The primary thyroid hormone that regulates the rate of metabolism is triiodothyronine, commonly known as T3. T3 is the biologically active form of thyroid hormone that influences basal metabolic rate (BMR) by stimulating ATP production and maintaining ion gradients critical for cellular metabolism. It acts on multiple metabolic pathways, including fat, glucose, and protein metabolism, and enhances energy expenditure by increasing the activity of ion pumps such as Na⁺/K⁺-ATPase and Ca²⁺-ATPase in cells
. T3 is mainly produced by the conversion of thyroxine (T4), the precursor hormone secreted by the thyroid gland, through the action of deiodinase enzymes in target tissues. This local activation of T4 to T3 is a key mechanism for regulating metabolism at the cellular level
. In summary, T3 is the primary thyroid hormone responsible for regulating metabolic rate by increasing energy expenditure and stimulating metabolic processes throughout the body
