When non-athletic young men commenced taking daily muscular exercise (running) peripheral degradation of thyroxine increased after a latent period, reaching 40% above resting level after 6 days. Athletes in moderately severe training had a thyroxine degradation/secretion rate 75% above that of resting, nonathletes. This fell significantly after 3 days' rest. PBI and free thyroxine levels were not significantly changed, the increase being primarily due to increased peripheral deiodination of thyroxine, which is believed to be due to repeated muscular exercise per se.
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C. H. G. Irvine (1968) studied this question.