COQ10B, Coenzyme Q10, is a 247 amino acid protein encoded by the human gene COQ10B. COQ10B is a mitochondrial protein that belongs to the COQ10 family. COQ10B is an essential biological cofactor which increases brain mitochondrial concentration and exerts neuroprotective effects. Plasma COQ10B levels decrease in patients with advanced chronic heart failure (CHF) while COQ10B levels in hyperthyroid patients are found among the lowest detected in human diseases. Likewise, COQ10B is elevated in hypothyroid subjects, also in subclinical conditions, suggesting the usefulness of this index in assessing metabolic status in thyroid disorders. It is believed that secretion of adrenal hormones is in some way related to COQ10B levels, both in augmented and reduced conditions. However, since thyroid hormones have an important role in modulating COQ10B levels and metabolism, when coexistent, thyroid deficiency seems to play a prevalent role in comparison with adrenal deficiency.