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 insulin-treated 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 X 100 g, and 0.034 +/- 0.006 h-1, respectively) were significantly lower (P less than 0.001) than in the control group (1.01 +/- 0.04 ml/h X 100 g and 0.056 +/- 0.004 h-1). Similarly, the MCR and K values of T3 were also significantly reduced (P less than 0.001) in the diabetic animals: 16.2 +/- 0.7 ml/h X 100 g and 0.088 +/- 0.007 h-1 in controls vs 13.0 +/- 1.2 ml/h X 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 less than 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.(ABSTRACT TRUNCATED AT 250 WORDS)