Answer:
Mitochondria
Step-by-step explanation:
Thyroid hormones affect metabolism, increasing oxidative phosphorylation in cell mitochondria. Basically, cells use ATP (adenosine triphosphate) as their energy source to metabolize all the nutrients we consume and it is the mitochondria that produces ATP.
Inside cells, thyroid hormones bind to mitochondria and send them signals to increase or decrease ATP production. Thus, the more thyroid hormones are present, the more ATP will be produced and therefore the faster the metabolism will be.