Our thyroid gland produces a naturally occurring hormone called levothyroxine- also known as T4. The thyroid hormones in the body consists in the regulation of burning fat, carbohydrates and protein. They are responsible for the amount of the metabolic turnover, which is burned in the resting state during a given period of time. Bodybuilders occasionally use this during the mass phase, to increase the metabolic turnover and burn more calories. Furthermore, bodybuilders take protein preparations and/or Clenbuterol with the use of this hormones to counteract the muscle breakdown.

Side effects include trembling, restlessness, dizziness, sweating, weight gain, hair loss, diarrhea, insomnia, nausea, increased heart rate etc.