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Changes in Serum Triiodothyronine Kinetics and Hepatic Type I 5'- Deiodinase Activity of Cold-Exposed Swine

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Swine exposed to cold air have elevated serum values of total triiodothyronine T3 and free T3 FT3. To characterize the mechanism of these increases, we measured in vivo kinetic parameters after a bolus intravenous injection of 125I-labeled T3 by use of both multicompartmental MC and noncompartmental NC methods and in vitro hepatic type I iodothyronine 5- deiodinase 5D-I activity. Ten ad libitum-fed 5-mo-old boars were divided into two groups, living for 25 days in either control 22 deg C or cold 4 deg C conditions. Cold-exposed animals consumed 50 more calories than control animals but showed no difference in total body weight, percent body fat, or plasma volume. Thyroid gland weight was increased 86 P 0.004, as was serum total thyroxine TT4 48, free T4 FT4 61, TT3 103, and FT3 107, whereas serum thyrotropin TSH was not different in cold-exposed compared with control animals. The T3 plasma clearance rate was similar between groups when both MC and NC techniques were used. However, T3 plasma appearance rate PAR was elevated in cold-treated animals 110 over controls by MC P 0.001 and 83 by NC methods P 0.001. The animal total hormone pool of T3 was increased 76 MC and 53 NC compared with control P 0.01. The Michaelis constant of hepatic 5D-I was not different between groups, but the maximum enzyme velocity increased 106 P 0.02. Therefore cold exposure for 25 days is associated with increased energy intake, thyroid size, T3 PAR, and hepatic 5D-I activity with little change in serum TSH.

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  • Biochemistry
  • Anatomy and Physiology

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