Objectives: Effects of exercise on circulating thyroid hormone (TH) values remain controversial. We sought to observe the effect of treadmill exercise on serum TH values in highly selected subjects. Methods: Twenty-six healthy male military recruits aged 23–27 (mean, 25) years were studied. All had maintained identical diet and physical activity for a week before the test. Serum samples were drawn before (baseline) and immediately, 1, 4, 24, 24 and 48 h after maximal exercise (on a treadmill, Bruce protocol). All subjects completed the protocol with normal ECG results. Specimens were analyzed to measure 3,3′,5-triiodothyronine (T₃), thyroxine (T₄), free T₄ (FT₄), free T₃ and thyroid-stimulating hormone (TSH) in the same assays. To determine the possible effect of hemodynamic changes, hematocrit (Hct)-adjusted data were also compared. Results: Hemoconcentration, as reflected by increased Hct, was found immediately after exercise. No significant changes of serum mean TH values before and after exercise were found except for TSH, which increased significantly immediately after exercise (1.72 vs. baseline 1.42 IU/l, p < 0.01). Values for T₃, T₄, and TSH increased significantly immediately after exercise, as compared to other postexercise values. However, the changes became insignificant after Hct adjustment. The FT₄ values showed a reciprocal increase after exercise that became significant after Hct correction. Significantly negative correlation was found between FT₄ and TSH values, but these values were still well within the normal range. Conclusions: Maximal treadmill exercise does not greatly affect the determination of concentrations of circulating THs.