Increase in rT3 serum levels observed during extended Alaskan field operations of Naval personnel

Members of a US Navy Special Warfare platoon had blood samples drawn by venipuncture and other baseline measurements carried out prior to departure for a three-month period of field operations in Alaska. Assays of serum reverse triiodothyronine (rT3), free thyroxine (fT3), free T4 (fT4), and thyroid stimulating hormone (TSH) were subsequently done at the Naval Medical Research Institute (NMRI), Bethesda, Maryland. In the field and at baseline, hematocrits and urine specific gravities were also measured to track hydration status, and body weights and fat were recorded to track nutritional status. After plasma volume change and weight change corrections, an approximate 30% increase in serum rT3 level and 20% decrease in free T3 level over 76 days of cold exposure were recorded. The necessity for an rT3 kinetic study is indicated. The mean residence time for rT3 in the circulation is about 8 hours with respect to about 36 hours for free T3, so rT3 kinetic studies would be advantageous with respect to long term cold exposure of military personnel.