Triiodothyronine (T₃) binding to immunoglobulin G subclasses in a slightly hypothyroid woman

Abstract

A 44-year-old woman, with an earlier history of myxoedema, had stopped taking thyroxine and was found to have both a slightly elevated basal TSH level and an increased TSH response after thyroliberin stimulation. The T₄ value was low borderline, and using a radioimmunoassay that separated free and antibody-bound hormone by polyethylene glycol precipitation, the T₃ value was found to be less than zero. This last finding was explained by the existence of T₃ antibodies in the patient's serum. The antibodies, being of the IgG class, did not bind T₄, thyroglobulin or cytoplasmic thyroid antigen. The subclass IgG₂, and to a lesser extent IgG₁, IgG₃ and IgG₄, did bind T₃. A Scatchard plot gave an antibody avidity constant of 4.28 × 10⁸ 1/mol and a binding site concentration of 3.8 × 10^-9 mol/1. Radioimmunoassay of total T₃, after extraction with ethanol gave a high value. The concentration of free T₃ was, however, just above the lower reference limit.

Key-words:

• autoantibodies
• avidity
• ethanol extractions
• free T₃ and T₄
• myxoedema
• radioimmunoassays