Serum thyroxine, triiodothyronine and thyroid stimulating hormone after prolonged heavy excerise

Abstract

Serum thyroxine (T₄), triiodothyronine (T₃) and thyroid stimulating hormone (TSH) was measured before, immediately after and on the following days after a 70 km cross-country ski race in two groups of ten well-trained men, one aged 21–29 years, taking 5.01–7.03 h, and one aged 51–57 years, taking 6.07–7.38 h. T₄ was definitely increased immediately after the race, but fell to below the initial level on the following day and was not restored to the pre-race level until 4 days after the race. T₃ showed essentially the same pattern, though less pronounced, a major part of the changes being possibly due to plasma volume variations. TSH showed a tendency to rise immediately after the race, but rose further to 175% of the initial level on the following day and was not restored to the initial level 4 days after the race. The pattern of changes were independent of age. It is suggested that the prolonged rise in TSH is probably due to an exercise-induced increased peripheral need for thyroid hormone.