Thyroxine is the primary hormone associated with which physiological processes?

• A. Immune response
• B. Calcium metabolism
• C. Blood sugar regulation
• D. Metabolism and heart rate

Answer: (D)

Thyroxine mainly sets the body's metabolic rate and heart activity. This thyroid hormone acts on many tissues to ramp up metabolism—driving faster energy use, greater heat production, and increased activity of enzymes involved in carbohydrate and fat metabolism. It also makes the heart more responsive to sympathetic signals, raising heart rate and the force of contraction. Together, these effects tie thyroxine closely to processes that control overall metabolism and heart rate.

Calcium metabolism is primarily regulated by parathyroid hormone and vitamin D, immune responses are driven by immune cells and signaling molecules, and blood sugar regulation is mostly controlled by insulin and glucagon. Thyroxine can influence energy use in various tissues, but its main role is in metabolism and cardiac activity.