What is the net effect of thyroid hormones on metabolic rate and heat production?

• A. No change in basal metabolic rate, heat production, or energy expenditure.
• B. Increase energy expenditure only.
• C. Increase basal metabolic rate, heat production, and energy expenditure.
• D. Decrease basal metabolic rate, heat production, and energy expenditure.

Answer: (C)

Thyroid hormones T3 and T4 ramp up the body's metabolism across many tissues. They act on nuclear receptors to increase the transcription of genes involved in mitochondrial function and oxidative metabolism, so cells consume more oxygen and burn more fuel even at rest. This boost in cellular respiration drives a higher basal metabolic rate. As metabolism runs hotter and more ATP turnover occurs, more heat is released as a byproduct, raising heat production. Because overall energy expenditure includes basal metabolic rate plus heat production and other energetic costs, the total energy use goes up as well. So the net effect is an increase in basal metabolic rate, heat production, and energy expenditure.