Reverse T3 increases with prolonged exercised and decreased food consumption

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or why eating less and exercising more doesn't work for many

THE THYROID FUNCTION IN YOUNG MEN DURING PROLONGED EXERCISE AND THE EFFECT OF

ENERGY AND SLEEP DEPRIVATION

http://www3.interscience.wiley.com/journal/119531428/abstract

Thyroid function has been investigated in 24 young military cadets participating

in a 5 d ranger training course with heavy physical excercise, calorie

deficiency and deprivation of sleep. The cadets were divided into three groups,

each differing in the amount of sleep and food consumption. The serum levels of

thyroid hormones (T4, FT4, T3, rT3) and TBG showed a biphasic pattern during the

course. Initially there was an increased secretion concomitant with an increased

deiodination of T4 to T3 and rT3 mainly due to physical exercise. When the

activities lasted for several days without sufficient food supply the thyroid

secretion decreased simultaneously with an alteration of the peripheral

conversion of T4 to rT3 instead of T3. A significant correlation was found

between the changes in total and free thyroxine (r= 0·9) and between the

increase in rT3 and decrease in T3 (r= 0·6). TSH decreased during the first day

of activities and remained low throughout the course. The TSH response to TRH

stimulation was greatly reduced during the course due to physical exercise and

calorie deficiency. The present investigation demonstrates that the thyroid

function is strongly affected by prolonged physical exercise and a negative

energy balance, whereas sleep deprivation does not have any significant

influence. The results indicate that the alteration observed is not regulated

just by the hypothalamo-pituarity-thyroid-axis alone.