In recent years, the prevalence of diabetes mellitus (DM) has been rising rapidly and it has become a prominent global public health problem. Currently, around 415 million people worldwide have diabetes, this accounts for 9% of the population, and this number is predicted to increase even further .
Insulin resistance is common in the pathogenesis of type 2 diabetes mellitus (T2DM) and metabolic syndrome [2, 3]. Metabolic syndrome is an accumulation of factors that increase the risk of disease, approximately 2-fold for cardiovascular disease and 5-fold or more for T2DM . Insulin resistance is also a risk factor for stroke and other diseases such as end-stage renal disease . Obesity is the basis of insulin resistance because of the destructive effects of excess fat accumulation on glucose metabolism, which causes functional impairments in metabolic pathways of several areas such as adipose tissue and peripheral organs, like liver, heart, pancreas and muscle .
Thyroid hormones, including thyroxine and triiodothyronine, regulate the synthesis, mobilization, and breakdown of lipids. Therefore, thyroid hormones are closely related to obesity, and slight changes of serum thyroid hormone level can cause local fat accumulation and increased body mass [7, 8]. The thyrotropin receptor (TSHR) has long been considered as a key regulator of thyroid function . Recent studies have shown that thyroid stimulating hormone (TSH) can also directly bind to TSHR in tissues outside the thyroid to exert external effects, such as in the adipose tissue and liver .
The role of thyroid function in energy metabolism has been extensively studied, but traditional viewpoints only emphasize the role of hypothyroidism or subclinical hypothyroidism in obesity. Among patients with obesity but normal thyroid function, thyroid hormones, especially TSH, are significantly different from that of people of healthy weight. This manifests as the TSH, free triiodothyronine (FT3), and free thyroxine (FT4) of obese patients being higher than that of healthy people [11, 12]. The incidence of thyroid diseases in patients with T2DM is significantly increased, especially subclinical hypothyroidism (SCH), and is more common in women than men [13, 14]. However, the relationship between TSH, FT3, FT4 and adipocytokines and lipid metabolism in T2DM patients with normal thyroid function and people with normal blood glucose has not been reported.
The purpose of this study was to explore the changes in thyroid hormones in patients with T2DM and normal thyroid function, compared with people with normal blood glucose. We then studied the correlation between thyroid hormone and lipid metabolism/body fat content.