Oxygen is of vital importance for the metabolism and function of all cells in the human body. Hypoxia, the reduction of oxygen supply, results in adaptationally appropriate alterations in gene expression through the activation of hypoxia-inducible factor 1 (HIF-1) to overcome any shortage of oxygen. Thyroid hormones are required for normal function of nearly all tissues, with major effects on oxygen consumption and metabolic rate. Thyroid hormones have been found to augment the oxygen capacity of the blood by increasing the production of erythropoietin (EPO) and to improve perfusion by vasodilation through the augmented expression of adrenomedullin (ADM). Since the hypoxic expression of both genes depends on HIF-1, we studied the influence of thyroid hormone on HIF-1 activation in the human hepatoma cell line HepG2 under normoxic and hypoxic conditions. We found that thyroid hormones increased HIF-1 protein accumulation by increasing HIF-1 protein synthesis rather than attenuating its proteasomal degradation. HIF-1 expression directly correlated with augmented HIF-1 DNA binding and transcriptional activity of luciferase reporter plasmids, while HIF-1 levels remained unaffected. Knocking down HIF-1 by siRNA clearly demonstrated that thyroid hormone induced target gene expression required the presence of HIF-1. Although an increased association of the two known coactivators of HIF-1, p300 and SRC-1 was found, thyroid hormone did not affect the activity of the isolated C-terminal transactivating domain of HIF-1. Increased synthesis of HIF-1 may contribute to the adaptive response of increased oxygen demand under hyperthyroid conditions.