['Visfatin, a visceral fat-derived adipocytokine, plays a significant physiological function in lipid metabolism. However, the precise function of visfatin and its regulation by thyroid hormones are still unknown. This study observed the plasma visfatin concentrations in subjects with hyperthyroidism and hypothyroidism in vivo, and investigated the possible regulation mechanism between visfatin and tri-iodothyronine (T3) in vitro as a further interpretation.', 'The experiment in vivo included clinical subjects (57 patients with thyroid dysfunction and 29 euthyroid healthy volunteers) and an animal model (24 Wistar rats). All subjects were divided into hyperthyroidism, hypothyroidism and euthyroidism groups, with plasma thyroid hormones, thyrotropin, visfatin and triglyceride concentrations determined. Visfatin mRNA expression in visceral fat and liver of rats was detected by quantitative real-time reverse transcription-polymerase chain reaction (RT-PCR). The experiment in vitro studied 3T3-L1 cells and visfatin mRNA expression under nine different T3 concentrations (0, 0.1, 0.25, 0.5, 1, 5, 10, 20, 100 nmol/L) using quantitative real-time RT-PCR.', 'Clinical subjects and animal models showed elevated plasma visfatin concentrations in the hyperthyroidism group (20.466 ng/ml (15.263, 26.795 ng/ml) and (1209.164±165.292) ng/L) and hypothyroidism group (12.457 ng/ml (11.115, 15.454 ng/ml) and (1205.425±109.200) ng/L) compared to euthyroidism group (6.891 ng/ml (5.888, 8.803 ng/ml) and (926.650±54.002) ng/L, P<0.001). For animal models, visfatin mRNA expression in visceral fat in the hyperthyroidism and hypothyroidism groups increased about 3.33-fold and 1.98-fold compared to the euthyroidism group (P<0.001), which was positively correlated with plasma visfatin concentrations (r=0.713, P<0.001). However, no significant group difference (P>0.05) and correlation (r=0.121, P=0.572) was found in the liver. T3 induced a remarkable increase of visfatin mRNA expression in 3T3-L1 cells at low concentrations (0-0.5 nmol/L T3) followed by a sharp decrease at higher concentrations (0.5-100 nmol/L T3), with an inflection point at 0.5 nmol/L T3.'] Is up-regulation of visfatin expression in subjects with hyperthyroidism and hypothyroidism partially relevant to a nonlinear regulation mechanism between visfatin and tri-iodothyronine with various concentrations?