Effects of fasting on thyroid hormone levels and kinetics of reverse triiodothyronine in cattle

Abstract

Abstract. The effects of starvation for 5 days on plasma levels, and for 3.5 days on kinetics, of 3,3′,5′-triiodothyronine (reverse T3, rT3) were studied in steers. During the 5 day starvation period alterations of plasma levels of thyroxine (T4), 3,5,3′-triiodothyronine (T3), immunoreactive insulin (IRI), glucose, free fatty acids (FFA), urea and protein were determined. During starvation rT3 did not change, while T4 and T3 (total and free), IRI and glucose decreased and FFA, urea and protein increased (P < 0.05). During re-alimentation changes of T4 and T3 (total and free), IRI, FFA and urea were partially reversed and rT3 slightly increased, but the difference was not statistically significant with respect to mean values observed during fasting. During intravenous rT3 infusions (3.85 pmol/kg/min) for 300 min in fed and fasted steers plasma rT3 levels increased rapidly, reaching steady-state levels around 2 nmol/l between 180 and 300 min, and decreased after termination of the infusions. In fed as compared to fasted animals the total clearance rate was higher (2.63 ± 0.30 and 1.72 ± 0.21 ml/kg/min, respectively; P < 0.025) and the exponential constant (k1) of the initial phase of disappearance of rT3 from plasma after the end of the infusions was greater (0.11 ± 0.02 and 0.05 ± 0.01, respectively; P < 0.025). The data show that plasma rT3 levels in steers do not increase during starvation, which is in contrast to man, rabbits and sheep. This may be due to reduced availability of T4, serving as a substrate for rT3 and to the relatively high clearance rate of rT3 even during fasting, as compared to that of rabbits and man. That energy deficiency was sufficiently severe, was suggested by the marked responses of T4, T3, IRI, glucose, FFA and urea to starvation.