Thyroid hormones are considered to be the major regulator of genomic and non-genomic pathways of mitochondrial biogenesis and consequently of mitochondrial bioactivity and oxygen uptake, but also a major factor in ROS production.We have recently established a flowcytometric method for determination of mitochondrial membrane potentials (MMP) and ROS production, as well as determination of mitochondrial related gene expression by PCR of PGC-1 a, PGC1b, TFAM and NRF2 in human cells, and demonstrated a direct fast effect on MMP as well as ROS production by bioactive iodothyronines.We investigated the relation between thyroid hormone and mitochondrial regulation with special reference to gender and age and explored the clinical relevance by examining patients with prediabetes and diabetes.Neither genomic, nor non genomic markers of mitochondrial function, nor TSH and thyroid hormone concentrations (T3 and T4) were gender dependent.When the significance of age was examined, we observed that TSH and T3 decreased with age (children TSH =2.7+1.2 mmol/1 vs adults 1.9+1.1 mU/l, p=0.001;children T3=6.4+0.7 pmol/1 vs adults T3=4.9+1.1 p=0.03).Gene expression of PGC-1 a, PGC-1b, TFAM and NRF2 was independent of age, whereas MMP was significantly higher in children (MMP =6913+2221 a.u.) compared to adults (MMP =6173+2015 a.u.p=0.034).These observations suggest an important significant relation between thyroid hormones and MMP, and suggest that thyroid hormones may play an important role as regulator of decreasing MMP with age.Examination of MMP in patients with prediabetes (raised HOMA 1-R but normal fasting blood sugar) and with type 2 diabetes revealed that MMP was depressed in adult subjects with prediabetes compared to controls (MMP=5756+1887 a.u.vs 7005+2065 a.u.p=0.02) and in patients with manifest type 2 diabetes (MMP=5050+1887 a.u.vs 7005+2065 a.u.p=0.002).We then wanted to examine the relation between thyroid hormones and ROS (reactive oxygen species), known to be involved in vascular damage, especially in diabetes.First we established a method for measuring ROS in human mononuclear blood cells and demonstrated that bioactive iodothyronines increased the ROS production in cells from healthy euthyroid, non diabetic subjects by ap 5%.When we compared the result from control persons with that of patients with type 2 diabetes, we observed that basal ROS production was increased and further that the T3 stimulated ROS production was significantly higher in cells from diabetic patients compared to controls.In conclusion: Thyroid hormones are major regulators of mitochondrial membrane potentials and also of ROS production.This regulation appears impaired in patients with diabetes type 2.