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

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

Answer: (C)

Thyroid hormones raise metabolic rate and heat production. They do this by ramping up energy use in nearly every tissue, increasing basal metabolic rate, and driving more calories through energy pathways even at rest. Mechanistically, T3/T4 boost transcription of genes involved in mitochondrial function and oxidative metabolism, increase Na+/K+ ATPase activity (which uses ATP continuously), and upregulate uncoupling proteins that make mitochondria leak energy as heat rather than store it as ATP. The combined effect is higher oxygen consumption, greater energy expenditure, and more heat generation. That’s why the net effect is an increase in basal metabolic rate, heat production, and energy expenditure. A statement of no change would contradict how thyroid hormones power these metabolic processes.