Abnormalities of Thyroid Hormone Turnover in Patients with Diabetes Mellitus before and after Insulin Therapy*

The effects of impaired glucose metabolism on thyroid hormone turnover were studied in four patients with insulin-independent diabetes mellitus and four patients with insulin-dependent diabetes mellitus before and after insulin therapy. Seven normal subjects were included as controls. All subjects were given T4 (0.2 mg daily) by mouth 3 weeks before and throughout the study. T4 turnover results from the untreated and treated diabetic subjects and from the normal subjects overlapped. However, both the mean free T4 fraction and mean free T4 concentration were reduced in the diabetic patients after insulin therapy. The mean MCRs of T3 were normal in both the insulin-independent and the insulin-dependent diabetic patients and were unaffected by insulin therapy. However, the mean serum T3 concentration, T3 disposal rates, and T3 conversion rates were low in both groups of diabetic patients. After insulin therapy, these values all increased but remained significantly lower than in the normal subjects. The mean serum levels, MCRs and disposal rates of rT3 were within the normal range in the insulin-independent diabetic patients and did not change with insulin therapy. In the insulin-dependent diabetic patients, the mean rT3 serum concentration and disposal rate were elevated and mean rTsMCR was low. These values reverted toward control values after insulin therapy; however, the changes were not statistically significant. Our results suggest a significant impairment of T4 monodeiodination of both phenyl rings in patients with uncontrolled diabetes mellitus. Good diabetic control with diet and insulin therapy for 2 weeks improved but did not totally correct the abnormal iodothyronine monodeiodination in these patients.