This study was undertaken to elucidate thyroid hormone metabolism in streptozotocin-induced diabetic rats. Diabetes mellitus was induced by the administration of a single intraperitoneal dose of streptozotocin (50mg/kg body weight). Two weeks later, rats whose fasting blood sugar levels were above 200 mg/dl were used in the present study. Serum thyroxine (T4), 3, 5, 3'-triiodothyronine (T3) and thyroid stimulating hormone (TSH) were determined by respective RIA. In the streptozotocin-induced diabetic rats, serum levels of T4 (2.1±0.4μg/dl, mean±SD) and T3 (80±20 ng/dl) were significantly (p<0.005) lower than those in controls (4.6±0.9μg/dl and 142±7 ng/dl), respectively. Serum levels of TSH were significantly (p<0.05) higher in streptozotocin-induced diabetic rats (4.1±0.7μIU/ml) than in controls (3.0±1.2μIU/ml). These results suggest that primary hypothyroidism may occur in streptozotocin-induced diabetic rats.
The acute effects of insulin on plasma triglyceride and cholesterol were estimated by applying glucose clamp for two hours in 58 patients with NIDDM. Glucose clamp studies were carried out by primed continuous insulin [human regular insulin (40 mU/m2/min)] infusion, and plasma glucose was maintained at the basal level by automatically infused glucose. The glucose infusion rate for the last 60 minutes was considered to be the insulin sensitivity for the glucose metabolism. Plasma triglyceride and cholesterol levels after glucose clamp studies decreased significantly compared with those of their starting levels and the rates of decrease were 15.2±2.2%(mean±SEM) and 8.3±1.4%, respectively. Moreover, we investigated the relationship between the changes of plasma triglyceride and cholesterol and the glucose infusion rate. As the glucose infusion rate was dependent on the plasma glucose concentration, we separated the patients into two groups. In group A, the plasma glucose concentrations ranged from 100 mg/dl to 130 mg/dl and in group B from 131 mg/dl to 160 mg/dl. The rate of decrement of plasma cholesterol during glucose clamp application, which was variable in each subject, was correlated with the glucose infusion rate (r=0.400, p<0.05 in A.r=0.585, p<0.01 in B). In contrast, the rate of decrement of plasma triglyceride was not correlated with the glucose infusion rate in either group. In conclusion, insulin reduced plasma cholesterol levels and the insulin-induced decrement of plasma cholesterol during glucose clamp application was well correlated with the insulin sensitivity for the glucose metabolism. The metabolism of plasma cholesterol may be regulated mainly by insulin with dependency on the concentration and the sensitivity.