The interrelation of thyroid hormones with blood rheology and lipid profile was studied in athletes (N = 12). A decrease in blood viscosity in the athletes was caused by a lower erythrocyte rigidity index, which correlated with an increased reticulocyte count (P = 0.022), a decreased mean concentration of hemoglobin per cell (MCHC) (P = 0.006), and an increased level of thyrotropin (TTH) (P = 0.040). By multiple regression analysis, MCHC was found to be the primary correlate of the erythrocyte rigidity index (P = 0.044). The interrelation of TTH with the rigidity index was explained by a positive correlation of TTH with the reticulocyte count (P = 0.022). The level of triiodothyronine (T3) was similar in the athletes and the control subjects and negatively correlated with total cholesterol (Ch) (P = 0.033) and low-density lipoprotein cholesterol (LDL Ch) (P = 0.048), which both correlated positively with blood and plasma viscosity and erythrocyte aggregation (P < 0.05 or 0.01). Thus, the decrease in LDL Ch in the athletes, which was closely related with the higher blood and plasma fluidities and with erythrocyte aggregation, was at least partly due to the effect of T3.