Hypersecretion of proinsulin in thyrotoxicosis.

The plasma insulin, C-peptide and proinsulin concentrations were investigated in thyrotoxic patients and in normal controls after an overnight fast, during a 36 h fasting period, an intravenous glucose tolerance test and an oral glucose tolerance test. The main finding was a significantly raised concentration of proinsulin in plasma of patients with thyrotoxicosis. After an overnight fast the plasma proinsulin was 0.048 +/- 0.005 pmol/ml in 15 thyrotoxic patients compared with 0.023 +/- 0.012 pmol/ml in 15 euthyroid controls. A twofold rise of plasma proinsulin concentration was also found in thyrotoxic patients during a prolonged fast, and during intravenous and oral glucose tolerance tests. The immunoreactivity of proinsulin in the insulin radioimmunoassay gave rise to slightly elevated concentrations of immunoreactive insulin in thyrotoxic patients in all the conditions investigated. When insulin values were corrected for proinsulin crossreactivity, they were similar in euthyroid controls and thyrotoxic patients. The concentration of plasma C-peptide was not significantly altered in thyrotoxic patients during intravenous and oral glucose tolerance tests.