Beyond T3: The Emerging Role of 3,5-Diiodothyronine in Mitochondrial Thyroid Hormone Signaling

Thyroid hormone physiology has traditionally been understood through the hypothalamic–pituitary–thyroid (HPT) axis and the genomic actions of triiodothyronine (T3). However, advances in thyroid hormone biology have expanded this classical paradigm, demonstrating that thyroid hormone signaling is regulated through a coordinated network involving tissue-specific deiodination, specialized membrane transporters, genomic and non-genomic signaling pathways, and mitochondrial regulation of cellular bioenergetics. Among the iodothyronine metabolites generated through thyroid hormone metabolism, 3,5-diiodothyronine (3,5-T2) has emerged as one of the most extensively investigated because of its reported ability to rapidly influence mitochondrial respiration, oxidative metabolism, and energy expenditure. Experimental studies suggest that 3,5-T2 enhances mitochondrial respiration, fatty acid oxidation, oxidative phosphorylation, and metabolic efficiency, particularly in metabolically active tissues such as liver and skeletal muscle. These findings have generated considerable interest in the potential role of 3,5-T2 in metabolic disorders characterized by mitochondrial dysfunction, including metabolic dysfunction-associated steatotic liver disease (MASLD), obesity, and insulin resistance. However, important translational challenges remain. Most available data derive from animal studies using pharmacologic doses, while the endogenous physiology of 3,5-T2 in humans, its molecular targets, tissue-specific regulation, and long-term endocrine effects remain incompletely understood. In addition, evidence of hypothalamic–pituitary–thyroid axis suppression following exogenous 3,5-T2 administration and limitations in accurately measuring circulating 3,5-T2 continue to complicate clinical translation. This review critically evaluates the emerging biology of 3,5-diiodothyronine, integrating current evidence regarding its biosynthesis, mechanisms of thyroid hormone signaling, mitochondrial actions, metabolic effects, translational challenges, and future research priorities. By synthesizing findings from primary experimental studies and emerging translational investigations, this review places 3,5-T2 within the broader framework of contemporary thyroid hormone biology while highlighting key knowledge gaps that must be addressed before its physiological and therapeutic significance can be fully established.