Thyroid hormones are important regulators of energy metabolism and may influence 
energy processes during physical exercise. There are controversial results 
concerning thyroid hormone metabolism during strenuous exercise in adult 
athletes and only scant data concerning the impact of strenuous exercise on 
thyroid hormone metabolism in children and adolescents. Although some studies 
demonstrate a transient change in thyroid hormones during intense physical 
performance, most studies agree that these changes are of minor impact, 
practically reflecting the relative negative energy balance during strenuous 
exercise. This state of hypometabolism during intense physical performance has 
also been confirmed in highly trained female young athletes, who may be also 
characterized by reproductive axis dysfunction, manifested either as 
luteal-phase deficiency or amenorrhea, alongside the typical constellation of 
low T3, insulin and leptin levels. More importantly, strenuous exercise during 
childhood or adolescence is mostly accompanied by a delay of skeletal 
maturation, and height and may have a long-lasting negative effect on growth and 
acquisition of maximum bone mass. In conclusion, although thyroid hormones are 
only transiently or insignificantly changed during strenuous exercise, adequate 
caloric intake should be guaranteed in highly performing young athletes in order 
to counteract the relative negative energy balance and prevent alterations in 
endocrine-metabolic profile. Moreover, when growth and pubertal progression in 
very young athletes are significantly impaired, a reduction in the intensity of 
the physical exercise should be advocated in order to guarantee better final 
height and adequate acquisition of bone mass.