Thyroid and Sports: A Connection Marking the Olympic Games in Athens 2004

On July 2 - 4 2004, a conference on Thyroid and Sports was held in Athens by the European Thyroid Association and the Medical Directory of ATHENS 2004. This was the first time for a meeting concerning thyroid and sports ever to be held before and in connection with the Olympic Games. Truly, the return of the Olympic Games to Greece has offered a unique opportunity to bring science and sports into context, studying the thyroid and its effects on Olympic athletic performance. The significance of this topic would seem to warrant a minor foray into etymological exploration, which can always be enlightening. Tracing words back to their origins can open up a world of hidden meaning, and prime examples are the words athlete, athleticism and epathlon (award), all originating from the terms athlos (feat, exploit) and athlon (prize, award). These three cognates combine the dual meaning of endeavor and reward that not only constituted fundamental components of ancient Greek ideology, but also embody the supreme values of humanity. The word thyroid also has its own history, stemming from either thyreos (shield) or thyra (door), the latter related to an ancient Minoan concept regarding the throat as the point of entrance of the soul.

The first paper (Pubmed) concerning thyroid hormones and sports was published in 1968, and reported on the effect of exercise on thyroxin degradation in athletes and non-athletes [1]. In 1970, important data were presented on the regulation of the metabolic adaptation and performance by thyroid hormones during prolonged physical training [2]. Since then, several studies have revealed the considerable importance for energy homeostasis of thyroid hormones which, together with leptin - an adipose tissue hormone whose plasma levels reflect energy stores - are heavily involved in adaptive thermogenesis and regulation of uncoupling proteins [3] [4] [5]. Furthermore, data indicate uncoupling protein-3 as a molecular determinant in the regulation of resting metabolic activity by thyroid hormones [6], and resting energy expenditure (REE) in humans is also reportedly highly sensitive to thyroid hormones [7]. Indeed, REE changed by 15 % at TSH secretion levels between 0.05 and 10 mU/l induced by varying the dose of LT₄ administration [7]. Therefore, correct thyroid function is a prerequisite for optimal performance in all athletes. Following this line of evidence, thyroid disorders were found in up to 1 % of subjects in an investigation as to whether underlying medical conditions may cause fatigue and the high incidence of infections that occur during repeated training [8].

However, as reported at the meeting, the impact of exercise on thyroid hormone metabolism in both children and adults remains somewhat controversial. The assumption is that thyroid hormone metabolism depends on the intensity, duration and form of exercise as well as on the individual’s physical and metabolic condition. Increased, decreased and even unaltered serum TSH and thyroid hormone concentrations have been measured during vigorous exercise. Three weeks of professional road competition led to a significant increase in T₄ and FT₃ levels by the last week of the course, while TSH and T₃ remained unchanged [9]. In contrast, high energy flux in trained rowers during high-resistance training followed by endurance training caused a decrease in the hypothalamic-thyroid axis (HTA), irrespective of BMI and body fat [10], whereas another study found that intensive physical training decreased thyroid volume in relation to body weight, lean body mass, and body mass index [11]. On the other hand, no statistically significant changes were found in serum concentrations of thyroid hormones, TSH and TBG during a one-year period with alternate strength training in elite weight lifters [12]. Prolonged exercise such as marathons or even a 1,100 km course may increase serum levels of TSH, FT₄ and rT₃ and decrease T₃ after the run, revealing a change of the peripheral conversion in favor of rT₃ [13] [14]. Finally, additional factors such as altitude and cold exposure may cause changes in total and/or free thyroid hormone levels [15] [16].

One major topic at this meeting was the question as to whether well-trained athletes should be given iodine supplements before strenuous exercise. Each liter of perspiration is known to result in the loss of about 15 % of the average daily intake of iodine. Thus, an athlete can lose 30 - 45 μg of iodine in two to three hours of training, diminishing his achievement. In addition, the iodine concentration of sweat is about 37 μg/l as stated at this conference by Peter Smyth (Dublin), so one could calculate an iodine loss amounting to about 100 - 180 μg; this may seriously compromise athletic performance. This question-a source of much animated discussion at the meeting-opens up a new field of research in clinical and nutritional thyroidology.

The considerable importance of these issues can clearly be seen, and there is much gratification in the thought that the 2004 Athens Olympics held in the historical birthplace of the ancient Games should have formed an inspiring background for the commemoration of Thyroid and Sports.

References

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