BACKGROUND: Mitochondrial function may be impaired in a number of diseases including metabolic syndrome, cardiovascular disease and endocrine disorders. Therefore it is important to be able to measure mitochondrial function in human cells. PURPOSE: The aim of the present study was to evaluate a method to measure mitochondrial function in human derived cells, which also would reflect regulation by thyroid hormones. METHODS: The MDA-MB-231 cell line (a human breast cancer cell line) was incubated with bioactive iodothyronines (T(4), 3'-3, 5-T(3), 3, 5-T(2)) 50 nmol/l for 3 h. Mitochondrial membrane potentials (MMP) were measured by a flow cytometer after staining with Tetramethylrhodamine methyl ester (TMRM). Also, the effect of TRIAC (a stimulator of thyroid hormone nuclear receptors) and L-Carnitine (an inhibitor of thyroid hormone passage into the nucleus) was examined. FINDINGS: It was possible to measure mitochondrial membrane potential (MMP) in human derived cells and to examine thyroid hormone effects using flow cytometry. Bioactive iodothyronines increased mitochondrial membrane potential. TRIAC had no effect and L-Carnitine only inhibited T(4) stimulation of membrane potential. CONCLUSION: Flow cytometry may be a valuable method for examining and testing mitochondrial function in human cells. Our findings demonstrate increase of mitochondrial membrane potential and an extra nuclear short time effect of 3, 5-T(2) on mitochondrial activity.