Which hormone is primarily produced by the thyroid to regulate metabolism?

• A. Calcitonin
• B. Melatonin
• C. Thyroxine
• D. Insulin

Answer: (C)

Metabolism is driven by thyroid hormones that set the body's base rate of energy use. The thyroid mainly releases thyroxine, or T4, which enters the bloodstream. In many tissues, T4 is converted to the more active form, T3, and this duo increases cellular metabolic activity across nearly all cells. The result is a higher rate of oxygen consumption and energy production, plus boosted metabolism of carbohydrates and fats and increased heat generation, all of which elevate the overall metabolic rate. This central role in driving metabolism is why thyroxine is the hormone most closely tied to regulating metabolic processes. Other hormones mentioned have different duties: calcitonin helps regulate calcium balance, melatonin governs sleep-wake cycles, and insulin controls blood glucose.