Problems on the Specificity of Iodinated Insulin Binding by Sera of Patients with Autoimmune Thyroid Disorders

Abstract

Binding of ¹²⁵I-insulin was detected by the polyethylene glycol method in three patients with Hashimoto's thyroiditis, who had very high titers of anti-thyroglobulin and anti-thyroid microsome antibodies. The specificity of this binding was analyzed. In contrast to serum of an insulin-treated diabetic patient which bound only ¹²⁵Iinsulin, these sera bound various ¹²⁵I-labelled peptides (insulin, C-peptide, gastrin, GH, prolactin and TSH) more than normal. The binding of ¹²⁵I-insulin by serum of the insulin-treated patient was inhibited by insulin and iodinated insulin. On the other hand, ¹²⁵I-insulin binding by sera of thyroiditis patients was decreased by the addition of iodinated insulin, iodinated albumin, thyroglobulin, MIT and DIT, but not by unlabelled insulin. Immunization of guinea pigs with thyroglobulin resulted in the production of antisera which bound ¹²⁵I-insulin, and this binding was inhibited by iodinated peptides, MIT and DIT, but not by unlabelled insulin. Binding of ¹²⁵I-insulin by sera of thyroiditis patients was not clearly demonstrated by gel filtration or paper hydrodynamic flow methods, suggesting the weakness of the binding.
These findings suggest that serum ¹²⁵I-insulin binding in Hashimoto's thyroiditis is not due to insulin antibody but due to some other factor, possibly to anti-thyroglobulin antibody with binding specificity to iodotyrosine residues. Such a binding could be a pitfall for the detection of hormone antibodies using radioiodinated hormones, as well as a source of error in some radioimmunoassays.
Serum of a patient with Graves' disease also bound ¹²⁵I-insulin but not the other iodinated peptides and the binding was inhibited to some extent both by unlabelled insulin and by iodinated compounds. The nature of this binding remains obscure.