Studies on the Thyroid Functions and the Secretion of Insulin by Arginine

Abstract

Although it has been known in general that abnormal glucose tolerance is observed in hyperthyroidism, there have been few reports on the change of plasma insulin and blood glucose by arginine infusion. In our present study performed by giving arginine to patients with hyperthyroidism and hypothyroidism, an inverse correlation was observed between the change of plasma insulin and glucose levels and the thyroid functions. The thyroid function was represented by serum thyroxine levels (T₄), resin sponge uptake of triiodothyronine (RSU), basal metabolic rate (BMR) and thyroidal ¹³¹I uptake (¹³¹I uptake). Arginine hydrochloride (0.5g/kg of body weight) dissolved in 200ml of physiological saline was administered by intravenous infusion from a cubital vein over 30 minutes to the patients with hyperthyroidism and hypothyroidism and to the normal controls fasting since early in the morning. Blood samples were taken before and 15, 30, 45, 60, 90 and 120 minutes after the start of the infusion. Plasma insulin was determined by the double antibody adioimmunoassay and the blood glucose was determined by autoanalyzer. Plasma insulin in normal individuals reached to its peak 30 minutes after the arginine infusion (ΔIRI=32.5±3.0μU/ml). The JIRI at 30 minutes in hyperthyroid patients was as low as (2.6±2.6μU/ml), whereas a considerably high JIM at 30 minutes (87.6+19. 9μU/ml) was observed in hypothyroid patients. A statistically significant inverse correlationship (p<0.01) was observed between ΔIRI at 15, 30, 45 and 60 minutes and the thyroid functions. This correlation was most marked at 15 minutes, and became less marked as time lapsed. Blood glucose levels in normal individuals showed a biphasic curve which reached its peak at 30 minutes and its nadir at 60 minutes. A flat blood glucose curve was obtained for hyperthyroidism, and a mcnophasic curve was observed with an extremely high peak for hypothyroidismus. A statistically significant inverse correlation was observed between the thyroid functions and the ΔBS at 15 and 30 minutes, although it was less marked than in the case of ΔIRI.