Thyroid hormones and mitochondria: with a brief look at derivatives and analogues.

Thyroid hormones (TH) have a multiplicity of effects. Early in life, they mainly affect development and differentiation, while later on they have particularly important influences over metabolic processes in almost all tissues. It is now quite widely accepted that thyroid hormones have two types of effects on mitochondria. The first is a rapid stimulation of respiration, which is evident within minutes/hours after hormone treatment, and it is probable that extranuclear/non-genomic mechanisms underlie this effect. The second response occurs one to several days after hormone treatment, and leads to mitochondrial biogenesis and to a change in mitochondrial mass. The hormone signal for the second response involves both T3-responsive nuclear genes and a direct action of T3 at mitochondrial binding sites. T3, by binding to a specific mitochondrial receptor and affecting the transcription apparatus, may thus act in a coordinated manner with the T3 nuclear pathway to regulate mitochondrial biogenesis and turnover. Transcription factors, coactivators, corepressors, signaling pathways and, perhaps, all play roles in these mechanisms. This review article focuses chiefly on TH, but also looks briefly at some analogues and derivatives (on which the data is still somewhat patchy). We summarize data obtained recently and in the past to try to obtain an updated picture of the current research position concerning the metabolic effects of TH, with particular emphasis on those exerted via mitochondria.