Triiodothyronine is the biologically active thyroid hormone responsible for regulation of metabolic activity, mitochondrial respiration, oxygen consumption, thermogenesis, cardiovascular performance, nervous system function, and cellular energy utilization. T3 strongly influences basal metabolic rate by increasing transcription of genes involved in oxidative metabolism, ATP generation, nutrient transport, and mitochondrial enzyme activity.
The hormone affects carbohydrate utilization, lipid oxidation, protein turnover, gastrointestinal motility, cardiac contractility, neural signaling, and thermoregulatory pathways. T3 is especially important during development because it supports maturation of the central nervous system, skeletal growth, and tissue differentiation. Through widespread receptor expression, T3 coordinates metabolic responsiveness across nearly every major organ system.
Triiodothyronine is produced both directly by the thyroid gland and through peripheral conversion of thyroxine by deiodinase enzymes. Most circulating T3 originates from enzymatic removal of an iodine atom from T4 within liver, kidney, skeletal muscle, brain, and additional tissues. Deiodinase enzymes determine local tissue thyroid hormone activation and therefore strongly influence metabolic activity.
Thyroid follicular cells synthesize T3 using iodinated thyroglobulin intermediates generated through thyroid peroxidase activity. Once released into circulation, T3 binds thyroid-binding globulin, transthyretin, and albumin, while the free fraction enters cells and activates nuclear thyroid hormone receptors.
T3 production is regulated by the hypothalamic-pituitary-thyroid axis together with tissue-specific deiodinase activity. Thyroid-stimulating hormone increases synthesis and secretion of thyroid hormones from the thyroid gland. Peripheral conversion of T4 into T3 is influenced by selenium availability, nutrient status, inflammatory signaling, stress hormones, caloric intake, circadian rhythm, and mitochondrial function.
T3 exerts negative feedback on hypothalamic TRH release and pituitary TSH secretion. Cellular signaling occurs primarily through thyroid hormone receptors alpha and beta, which regulate transcription of metabolic enzymes, mitochondrial proteins, transport systems, and oxidative phosphorylation pathways. Through these integrated endocrine systems, T3 serves as the dominant regulator of metabolic intensity, thermogenesis, and cellular energy adaptation.
T3 is the active form of thyroid hormone that directly regulates cellular metabolism.
