REVIEW Thyroid hormone action in mitochondria

Triiodothyronine (T3) is considered a major regulator of mitochondrial activity. In this review, we show evidence of the existence of a direct T3 mitochondrial pathway, and try to clarify the respective importance of the nuclear and mitochon-drial pathways for organelle activity. Numerous studies have reported short-term and delayed T3 stimulation of mitochondrial oxygen consumption. Convincing data indicate that an early influence occurs through an extra-nuclear mechanism insen-sitive to inhibitors of protein synthesis. Although it has been shown that diiodothyronines could actually be T3 mediators of this short-term influence, the detection of specific T3-binding sites, probabl