While it is well known that hyperthyroidism leads to weight loss, and hypothyroidism is associated with weight gain, there has been an increasing focus on the relationship between thyroid function and body weight in recent years. A moderate elevation of TSH concentrations, which is associated with triiodothyronine (T3) values in or slightly above the upper normal range, is frequently found in obese humans. These thyroid hormone alterations in obesity seem rather a consequence than a cause of obesity since weight loss leads to a normalization of elevated thyroid hormone levels. The underlying pathways are not fully understood. White adipose tissue, the largest energy store in the body, actively produces various hormones, cytokines, and chemokines, which together exert important roles in homeostasis and also in thyroid hormone regulation. The adipokine leptin seems to be the most promising link between body weight and TSH levels since leptin stimulates the hypothalamicpituitarythyroid axis.
High serum T3 levels in obesity and overfeeding suggest a role for T3 in metabolic adaptation to these situations: increased basal metabolic rate and thermogenesis in obesity and as a consequence the availability of accumulated energy for conversion into fat is diminished. 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.
Declaration of interest: The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.
Funding: This work was supported, however funding details are unavailable.