Thyroid hormone profile and biological variation in a population of athletes during an entire sport season

Abstract

Background: Biological variation (BV) refers to fluctuations in biomarker concentrations around an individual's homeostatic set point and supports tools such as the reference change value and personalized reference intervals for longitudinal monitoring. Physiological variations in athletes may influence these estimates. This study aimed to analyze thyroid hormone profiles and determine BV estimates in a reference population of athletes. Methods: Fasting blood samples were collected monthly over 11 consecutive months from 30 athletes (15 women, aged 19-53 years) training at least 13 h per week. TSH, FT4, and FT3 were measured in duplicate using chemiluminescent immunoassays. Hormone concentrations were analyzed in relation to age, season, sex, and recent exercise (within 24 h prior to sampling). The transferability of reference intervals used in our laboratory to an athletic population was assessed. BV estimates were calculated using classical ANOVA, a linear mixed-effects model, and a Bayesian approach. Results: A negative association between TSH and age was observed. Significant sex-related differences were found in concentrations of all three hormones. Although some seasonal differences were detected, hormone profiles appeared more closely related to training load. BV estimates for FT3 differed significantly between sexes. Overall, BV estimates in athletes differed from those reported in the general population. Conclusions: Thyroid hormone BV in athletes shows distinct patterns influenced by sex and exercise load, highlighting the need for population-specific reference data when interpreting results in athletic contexts.