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The correlation between BMI and the various thyroid hormones differed in both sexes. In women, there was a statistically significant association between BMI and TT3 and TT4, while in men BMI was positively associated with FT3 levels and negatively with TSH.

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It is widely accepted that hypothyroidism causes weight gain associated with a decrease in basal metabolic rate (BMR) and thermogenesis, and that there is a perceived association between thyroid hormones and body composition.

This emanates from the function of the thyroid gland in the regulation of basal metabolism and thermogenesis, thereby affecting energy intake, and the glucose and lipid metabolism (Longhi e Radetti, 2013). The most common form of TH replacement therapy is synthetic T4 hormone, which is generally known as levothyroxine (LT4) and it is used to suppress TSH.
FT4 has been closely associated with metabolic syndrome factors independently of insulin resistance. The serum FT4 (not TSH) was found to have a negative correlation with BMI. It was also negatively associated with total cholesterol and triglycerides and positively associated with HDL. The serum TSH levels were positively related only to triglycerides A. Roos,.
The thyroid hormone-induced increase in thermogenesis is explained, among other things, by an increased need for ATP due to increased activity in most cells and reduced efficiency of ATP synthesis, but the specific mechanism has not been settled. Increases are observed for obligative, as well as facultative, thermogenesis.

Even small differences in thyroid function with TSH variation within the normal laboratory range for patients on T4 substitution therapy are associated with measurable differences in resting energy expenditure (REE), but the impact on body mass index (BMI) remains unsettled (al Adsani H.);
However, the association between small differences in thyroid hormone levels, as seen in the general population without thyroid dysfunction, and body weight or BMI has only been described in few previous studies.
In a recent study of old participants, an association was found between serum TSH and BMI, but also subjects with overt thyroid dysfunction were included (Gussekloo J 2004). In a study of elderly subjects, a possible association between mild hypothyroidism and BMI was found among women (Lindeman RD 2003), however with the opposite tendency among men. In another recent study, no association was identified (Hak AE 2000).
A study by Lee and collaborators (2014) demonstrated no significant change in weight following treatment with L-T4 in patients with primary hypothyroidism. The authors observed that 52% of the investigated patients lost weight, with a median weight loss of 3.8kg following treatment. However, taking into account all patients, the overall weight change was only – 0.1kg. An earlier clinical trial showed that in 18 hypothyroid patients, there was an initial
weight loss within the first six months of treatment, but by 24 months, patients had reverted to their pre-treatment weight (Hoogwerf and Nuttall, 1984).

Although L-T4 is considered the mainstay of primary hypothyroidism treatment, combination therapy adding liothyronine (L-T3) has been used in selected group of patients (Chakera and collaborators, 2012). The combined therapy was associated with more weight loss compared to monotherapy with L- T4 (Bente and collaborators, 2005). However, it had a TSH endpoint below the reference range, which is not recommended, as there is a linear correlation of suppressed TSH with cardiovascular dysfunction and decreased bone mineral density (Kostoglou-Athanassiou and Ntalles, 2010).
The evidence suggests that such a relationship is in existence. High TSH levels were shown to be significantly associated with high BMIs, whilst FT4 was inversely associated with body mass. Increased leptin levels and insulin resistance accompanied this profile. This association has led to the unsubstantiated use of thyroid hormone in the treatment of obesity in euthyroid subjects. Experts  agree that  thyroid hormonal therapy should be reserved for obese patients with overt hypothyroidism.