Statistical evidence that endogenous reverse triiodothyronine (rT3) may affect oxygen consumption in food deprived...

Exogenous rT3 decreases O2 consumption in mammals and birds. Until now a correlation coefficient and a regression equation have not been presented. Statistical evaluation seems to be requisite for verifying the answer to the question of whether endogenous rT3 may be able to reduce O2 consumption in birds where the normal level of plasma rT3 is 10 times less than the corresponding level of T3. Food deprived chickens (for 48 h) were used in this study because fasting enhances plasma rT3. The results revealed a reciprocal relation of T3 and rT3 in the circulation. Reverse T3 began to increase when T3 decreased to a plateau at 53.9% of initial level. As expected a reciprocal relationship was obtained (r = -0.749; n = 36) between plasma rT3 and O2 consumption. The regression line was calculated according to the equation: Y = -0.388X + 0.856. This relation differs from the linear relationship between T3 and O2 consumption (r = 0.796; Y = 0.107X + 0.449). The regression line lies in the range of 0.122-0.778 nmol rT3/l, which is found in some physiological conditions in birds where elevated plasma rT3 occurs. This suggests that endogenous rT3 may participate in modifying O2 consumption in birds. Using the rT3:T3 ratio the correlation coefficient was somewhat higher (r = -0.831; Y = -0.673X + 0.831) suggesting common involvement of both triiodothyronines in the reduction of O2 consumption during food deprivation. The drop in O2 consumption after 48 h of food deprivation was 28.4%; decreased T3 and increased rT3 may participate in 15.4% and 13.0% of this fall, respectively. The hypometabolic effectiveness of rT3 seems to be greater than the hypermetabolic effectiveness of T3, since a smaller increase of plasma rT3 was needed to reduce O2 consumption compared to the amount of T3 necessary to enhance it.