Serum total and free thyroxine and triiodothyronine during dynamic muscular exercise in man

The effect of strenuous dynamic muscular exercise on serum-free thyroxine (FT4) and free triiodothyronine (FT3) concentrations has been studied in 10 healthy subjects. In the course of 7.8-19 min of treadmill exercise (protocol of R. A. Bruce), the group showed a mean increase in serum albumin concentration of 11.4% (P less than 0.01). Five subjects demonstrated an increase in FT4 concentrations (x = +54%) and five showed decreased FT4 (x = -36%). The difference in FT4 concentrations was accounted for primarily by acute alterations in dialyzable fraction T4 (DFT4) rather than total serum T4. Mean changes in DFT4 were statistically significant (P less than 0.02) when increased FT4 and decreased FT4 groups were compared. Acute exercise was accompanied by a 9% decrease in dialyzable fraction T3 (DFT3) (P less than 0.02) in the 10 subjects, but the concomitant decrease in FT3 concentration was not significant. Changes in DFT4 and DFT3 and FT4 and FT3 concentrations were not correlated. Trends in double product (heart rate times blood pressure) with maximal exercise and in double-product recovery after exercise suggested that increased FT4 concentrations developed in subjects who were physically better trained.