What regulates metabolism, enhances protein synthesis, and stimulates lipid breakdown?

The correct choice focuses on follicular cells, particularly their role in producing thyroid hormones. These hormones, such as thyroxine (T4) and triiodothyronine (T3), play a crucial role in regulating the body's metabolism. They enhance protein synthesis by increasing the rate at which the body converts amino acids into proteins. This is essential for growth, tissue repair, and overall metabolic function.

Additionally, thyroid hormones stimulate lipid metabolism by promoting the breakdown of stored fats, which releases energy and provides substrates for other metabolic processes. This action contributes to maintaining energy homeostasis and proper metabolic rate.

Other options provided in the question have distinct functions. While adrenaline does play a role in metabolism by stimulating the breakdown of glycogen and fats during stress responses, it does not primarily regulate metabolism in the way thyroid hormones do. Parathyroid hormone primarily regulates calcium levels in the body, and while follicular cells are involved in the synthesis of thyroid hormones, they do not have a direct role in lipid breakdown or protein synthesis regulation on their own.

Understanding the specific roles of follicular cells and the hormones they produce helps clarify why they are integral to metabolic regulation, making this option the most accurate response to the question.