The effect of thyroid disease on proinsulin and C-peptide levels.

C-peptide and proinsulin levels were studied in hyper and hypothyroidism both pre and post-treatment and in comparison to matched normals. Fasting C-peptide was reduced in untreated hyperthyroidism (0.4 +/- 0.2 (mean +/- SEM) vs 0.7 +/- 0.2 nmol/l, P less than 0.05) but returned to normal levels following treatment. Fasting proinsulin was elevated in untreated hyperthyroidism (3.6 +/- 0.7 vs 2.4 +/- 0.5 pmol/l, P less than 0.05) also returning to normal after treatment. A similar pattern was seen after oral glucose. The increased proinsulin and reduced C-peptide suggest there may be a defect of proinsulin processing in hyperthyroidism. Fasting C-peptide was reduced in untreated hypothyroidism (0.4 +/- 0.1 vs 0.7 +/- 0.1 nmol/l, P less than 0.05) and also returned to normal after treatment. Fasting proinsulin did not differ significantly from controls. However, proinsulin was reduced after oral glucose (4.7 +/- 0.7 vs. 7.9 +/- 2.0 pmol/l, P less than 0.05) as was C-peptide (0.9 +/- 0.2 vs 2.6 +/- 0.3 nmol/l, P less than 0.05). Both returned to normal after treatment. These findings suggest there are abnormalities of proinsulin and C-peptide levels in both hyper and hypothyroidism.