Effect of long term differences in energy intake on metabolic rate and thyroid hormones.

Long term effects of differences in energy intake on thyroid hormone metabolism and the rate of oxygen consumption were studied in two groups of young pigs. The pigs were kept at 25 degrees C and fed either a high (H) or low (L) energy intake such that H = 2L for six weeks. The rate of oxygen consumption, the plasma levels of thyroxine (T4) and 3,5,3'-triiodothyronine (T3) and the fractional rate of disappearance of T4 and T3 were measured each week. The metabolic rate at thermal neutrality was lowest in the animals on the L intake and the critical temperature was lowest in the pigs on the H intake. These differences tended to increase with time. The fractional disappearance rate (K) for T4 began by being greater in the animals on the H intake, but after four weeks the difference reversed. The catabolic rate of T4 in nmol.kg-1h-1 showed similar changes. The k values for T3 were not significantly different between treatments. It is tentatively suggested that the different energy intakes lead to a change in the tissue sensitivity to thyroid hormone.