the lower rib and the upper iliac crest. Body mass index (BMI) was calculated as weight in kilograms divided by the square of height in meters. insufficient (UIC<100µg/L), sufficient (100-199µg/L), more than adequate (200-299µg/L) and excessive (≥300µg/L). Each participant was also asked to bring a salt sample of more than 20 g from home and salt iodine concentration (SIC) was measured by using a national standard method with a proper quality control (GB/T 13025.7-2012). 22 Iodized salt consumption status was grouped into two categories: non-iodized-salt (SIC<5mg/kg) and iodized-salt (≥5mg/kg). 23

Statistical analysis
Statistical analysis was based on data from 474 girls with complete information of thyroid hormones and anthropometric measurements. All participants were categorized into three groups of under / normal weight, overweight, and obesity status according to the BMI growth reference values for Chinese children suggested by Li H et.al. 24 The BMI (kg/m 2 ) cut-offs for overweight and obesity in girls were 19.6 and 22.7 for age 11, 20.5 and 23.9 for age 12, 21.4 and 25.0 for age 13, and 22.2 and 25.9 for age 14, respectively. Subjects were also divided into two groups of central obesity according to the cutoff values of 85 th and 95 th percentiles of WC data. 25 Due to the lack of reference range of TSH for girls during puberty, the serum TSH concentration was categorized into three tertiles by corresponding cut-off values: tertile 1 (<1.53 mU/L), tertile 2 (1.53 mU/L≤TSH≤2.37 mU/L), and tertile 3 (>2.37 mU/L). Youth development assessment was according to the total scores of three items including menarche, breast development and body hair growth, and then converted into five stages. In the current analysis, we labelled "prepubertal" for stage 1, "pubertal" for stages 2, 3 and 4 combined and "postpubertal" for stage 5. 21 26 Wilcoxon test and χ 2 test were used to analyze continuous variables and categorical variables, respectively. Multivariate logistic regression models were utilized to estimate the odds ratios for BMI-or WC-based overweight and obesity in relation to TSH levels after adjustment for age, puberty stage, family history of thyroid disease, iodized-salt consumption, income, parents' education, sleeping duration, physical activities, and serum FT4 concentrations. Potential effect modifications by age, puberty stage and iodized-salt consumption on the associations of interest were also examined by including associated interaction terms into the multivariable analysis. Due to a considerable day-to-day variation in urine iodine excretion, one-spot urinary iodine level was appropriate to evaluate the iodine status for population, but not for individuals. 27 Therefore, in above analysis, iodized-salt consumption instead of urine iodine concentration was considered as an adjustable variable into the models. Goiter (1.90%) was not common among these girls and were not included in the analysis. All analysis were performed by using SPSS software for Windows (version 24.0, IBM Crop., Armonk, New York, USA).

Results
The mean age was 12.48(±0.69) years for the study population. Of the 474 school-aged girls, 94 (19.8%) were overweight or obesity based on BMI, while 103 (21.7%) had central obesity based on WC (Table 1). The prevalence of central obesity measured by WC increased with age (P=0.023) and showed area disparity (P=0.001), while the prevalence of general obesity measured by BMI was similar across age and area groups. Girls of postpuberty were most likely to be overweight or obesity compared with other groups (P<0.001). The median serum TSH concentration in BMI-based overweight or obese girls (2.16mU/L) was higher than that in normal weight girls (1.85mU/L) (P=0.037), while TSH concentration was comparable between girls with /without central obesity (P=0.173).  Table 3). The association between TSH and the BMI-based risk of overweight or obesity was not significantly modified by age, puberty status and iodized salt consumption. In stratified analysis, the association tended to be stronger in girls aged 13 to 14 years, with consumption of iodized salt and during puberty.

Discussion
In this population-based study of pubertal girls in East China, we observed that girls with a higher serum TSH concentration had a higher risk of obesity or overweight measured by BMI or WC. Previous studies demonstrated that TSH was significantly positive related to obesity measured by BMI in pediatric outpatients, 16 28 and measured by WC in adolescents. 29 Increased WC, as a confirmed risk factor for many chronic conditions, is more prevalent in children with mild subclinical hypothyroidism (featured with higher serum TSH levels) than healthy euthyroid children. 30 Also, a significant relationship between TSH changes and alterations in WC was observed in Tehran women during longterm follow-up. 31 TSH, also known as thyrotropin, is produced by the anterior pituitary. TSH secretion is regulated by the thyroid releasing hormone (TRH), and limited by negative feedback from the thyroid hormones (THs). 32 TH regulate basal metabolism, which constitutes approximately 66% of total daily energy expenditure. 33 In addition, THs are primarily involved in lipid and glucose metabolism with a mediator named sterol regulatory element-binding proteins (SREBP-2). 34 35 Nader et al. found that the increase in TSH level within the reference range was associated with an increase in insulin and an increase in homeostasis model assessment (HOMA) levels of insulin resistance(IR), 36 which may be caused by obesity, and may also contribute to the development of obesity in childhood. 37 Insulin is a critical regulator of adipoctye biology, and promotes adipocyte triglyceride storage by some mechanisms, including transportation of glucose, differentiation of preadipocytes to adipocytes, and synthesis of triglyceride (lipogenesis). 38 TSH was positively related to triglyceride and non-HDL lipoproteins in a large population-based study of children and adolescents. 39 Moreover, central depots of fat was much more sensitive to IR. 38 On the other hand, obesity may be a cause of thyroid dysfunction and elevated TSH levels 12 . One plausible explanation is that adipose tissue secretes inflammatory cytokines into the general circulation, such as tumor necrosis factor (TNF-α) and interleukin (IL-1, IL-6). 40 These cytokines impede the expression of sodium iodine transporter mRNA and the activity of iodine uptake in human thyroid cells, which reduces the secretion of THs, and then leads a compensatory rise in TSH levels.
Another hypothesis is that the production of leptin-mediated pro-thyrotropin-releasing hormone (pro-TRH) increases with weight gain. 41 Leptin is predominantly released by adipocytes and stimulates TSH secretion by hypothalamic-pituitary axis. 42 Non-synonymous mutations in thyroid stimulating hormone receptor (TSH-R) gene also plays a role in elevated TSH levels in relation to obesity. 43 In stratified analysis, the positive association between TSH and obesity was only observed in girls aged 13 to 14 years. In general, timing of puberty in girls relies on age at menarche, and earlier menarche was related to an increased risk of adulthood obesity. 44 It seems that older girls were likely to be at the later stage of puberty. Aromatase activity increases with elevated adiposity in puberty, leading to increased conversion of androgens to estrogens, 45 which has a growth-promoting effect on thyroid hormones. 11 The strengths of our study include population-based school-aged girls, directly-determined thyroid hormones and objectively anthropometric measurements following a standardized protocol. To our knowledge, this is the first study to observe the relationship of thyroid function with adiposity in girls around puberty. The study has several limitations. First, we did not have information on TPO-Ab (thyroid peroxidase antibody), which is better representative of thyroid function. In practice, determination of serum TSH level should be repeated within 3 months after the initial test. Moreover, we did not measure body composition, although BMI and WC are common indicators to define adiposity in population-based studies. In addition, cross-sectional design does not provide evidence for a causal association between TSH and obesity. In our study, But the changes of serum TSH level and body weight of these girls will be prospectively observed in the follow-up visit for three years, so the definite relationship between the above two variables will be further interpreted.

Conclusions
TSH level was positively associated with both general and abdominal obesity. Obesity was more prevalent in postpubertal than prepubertal. The association of TSH with obesity was independent of puberty.