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

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 x 10(8) l/mol and a binding site concentration of 3.8 x 10(-9) mol/l. 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.