Background Currently, there is no consensus on the frequency of subclinical hypothyroidism in athletes and its effect on exercise tolerance.

Objective The purpose of the study was to explore the prevalence of subclinical hypothyroidism in elite athletes and to identify its impact on physical performance indicators.

Main Outcome Measurements A retrospective analysis of data from a random sample of outpatient records of 1000 elite athletes aged 15 to 36 years who underwent medical screening, including clinical, laboratory and instrumental examinations.

Results According to the results of a laboratory study, subclinical hypothyroidism was detected in 95 (9.5%) athletes in the sample. In athletes with subclinical hypothyroidism, the average thyroid stimulating hormone (TSH) level was 5.53±0.24 mME/l, while in unaffected athletes it was 1.89±0.31 mME/l (p<0.01). The level of free T4 was within normal values and in the group with hypothyroidism was 12.0±0.48 pM/l, whereas in unaffected athletes 17.2±1.13 pM/l (p<0.05).

When analyzing bicycle ergometry data, a significant correlation was found between hypothyroidism and heart rate at 1st, 3rd and 5th minutes of recovery after the test (p<0.001), and with diastolic blood pressure at the 3rd minute of recovery (p<0.001). A significant positive correlation was also found between TSH level and the same set of recovery indicators (p<0,0001). There was also a significant negative relationship between the level of TSH and the intensity of the training regime (p<0.005) and with sports proficiency grade (p<0.0001).

We did not find any significant effect of TSH and hypothyroidism on exercise tolerance and aerobic reserve.

Conclusions The study showed a high prevalence of subclinical hypothyroidism in elite athletes, affecting 9.5% of the sample. The presence of subclinical hypothyroidism significantly contributed to a slower recovery of parameters of the cardiovascular system after at bicycle ergometer test, without affecting exercise tolerance.