Impaired pituitary thyrotroph function in uncontrolled type II diabetes mellitus: normalization on recovery.

Altered thyroid hormone metabolism with decreased serum T3 and increased rT3 concentrations in patients with uncontrolled diabetes mellitus has been well documented. However, data regarding TSH secretion are sparse, especially the influence of glycemic control. Therefore, we examined serum T4, free T4, T3, rT3, T3 resin uptake, and TSH as well as the TSH response to TRH administration [expressed as TSH increment (delta TSH) and area under the curve (theta TSH)] in 29 newly discovered type II diabetic patients (DM) before treatment and in 12 normal subjects. The study was repeated in the DM patients after attainment of euglycemia and normalization of glycosylated hemoglobin (HbA1C) following therapy with diet and tolazamide for 8-12 weeks. Serum T4, free T4, and T3 resin uptake were not significantly different in DM compared to those in normal subjects. Serum T3 was low and rT3 was high in DM before treatment, and both normalized on achieving the euglycemic state. Basal TSH in uncontrolled DM was not significantly different from that in normal subjects and remained unchanged during treatment. However, delta TSH and theta TSH were significantly reduced (P less than 0.01) in uncontrolled DM. Both fasting plasma glucose (FBS) and HbA1C levels correlated inversely with delta TSH as well as theta TSH (FBS vs. delta TSH, r = -0.42; FBS vs. theta TSH, r = -0.38; HbA1C vs. delta TSH, r = -0.40; HbA1C vs. theta TSH, r = -0.42; P less than 0.05 for all correlations). Finally, TSH responses returned to normal on attainment of euglycemia and normal HbA1C concentrations. These studies indicate that regulation of TSH secretion is altered in DM during the decompensated state and normalizes when euglycemia is achieved.