How do thyroxine and triiodothyronine affect metabolism?

Thyroxine and triiodothyronine are the primary regulators of metabolism and tissue oxidation in the human body. While the thyroid gland produces both, thyroxine acts largely as a circulating precursor that is converted into the biologically active form, triiodothyronine, within target tissues by enzymes called deiodinases.12

Triiodothyronine exerts its metabolic effects by entering the cell nucleus and binding to receptors, which alters the transcription of DNA and the subsequent synthesis of various proteins. Its specific metabolic actions include:2

• Energy Regulation: It stimulates the production of proteins that regulate nutrient utilization and oxygen consumption in mitochondria, increasing the proportion of energy dissipated as heat.3
• Nutrient Metabolism: It promotes carbohydrate utilization and lipid metabolism, which increases the utilization of cholesterol.3
• Systemic Activation: It activates the nervous system, leading to increased heart rate and cardiac muscle contraction.3

During periods of illness or starvation, the body may reduce the conversion of thyroxine to triiodothyronine to conserve energy and preserve tissue integrity.4