Which organ is primarily responsible for controlling metabolism?

The thyroid gland is primarily responsible for controlling metabolism because it produces hormones, particularly thyroxine (T4) and triiodothyronine (T3), which regulate the body's metabolic rate. These hormones influence how the body uses energy, impacting various functions such as heart rate, body temperature, and overall energy levels. The thyroid gland acts as a critical regulator of metabolism by responding to signals from the hypothalamus and pituitary gland to adjust hormone levels based on the body’s needs.

Other organs mentioned, while important for various bodily functions, do not primarily control metabolism in the same way. The heart's main role is to pump blood and maintain circulation. The liver does participate in metabolic processes, like detoxification and nutrient storage, but it does not regulate metabolism as a whole. The pituitary gland produces hormones that influence other endocrine glands, including the thyroid, but it serves more as a regulatory center rather than the primary controller of metabolic functions. Thus, the thyroid gland holds the central role in metabolic regulation among the choices provided.