title: The effect of endurance training on serum triiodothyronine kinetics in man: physical conditioning marked by enhanced thyroid hormone metabolism.
abstract: OBJECTIVE
  We studied the relationship between endurance training, aerobic capacity, and T3 metabolism in healthy euthyroid men.
  
  
  METHODS
  T3 kinetic studies performed on two groups of subjects differentiated on the basis of physical activity status and aerobic capacity.
  
  
  METHODS
  Five endurance-trained athletes and five sedentary controls (mean +/- SD VO2 max = 48.2 +/- 7.1 vs 23.2 +/- 4.5 ml/kg/min, respectively) matched for age, body surface area, lean body mass, and baseline thyroid function.
  
  
  METHODS
  Kinetic analysis performed using serial serum T3 levels measured following oral T3 administration. Metabolic clearance rate, total volume of distribution, disposal rate, and total body pool calculated using non-compartmental analysis.
  
  
  RESULTS
  When normalized for lean body mass, all kinetic parameters were 25-38% greater in the athletic group compared to controls (P < 0.05). Total volume of distribution, disposal rate, and total body pool were positively correlated with aerobic capacity (r = +0.69 to +0.79; P < 0.05). Metabolic clearance rate was positively correlated to a non-significant degree.
  
  
  CONCLUSIONS
  These results confirm the findings of prior studies that thyroid hormone metabolism is altered by physical conditioning. In addition, we demonstrated a positive correlation between aerobic capacity and several parameters of T3 kinetics. Differences in absolute lean body mass cannot explain these findings; rather it appears that there is something qualitatively different in the way endurance-trained tissue processes thyroid hormone, compared to untrained tissue. The study was not designed to elucidate these differences at the cellular level; however, it does support a link between muscle physiology and T3 activity and may suggest a physiological role for thyroid hormone in physical conditioning.
authors: Department of Endocrinology & Metabolism, Wilford Hall USAF Medical Center, Lackland Air Force Base, Texas.
authors: J K
authors: JK
authors: Rone
authors: R F
authors: RF
authors: Dons
authors: H L
authors: HL
authors: Reed
mesh_terms: D000328:Adult; D003430:Cross-Sectional Studies; D015444:Exercise / Q000502:physiology; D006801:Humans; D008297:Male; D008657:Metabolic Clearance Rate / Q000502:physiology; D010807:Physical Endurance / Q000502:physiology*; D013960:Thyroid Function Tests; D013961:Thyroid Gland / Q000378:metabolism / Q000502:physiology*; D014284:Triiodothyronine / Q000097:blood* / Q000378:metabolism / Q000493:pharmacokinetics
chemical_list: D014284:Triiodothyronine