What is the primary effect of increased thyroxine (T4) secretion on basal metabolic rate (BMR) and on long-term energy balance?

• A. T4 has no measurable effect on metabolism but only changes appetite hormones such as ghrelin and leptin
• B. T4 directly causes immediate loss of muscle mass without affecting caloric expenditure, thereby causing weight loss that is unhealthy and unrelated to energy balance
• C. Increased T4 lowers BMR and therefore promotes weight gain by reducing resting energy expenditure
• D. Increased T4 raises BMR, increasing background energy expenditure and tending to reduce energy storage (weight) over time if intake is unchanged

Answer: (D)