THYROID HORMONE METABOLISM AFTER ACCLIMATIZATION TO A WARM OR COLD TEMPERATURE UNDER CONDITIONS OF HIGH OR LOW ENERGY INTAKE

¹ Department of Applied Biology, A.R.C. Institute of Animal Physiology, Babraham, Cambridge CB2 4AT
² Department of Medicine, University of Birmingham, Queen Elizabeth Hospital, Birmingham B15 2TH

The separate effects of ambient temperature and energy intake on the metabolism of thyroid hormones in the young pig have been investigated. Piglets were kept singly from 4 weeks of age at 10 or 35 °C on a high (H), or low (L) energy intake, such that H = 2L. At the tenth week of age blood samples were taken 16-21 h after the last meal for the determination of plasma concentrations of thyroxine (T₄) and 3,5,3'-triiodothyronine (T₃). At the end of the tenth week of age the fractional disappearance rate (K) of T₄ was determined and finally the K value for T₃ was estimated.

Plasma concentrations of T₄ and T₃ were greater in animals on a high intake than in those on a low intake, (P 0·02), but the temperature at which the animals lived had no significant effect. The values of K for T₄ and T₃ were greater at 10 than at 35 °C (P 0·010 and P 0·05), and energy intake had no significant effect. Calculations of the catabolic rates in nmol/kg.h revealed that for T₄ the rate was greater at 10 than at 35 °C (P 0·001) and on a high than on a low intake (P 0·01). Similar differences were found between the mean values of T₃ with respect to both temperature and energy intake, but the variance was large and they did not reach statistical significance.

It is concluded that the metabolism of thyroid hormones is influenced independently by both ambient temperature and energy intake.