Changes in Serum Concentrations of 3,3',5'-Triiodothyronine and 3,5,3'-Triiodothyronine during Prolonged Moderate Exercise*

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Abstract
The effect of moderate bicycle exercise (3.5 h) on peripheral thyroid hormone metabolism was studied under two conditions (with and without glucose infusion) in four normal (males. Serum T3, rT3, T4, total protein, plasma glucose, and FFA (were determined. Exercise induced an increase in rT3 from 29 to 40 ng/dl (P < 0.01), a decrease (in T)3 from 154 to 147 ng/dl(P< 0.01), and an increase in T4 from 7.1 to 7.5 ug/dl (P < 0.05). When glucose was infused during exercise, the changes (in rT3 were blunted (P < 0.01) and the changes in T3 and T4 were (diminished. During exercise, rT3 correlated with FFA (r = 0.95) and plasma glucose (r = -;0.87). When glucose was infused during exercise, these correlations decreased (r = 0.81 and -0.56, respectively). Since moderate, prolonged exercise induces a state of early or acute starvation it is concluded that the changes in peripheral thyroid hormone metabolism reported here are similar to those found in starvation. The temporal changes of rT3, FFA, and plasma glucose during exercise suggest a relationship between thyroid hormone metabolism and the uptake and utilization of FFA and glucose or the mixture of these body fuels.