The effect of STZ-induced diabetes on serum triiodothyronine (T3) and thyroxine (T4) levels in the rat: a seven week time course.

This study examined the effect of a seven week period of diabetes on serum triiodothyronine (T3) and thyroxine (T4) levels in male Sprague-Dawley rats. Diabetes was induced by a sublingual injection of streptozotocin (80 mg/kg) in one-half of the animals (D) while the other half of the animals received a sham injection of 40mM citrate (pH 4.5) (C). Blood was sampled weekly via a cardiac puncture and serum T3 and T4 levels were determined by radioimmunoassay (Pharmacia) techniques. For the first two weeks after diabetes inducement T3 levels were significantly (p < or = .05) less in the D animals, after which time T3 levels did not differ significantly (p < or = .05) between the two groups. A significant (p < or = .05) decrease in serum T4 levels occurred by the first week of diabetes and was sustained through the seventh week. T3:T4 ratio was significantly (p < or = .05) higher in the diabetic animals at weeks 2 and 5. These findings suggest that the organism attempts to maintain levels of the more biologically active of the two thyroid hormones, T3, at the expense of circulating levels of serum T4 during prolonged states of STZ-induced diabetes.