What organ controls the rate of metabolism?

The thyroid gland plays a crucial role in regulating metabolism in the body through the secretion of thyroid hormones, primarily thyroxine (T4) and triiodothyronine (T3). These hormones increase the metabolic rate by influencing the speed at which cells use energy from nutrients, which affects various bodily functions such as heart rate, body temperature, and even the management of weight. A well-functioning thyroid gland is essential for maintaining the body's overall energy balance and metabolic health.

The other glands mentioned in the choices, such as the thymus and parathyroid glands, have different roles. The thymus is primarily active in developing the immune system during childhood and does not directly influence metabolic rate. The parathyroid glands regulate calcium levels in the blood but do not have a direct role in controlling metabolism. Thus, the thyroid gland is the key organ in governing metabolic processes, making it the correct answer.