Correlation between Body Mass Index and Thyroid Function in Euthyroid Individuals in Greece

Although the effects of hypothyroidism and hyperthyroidism on body weight have been clearly demonstrated, there is no sufficient data on the relationship between the body mass index (BMI) and minor differences within the normal range of thyroid function. The present study aims to investigate the relationship of fluctuations of the thyroid stimulating hormone (TSH) and thyroid hormones with BMI in euthyroid subjects. The study included 736 euthyroid healthy individuals of known age, weight, height, and biochemical picture of the thyroid function. Individuals were classified according to BMI and thyroid hormones’ values. The variations of normal thyroid function in euthyroid individuals were associated with body weight changes. A statistically significant positive correlation between BMI and thyroid function in women was found, while in men the correlation was not statistically significant. The alterations in thyroid function are mainly primary, while changes in body weight are secondary. The reason may be simple or multifactorial, and the biological mechanism is not completely known. Finally, the thyroid function disorders in conjunction with the strong influence of various environmental factors can increase body weight and lead to obesity. 1. Introduction The relationship between thyroid function and body weight in euthyroid individuals has been given a great medical concern. Various researchers have studied the effect of the thyroid hormones on body mass index (BMI), and it has been demonstrated that overt thyroid dysfunction affects body weight. Clinical hypothyroidism causes an increase in body weight, while hyperthyroidism reduces it [1]. However, variations in thyroid function exist also between individuals with thyroid hormones’ levels within the reference (physiologic) range [2]. These slight differences within the normal thyroid function may have important implications for the regulation of body weight and thus the prevalence of obesity. Although the optimal values for thyrotropin (TSH), T4 and T3 are not firmly established, there is a modern trend towards the narrowing of the reference normal range, especially for TSH [3]. In 2003, the American Association of Clinical Endocrinologists (AACE) defined the boundaries of the normal thyroid function and proposed the treatment of thyroid dysfunction when the serum TSH levels are off the narrow limits of 0.3–3.0？mIU/L [4]. Until now, data has led to the view that an increase in body weight can be attributed to diverse thyroid function (as it is expressed by TSH levels) even in euthyroid subjects [5,