Final answer:
Thyroid hormones, mainly T4 and T3, stimulate metabolic activity by regulating gene expression, which increases basal metabolic rate and body heat production, thereby enhancing energy use at the cellular level.
Step-by-step explanation:
Thyroid hormones stimulate metabolic activity throughout the body by regulating gene expression. The two main hormones produced by the thyroid gland, tetraiodothyronine (T4) and triiodothyronine (T3), affect nearly every cell. They increase metabolism by triggering the use of carbohydrates and fats for energy, resulting in increased ATP production. Additionally, T3 and T4 hormones influence the basal metabolic rate (BMR), which is the rate at which the body uses energy at rest. These hormones also play a crucial role in glucose oxidation, contributing to body heat production and increasing the rate and force of the heartbeat. Thyroid hormones do not inhibit enzyme activity, increase blood glucose directly, or activate the sympathetic nervous system as their primary mechanism of action; instead, they function mainly by influencing gene expression to increase metabolic activity.