A moderate elevation of thyrotropin (TSH) concentrations, which is associated with triiodothyronine (T3) values in or slightly above the upper normal range, is frequently found in obese humans. These alterations seem rather a consequence than a cause of obesity since weight loss leads to a normalization of elevated thyroid hormone levels. Elevated thyroid hormone concentrations increase the resting energy expenditure (REE). The underlying pathways are not fully understood. As a consequence of the increased REE, the availability of accumulated energy for conversion into fat is diminished. In conclusion, the alterations of thyroid hormones in obesity suggest an adaptation process. Since rapid weight loss is associated with a decrease of TSH and T3, the resulting decrease in REE may contribute towards the difficulties maintaining weight loss. Leptin seems to be a promising link between obesity and alterations of thyroid hormones since leptin concentrations influence TSH release.