Influence of the degree of obesity in obese euthyroid: An observational study of the correlation between body mass index (BMI) and thyroid stimulating hormone (TSH) in patients undergoing bariatric surgery.

doi:10.21203/rs.3.rs-1978628/v1

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Influence of the degree of obesity in obese euthyroid: An observational study of the correlation between body mass index (BMI) and thyroid stimulating hormone (TSH) in patients undergoing bariatric surgery.

https://doi.org/10.21203/rs.3.rs-1978628/v1

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Objective: Although controversial, there may be a positive correlation between the body mass index (BMI) of individuals with obesity in euthyroidism and serum levels of thyroid stimulating hormone (TSH). This study aimed to evaluate the correlation between BMI and serum levels of TSH in individuals with morbid obesity undergoing bariatric surgery.

Patients and methods: The medical records of patients treated between the years 2012 and 2016 were used. A total of 96 patients with obesity, pre-surgery BMI ≥ 40 kg/m2, being followed up in the endocrinology unit, with mean age of 50 years, were evaluated pre and post operatively. In addition to the plasma TSH dosage by IRMA and plasma free T4 by RIE, age, BMI and biochemical parameters (glycaemia, total cholesterol and triglycerides) were analyzed.

Results: Patients with BMI > 40 kg/m2 prior to surgery showed higher serum TSH than subjects with a BMI <40 kg/m after surgery (2.48 ± 0.2 vs. 1.81 ± 0.25, p < 0.001). The decrease in BMI was accompanied by an improvement in the glycemic and lipid profiles, as well as free T4 and TSH levels are shown to accompany BMI.

Conclusion: Bariatric surgery was efficient in controlling obesity, since 100% of the patients had their degree of obesity decreased with concomitant metabolic improvement. We found that BMI and TSH are positively related, as post-surgical patients had both BMI and TSH decreased.

TSH

Thyroid

Obesity

Bariatric surgery

Body Mass Index

Exogenous obesity is a clinical pathology defined by excessive accumulation of body fat, increasing health risk¹. Studying and grading the overweight and obese population is important as it enables the identification and management of risk groups, as well as the creation of control and intervention strategies².

The body mass index (BMI), which relates the individual's body mass and height (kg/m²), is still considered the most useful parameter in the classification of obesity². Studies reveal that the prevalence of obesity grows at an alarming rate, affecting both developed and developing countries². Considered the most prevalent metabolic disease, obesity is responsible for about 80% of cases of type 2 Diabetes Mellitus, 35% of coronary syndromes and 55% of systemic arterial hypertension in European adults³. Currently, Brazil is the fourth country in the world with the highest prevalence of obesity⁴, with an incidence of 17.8% in the male population and 20.7% among women⁵.

The etiology of obesity is complex and multifactorial, involving the socio-environmental context, genetic characteristics and a long period of positive energy balance, among other factors⁶. Several endocrine alterations have been described in obesity, so that their association with the hypothalamic-pituitary-thyroid axis is widely studied⁷.

Previous studies have shown that leptin, a hormone secreted by adipose cells, may play an important role in obesity changes in this axis⁸. Fasting has been shown to cause a drop in thyroid- stimulating hormone (TSH) levels in animals; it causes a drop in leptin levels, which consequently does not stimulate the neurons of the hypothalamic paraventricular nucleus, who produce thyrotropin- secreting hormone (TRH)⁹. Furthermore, leptin can increase the conversion of thyroxine (free T4) to triiodothyronine (T3) through the action of deiodinases⁸. In the present study, the leptin levels of patients were not measured.

Thyroid hormones (TH) are responsible for regulating the metabolism, so the correlation between thyroid changes, weight gain and appetite has already been evidenced in the literature¹⁰. Excessive TH – thyrotoxicosis - is characterized by increased energy expenditure and, consequently, weight loss. Conversely, in the lack of TH that occurs in hypothyroidism, energy expenditure decreases and weight gain occurs¹⁰. Although thyroid function in obese individuals is normal in most cases, previous studies demonstrate a positive correlation between BMI and serum TSH levels¹¹. The increase in TSH may be associated with adipogenesis by stimulating the differentiation of preadipocytes into adipocytes¹². In a study carried out with obese patients, it was found that obese individuals with a BMI greater than 40 kg/m² had higher TSH levels than those with mild obesity (BMI values below 40 kg/m²)¹³, and many times in clinical practice, it can be difficult to distinguish an obese individual with slightly altered TSH levels from a physiological increase in subclinical hypothyroidism⁸. TSH may represent a marker of altered energy, with elevated TSH levels being a homeostatic adjustment for excessive adiposity.

The treatment for obesity includes pharmacological and non-pharmacological modalities¹⁴ and in some selected cases, the surgical modality of obesity treatment is indicated, the most used being the Roux-en-Y gastric bypass¹⁴.

Although controversial, there may be a positive correlation between the BMI of obese euthyroid individuals and the serum levels of TSH and thyroxine, an alteration that may represent a challenging situation in clinical practice, since obese patients diagnosed with subclinical hypothyroidism actually present a physiological alteration of increased TSH secondary to such mechanisms described above. According to the literature review carried out for this project, it is expected to find a correlation of the increase in the degree of obesity with high levels of TSH and normal levels of thyroid hormones.

This observational retrospective study was carried out by reviewing the medical records of patients admitted from 2012 to 2016 at the University Hospital of The Medical School (FMB) of São Paulo State University (Unesp), Botucatu Campus, at the Obesity Outpatient Clinic. Patient data between 2016 and 2019 were also observed in order to update them and expand this analysis. This research was approved by the Research Ethics Committee of the institution, process no. 77947417.2.0000.5411. The study included patients diagnosed with obesity who did not have thyroid disease (euthyroid) and who underwent bariatric surgery as the treatment of their obesity.

Patients and exclusion criteria

196 medical records of patients treated at the Obesity Outpatient Clinic were analyzed and the following exclusion criteria were applied: patients with diagnosed thyroid dysfunction, users of thyroid hormones, patients undergoing radioiodine therapy and patients with absence of anthropometric and biochemical data from the pre and/or post bariatric period in the medical records system of the University Hospital. Thus, the study sample consisted of medical records of 96 eligible individuals, 12 (12.5%) male and 84 (87.5%) female with homogeneous age, mean of 50 years. Table 1 shows the general characteristics of eligible patients before bariatric surgery.

Table 1

General characteristics of patients before bariatric surgery
		Patients with obesity in euthyroidism, n(%)
Gender	Male	12 (12.5)
Gender	Female	84 (87.5)
Mean age (years)		50
Nutrition status*	Class II Obesity	14 (14.6)
Nutrition status*	Class III Obesity	82 (85.4)
Type 2 Diabetes Mellitus		20 (21)
Total Cholesterol > 190mg/dL		45 (47)
Triglycerides > 150mg/dL		45(47)

Study design

Patients were classified into four groups, based on the WHO classification, which are: G1 (BMI ≤ 18.5–24.9), G2 (BMI 25–29.9), G3 (BMI 30–34.9) and G4 (BMI 35 ≥40). At the pre-surgery moment, these individuals presented BMI 35-39.9kg/m² (Class II Obesity) or ≥40 kg/m² (Class III Obesity) and were reclassified in the 6-month follow-up after bariatric surgery. Of the ninety-six eligible participants, eighty-two individuals had a BMI ≥40 kg/m² and 14 individuals had a BMI ≥ 35- 39.9kg/m².

Patients were analyzed for weight and BMI two years after surgery and for their most current weight and BMI, up to the year 2019. The percentage of excess weight loss in these two periods was also evaluated.

Variables of interest and evaluated outcomes

The following parameters were analyzed to relate the degree of obesity with the serum levels of TSH, aiming to establish a new level of TSH normality in subjects with obesity and in euthyroidism: BMI (kg/m²), TSH (mU/l, reference value: 0.3–4.0) by immunoradiometric assay (IRMA), free T4 (ng/dl, reference value: 0.7–1.8) by radioimmunoassay (RIE), total cholesterol (TC, mg/dl, reference value <190), triglycerides (TGL, mg/dl, reference value < 150) and blood glucose (mg/dl, reference value 70-99); percentage of excess weight loss (according to the calculation: Preoperative weight – Current weight*100/Preoperative weight – Ideal weight) using a BMI= 25 kg/m².

Statistical analysis

The analysis of pre and post operative parameters was performed using the technique of multivariate analysis of variance for the model of repeated measures in independent groups, complemented with the Bonferroni test of multiple comparisons¹⁵.

The study of the association between the qualitative variables involved the Goodman association measure for contrasts between and within multinomial populations ^16,17. When the association involved quantitative variables, the option was the linear Pearson or Spearman correlation, according to the adherence of the variables to the normal distribution of probabilities. In case the association involved qualitative and quantitative variables simultaneously, the technique of parametric or non-parametric variance was used simultaneously according to the distribution of variable probabilities under analysis¹⁸. The study was complemented with descriptive measures of the variables (position and variability measures) and the association between the pairs of variables was performed using Pearson's linear correlation coefficient. All discussions are held at the 5% significance level¹⁹.

Bariatric surgery improves anthropometric and metabolic characteristics of obese euthyroid patients

Bariatric surgery was effective in morbid obesity; patients had a significant change in post- surgical BMI compared to preoperatively rates. According to Table 2, one can verify that after surgery the patients had a reduction in BMI leaving class III obesity (BMI ≥ 40), with approximately 35% of the individuals progressing to overweight and 12% to eutrophication.

Table 2

BMI of patients after bariatric surgery
Groups	BMI*	BMI reclassification after bariatric surgery: n(%)
G1	≤ 18.5–24.9	12 (12.5)
G2	25–29.9	34 (35.4)
G3	30–34.9	27 (28.1)
G4	35 ≥ 40	23 (24)

* Body mass index (BMI), calculated as current weight (kg) / height² (m²). Patients were classified according to BMI as underweight (< 18.5 kg/m²), normal weight (18.5–24.9 kg/m²), overweight (25.0-29.9 kg/m²), or obese (≥ 30.0 kg/m²).

Reduction in BMI accompanied by improvement in glycemic profile

Weight reduction leads to an improvement in the patient's metabolic profile, as shown in Figure 1.

Figure 1: Relationship between blood glucose and BMI reduction. Reduction of glycemic levels in the Post Bariatric period of the four BMI classification groups.

* = p <0.005 , **= p <0.001.

Reduction in BMI accompanied by improvement in lipid profile:

Figure 2 shows the decrease in total cholesterol (TC) and triglyceride levels accompanied by a reduction in BMI after bariatric surgery.

Figure 2: BMI vs Total Cholesterol (TC) and Triglycerides.

* p< 0.05 post BMI in relation to pre-bariatric surgery; # p > 0.05 CT post compared to pre-bariatric surgery; & p > 0.05 Post triglycerides compared to pre-bariatric surgery.

Thyroid function and BMI

Free T4 levels were lower in the postoperative period compared to the preoperative period in groups G2 and G4; in addition, a positive correlation between higher BMI and higher levels of free T4 was observed in the pre-surgical exams. TSH values also decreased with the change in BMI.

Table 3. Level of free T4 before and after bariatric surgery

Table 3

Level of free T4 (ng/dL) before and after bariatric surgery.
Moment of assessment
BMI	Pre	Post	p
G1	1.07a	1.03	p > 0.05
G2	1.13a	1.04	p < 0.05
G3	1.15ab	1.08	p > 0.05
G4	1.29b	1.08	p < 0.001
	p < 0.01	p > 0.05

Decreased levels of free T4 with the reduction of BMI promoted by bariatric surgery: Positive correlation between reduction of adipose tissue and reduction of free T4 levels.

Table 4 shows the decrease in TSH levels after surgery (G1-G4 groups) compared to the preoperative period (2.48 ± 0.2 vs 1.81 ± 0.25 p < 0.001) and in Fig. 3 we can observe a positive correlation between BMI and TSH.

Table 4

TSH level (mcg/ml) before and after bariatric surgery
Moment of assessment
BMI	Pre	Post	p
G1	2.64	2.11	p > 0.05
G2	2.22	1.57	p < 0.001
G3	2.41	1.64	p < 0.001
G4	2.64	1.94	p < 0.005
	p > 0.05	p > 0.05

Table 4. TSH level before and after bariatric surgery

Decreased TSH levels with the reduction of BMI promoted by bariatric surgery: Positive correlation between reduction of adipose tissue and reduction of TSH levels.

Figure 3. TSH vs BMI: linearity aspect between variables p < 0.0001.

Association of BMI with TSH, free T4, glucose, cholesterol, triglycerides and age

The association between the variables evaluated in this study in relation to BMI can be seen in Table 5, where Pearson's linear association measure was performed according to pre and post bariatric moments. BMI showed a statistically significant relationship to TSH at the time of pre-surgical evaluation (0.291, p < 0.01) and evidenced a decrease in the value of this correlation in the postoperative period. The association between BMI vs free T4 showed a decrease in the value in the postoperative period. Regarding glycemic metabolism, an increase in the values of the association between BMI and fasting glucose was observed, which was statistically significantly at the postoperative moment (0.328, p < 0.01). The association between BMI and lipids was significant at the preoperative moment when we analyzed total cholesterol and at the postoperative moment when analyzing triglycerides. The association of BMI and age was significant after bariatric surgery.

The percentage of excess weight loss at the time of surgery and two years after was statistically significant when analyzed in association with BMI before (-0.324 p < 0.005) and after (-0.790 p < 0.001) surgery, and this association was reversed when analyzing such variables.

Table 5

Association of BMI with TSH, Free T4, fasting glucose, total cholesterol, triglycerides and age.
	Moment of assessment
Association	Pre	Post
BMI vs TSH	0,291 (p < 0,01)	0,108 (p > 0,05)
BMI vs Free T4	0,153 (p > 0,05)	0,075 (p > 0,05)
BMI vs Fasting glucose	-0,050 (p > 0,05)	0,328 (p < 0,01)
BMI vs Total Cholesterol	-0,211 (p < 0,05)	-0,022 (P > 0,05)
BMI vs Triglycerides	-0,141 (p > 0,05)	0,237 (p < 0,05)
BMI vs Age	0,068 (p > 0,05)	0,246 (p < 0,05)

Table 5 - Association of BMI with TSH, free T4, blood glucose, cholesterol, triglycerides and pre and post-bariatric age.

The measure of the association between the variables in the moments was established by Pearson's linear correlation coefficient (ZAR, 2007).

Obesity is considered the most common metabolic disorder in the world^21,22, being considered a public health problem that affects both developed and developing countries²³. Obesity occurs when energy consumption exceeds energy expenditure²¹ and is mainly characterized by excessive adipose tissue (AT)^24,25. The amount and distribution of adipose tissue are associated with many adverse consequences, such as hypertension, type 2 Diabetes Mellitus or cardiovascular diseases²⁶.

Approaches used to deal with individuals with obesity include food restriction and physical exercise²⁷. According to some authors, the determination of individualized epigenetic profiles can be used to guide clinical decision-making, in the progression of treatment to improve the health and quality of life of people with obesity²⁸. Pharmacological therapy has been used to treat obesity, but its success is relative and sometimes it does not overcome their adverse effects²⁹. Thus, bariatric surgery is the most effective treatment for weight loss and maintenance. Weight loss with bariatric surgery can reach 50–75% of excess body weight and can be maintained for a long period or for life depending on the patient's lifestyle^30,31.

In our study, bariatric surgery proved to be efficient in controlling the obesity of the evaluated patients, since 100% of the patients had a decrease in the degree of obesity, leaving morbid obesity in the 4 groups presented. Concomitantly, there was an improvement in lipid profiles with a reduction in total cholesterol and triglyceride levels, which indicates that a smaller amount of adipose tissue combined with a reduction in caloric intake leads to metabolic improvement³². The reduced levels of blood glucose in the postoperative period also indicate the influence that adipose tissue can promote on carbohydrate metabolism, acting mainly on the insulin resistance of patients³³.

Studies show improvement in metabolic parameters in the postoperative period as a result of normalization of fasting blood glucose, decrease in insulinemia and calculation of insulin resistance by the Homeostasis Assessment Model of Insulin Resistance (HOMA-IR)³⁴, in addition to an increase in the levels of glucagon-like peptide 1 (Glucagon-like peptide-1, GLP-1) and Peptide YY, which are hormones produced by the intestines that play an anorectic role in controlling hunger and satiety³⁵. The increase in these hormones was attributed to the faster arrival of food in the ileum without being completely digested³³.

Several studies have demonstrated the relationship between thyroid function and weight status, especially in obesity. Different studies, mainly of cross-sectional design, have found an association between changes in TSH and thyroid hormone levels, even within the normal range, and a higher body mass index 13,36,37,38.

The relationship between serum TSH levels and obesity is widely studied. It is described in the literature that obese populations have increased TSH levels²³; however, the pathophysiological mechanism responsible for this metabolic adjustment is still undetermined³⁹. The present study demonstrated a significant positive relationship between the change in BMI classification due to weight loss, and the decrease in serum TSH levels. Many authors have studied the influence of adipokines in this process. Among them, leptin would play an important role in this scenario. It is believed that leptin can mediate both obesity and TSH factors. Produced by adipocytes, leptin is a substance that acts on hypothalamic cells informing the central nervous system about satiety¹³, as well as at the hypothalamic level, modulating the stimulation of pituitary secretion of TSH⁴⁰.

At the same time, it is known that TSH has an effect on adipose tissue and directly promotes that is, without the involvement of thyroid hormones - the differentiation of preadipocytes into adipocytes⁴¹. Thus, obesity is a scenario in which the individual has high levels of leptin¹³ with greater stimulus for TSH secretion, which will act on adipose tissue, stimulating adipogenesis or accumulation of lipids in existing adipocytes that will secrete more leptin, who in its turn stimulates neurons in the hypothalamic supraventricular nucleus to secrete more TRH and, consequently, an even greater TSH secretion. This mechanism could be part of an adaptive response to supply the high thermogenesis resulting from the increase in the amount of body fat¹³.

TSH may represent a marker of altered energy, with high levels of TSH being a metabolic adjustment for adiposity, as it directly stimulates adipogenesis independent of thyroid hormones.

This alteration in the thyroid axis in obesity may represent a metabolic alert of these individuals, suggesting that obesity is the cause of this alteration and instead of initiating treatments for subclinical hypothyroidism, one should first treat the obesity status of these individuals. Studies have shown that individuals with an established diagnosis of subclinical hypothyroidism who did not receive hormone treatment had a reduction and consequent normalization of TSH within a 12 months period after bariatric surgery⁴². However, our study demonstrated that the treatment of obesity through bariatric surgery already results in benefits for the patient within the first six months of evolution, resulting in the reduction of TSH levels in obese euthyroid individuals.

Bariatric surgery was efficient in controlling obesity, since 100% of the patients had their degree of obesity decreased. Our study was able to achieve the objective of positively correlating BMI and TSH since post- surgical patients had a decrease in their BMI and TSH, accompanied by metabolic improvement, evidenced by the reduction of total cholesterol, triglycerides and blood glucose parameters.

Ethical Approval and Consent to participate:

The local ethics committee approved this present study, by the registration number: 2387916 and CAAE number: 77947417.2.0000.5411 registered on Plataforma Brasil.

Availability of data and materials:

The data available here were evaluated by the internal system of the Hospital das Clínicas da Faculdade de Medicina de Botucatu, where the electronic medical records and records of the patients are located.

Competing interests

There is no conflict of interest of the authors in this study.

Funding:

This research received an incentive grant from PIBic CNPq

Authors' contributions:

All authors contributed equally to this research.

Acknowledgements

We would like to thank all researchers and patients who collaborated in this study.

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Influence of the degree of obesity in obese euthyroid: An observational study of the correlation between body mass index (BMI) and thyroid stimulating hormone (TSH) in patients undergoing bariatric surgery.

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Abstract

Figures

Introduction

Patients And Methods

Study design

Variables of interest and evaluated outcomes

Statistical analysis

Results

Bariatric surgery improves anthropometric and metabolic characteristics of obese euthyroid patients

Reduction in BMI accompanied by improvement in glycemic profile

Reduction in BMI accompanied by improvement in lipid profile:

Thyroid function and BMI

Association of BMI with TSH, free T4, glucose, cholesterol, triglycerides and age

Discussion

Conclusion

Declarations

References

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