Which of the following factors affect metabolic rate?

Thyroid hormone plays a crucial role in regulating metabolic rate. It is primarily responsible for controlling the body's energy expenditure and affects how quickly the body converts nutrients into energy. Thyroid hormones, particularly thyroxine (T4) and triiodothyronine (T3), increase the metabolic rate by stimulating the metabolic processes in various tissues, including the heart, liver, and skeletal muscle. Higher levels of these hormones generally lead to an increased basal metabolic rate, while lower levels can lead to a decreased rate. This direct impact on cellular metabolism highlights the importance of thyroid hormones in determining overall metabolic rate.

The other factors such as physical activity, absorptive/postabsorptive state, and mental state certainly influence metabolism, but they are not as directly linked to the intrinsic hormonal regulation of metabolic rate as thyroid hormones are. Physical activity can increase energy expenditure temporarily, while the absorptive and postabsorptive states relate to nutrient availability and metabolism after eating or fasting. Mental state can also influence metabolism through stress and hormonal outputs, but these effects are more secondary compared to the primary role of thyroid hormones.