Thyroid hormones are primarily responsible for regulation of metabolism. Thyroid hormones affect basal metabolic rate, heat balance, fat and sugar metabolism, burning of fats and food consumption.

When thyroid functions are disordered, changes occur in weight and energy consumption of the body.

Energy consumption (energy expenditure) is 60% of total energy consumption during rest. In case of excessive secretion of thyroid hormones, in other words hyperthyroidism, the energy consumption increases during rest. In the case of low secretion of thyroid hormones, in other words hypothyroidism, the energy consumption reduces during rest.

In hypothyroidism , 7-8% change in energy consumption also causes a change in fat deposits.  Although TSH levels are within normal limits, weight gain may occur.

The normal range of TSH level is 0.5-4 mIU/l. It has been shown that individuals whose TSH levels are close to the upper limit may gain weight. However, there are also papers that support the opposite.  Although weight gain and high cholesterol are the known results in patients with significant hypothyroidism, the cause of weight gain is still being researched in people with normal TSH levels and without clinical signs.

In people whose TSH levels are close to the upper limit, weight gain is explained with two theories. TSH stimulates the fat production and affects leptin hormone. Starting thyroid hormone treatment is not currently included in the guidelines for the people with such complaints. However, as a result of individual evaluations of patients, starting thyroid hormone at low doses can be considered.