The effects of endurance training and season on the function of the anterior 
pituitary-thyroid axis were studied in 18 female runners and their 12 controls, 
and in 13 joggers and their 11 controls in Northern Finland, with a large 
seasonal difference in environmental factors. The serum concentrations of 
thyrotropin (TSH), thyroxine (T4), free thyroxine (fT4), triiodothyronine (T3), 
thyroxine binding globulin (TBG) and oestradiol (E2) were measured during one 
menstrual cycle in the light training season (autumn) and in the hard training 
season (spring). The responses of TSH to intravenous TRH stimulation were also 
measured in the luteal phase of the cycle during the hard training season. 
Endurance running did not affect the basal or TRH-stimulated serum TSH 
concentrations, while those of T4 and fT4 in runners were lowered in both 
seasons and that of T3 in the light training season in relation to control 
subjects. The serum concentrations of TBG were also significantly lower in 
runners than their controls in the luteal phase in both seasons. The effect of 
jogging on thyroid hormones was less pronounced. Serum concentrations of TSH, 
T4, fT4, T3 and TBG were generally slightly higher in spring than in autumn. 
Strenuous endurance training seems to have minor changes on the function of the 
thyroid gland. Depressed T4 levels in runners may rather be due to lowered TBG 
levels than due to direct effect of training. In spring the function of anterior 
pituitary-thyroid axis is more active than in autumn.