Secretion and metabolic clearance rates of thyroxine and triiodothyronine in streptozotocin-diabetic rats

Abstract

Abstract. This study was undertaken to evaluate the effects of streptozotocin-induced diabetes on T4 and T3 production rate (PR) and metabolism in rats. [125I]T4 and [125I]T3 were injected iv and sequential blood samples were obtained. Plasma ethanol extracts were analyzed for [125I]T4 and [125I]T3 by thin layer chromatography. T4 and T3 production rates and kinetic parameters of T4 and T3 metabolism in control, diabetic and insulintreated diabetic rats were assessed by applying kinetic analyses to measurements of disappearance of injected T4 and T3 radiotracer from plasma. The metabolic clearance rate (MCR) and the fractional disappearance rate (K) of T4 in diabetic rats (0.72 ± sd, 0.02 ml/h · 100 g, and 0.034 ± 0.006 h−1, respectively) were significantly lower (P 0.001) than in the control group (1.01 ± 0.04 ml/h · 100 g and 0.056 ± 0.004 h−1). Similarly, the MCR and K values of T3 were also significantly reduced (P < 0.001) in the diabetic animals: 16.2 ± 0.7 ml/h · 100 g and 0.088 ± 0.007 h−1 in controls vs 13.0 ± 1.2 ml/h · 100 g and 0.058 ± 0.009 h−1 in diabetics, respectively. Insulin therapy significantly reversed these alterations. There was also a significant reduction (P < 0.001) in plasma T4 (18 ± 6 ng/ml) and T3 (0.20 ± 0.05 ng/ml) concentrations in the diabetic rats compared to control values, whose T4 and T3 levels averaged 65 ± 10 and 0.68 ± 0.09 ng/ml, respectively. In addition, the T4 and T3 production rates in diabetic rats (12.9 ± 5.6 and 2.6 ± 0.2 ng/ml · 100 g, respectively) were significantly different (P < 0.001) from control values (65.6 ± 10 and 11.0 ± 0.7 ng/ml · 100 g). In conclusion, the dramatic fall in plasma T4 and T3 concentrations associated with diabetes appear to be the result of the reduction in the PRs rather than to changes in MCRs.