One of the common features of thyroid hormone deficiency is a weight gain. Many patients complain of increased numbers on scales during the treatment or even after thyroid hormone level has been stabilized. The underlying mechanism of this persistent weight gain remains unclear (Hoogwerf & Nuttall, 1984). A great number of studies have been conducted to identify the correlations between hypothyroidism treatments and BMI. Levothyroxine (L-T4) Levothyroxine replacement therapy is a common choice to treat hypothyroidism. Levothyroxine (L-T4) is one of the most prescribed medication in the UK after aspirin (Chakera, et al, 2012). L-T4 replacement aims to normalize TSH circulating levels and reach the euthyroidism state (Biondi & Wartofsky, 2012).Several studies reported a strong correlation between L-T4 treatment and energy expenditure, therefore, body composition and body weight. Al-Adsani et al (1997) suggested that hypothyroid-treated person might have increased BMI and body fat percentage due to various factors. Howbeit, he suggests that slight changes in TSH levels by minimal L-T4 dosage variations significantly improves REE and lower BMI, as a result. The elimination of excess body water has been suggested by Karmisholt (2011) to be one of a reason for observed weight decrease in L-T4 treated patients. In addition, three months long pilot-study by Gluvic et al (2015) on the effect of L-T4 therapy on parameters of metabolic syndrome demonstrates lower BMI in hypothyroidism-treated subjects by Levothyroxine. Jonklaas et al (2014) and Bakiner et al (2013) reported that in spite achieving euthyroidism state in their patients they haven’t observed weight decrease as was expected. There have been proposed two reasons to explain the results: orexigenic response increase due to elevated thermogenesis; hypothyroid milieu in the tissues wasn’t affected by TSH normalizing. The similar findings reported by Lee, Braverman, & Pearce (2014): there were no significant body mass changes observed after initiation of L-T4. Increased BMI after L-T4 treatment has been found by Ruhla et al (2012). This study has emphasized that higher BMI in patients seen no relation to TSH circulating levels. Triiodothyronine (Liothyronine) L-T3L-T3 monotherapy has shown to be more favorable among obese and overweight patients with hypothyroidism. There is numerous evidence from the studies suggesting that L-T3 treatment reduces body weight and improve lipid profile (Celi, et al, 2011). Weight loss of 2,1 kg and LDL-cholesterol level lowering at 13.3% in L-T3-treated subjects have been noted, comparing to L-T4 therapy (Jonklaas et al, 2014). However, more long-term studies are required, as the risks of skeletal and cardiac toxicity have been reported. Nevertheless, L-T3 treatment regarded as less beneficial comparing to L-T4 due to the usage complications caused by its strict regimen and dosage of administration (Chakera, et al, 2012).Triiodothyronine-levothyroxine combination Over the last decade, new evidence data guided researchers to investigate the combination of L-T3 and L-T4 therapy. The double-blind randomized study, assessed body mass changes in a patient treated with combined T3-T4 therapy and patients with L-T4 therapy alone, shows the benefits in combination use in a decrease of body weight of 1,7 kg, compared to L-T4 therapy (Appelhof B, 2005). However, comprehensive analyses of 11 randomized trials with 1216 subjects found that T3-T4 therapy isn’t more effective than standard L-T4 treatment (Grozinsky-Glasberg, et al, 2006). It hasn’t found enough proof to be more beneficial in lowering BMI or body fat as per the American Thyroid Association Task Force Guidelines (Jonklaas, et al, 2014).