INAPPROPRIATE TRIIODOTHYRONINE (T3) AND THYROXINE (T4) RADIOIMMUNOASSAY LEVELS SECONDARY TO CIRCULATING THYROID HORMONE AUTOANTIBODIES

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Abstract
SUMMARY: A 16‐year‐old boy with chronic lymphocytic thyroiditis was noted to have a low free thyroxine (T4) level, low triiodothyronine resin uptake (T3U), and high serum thyrotropin (TSH) values. Unexpectedly, markedly elevated T3 radioimmunoassay (RIA) and T4 (RIA) values, using a double antibody technique were obtained when performed directly on unextracted serum samples. Extremely low T4 (RIA) values were noted when polyethylene glycol (PEG) was used to separate bound from free hormone. The presence of circulating T3‐ and T4‐binding immunoglobulins was suspected and confirmed with the following special studies. With undiluted serum in a T3 (RIA) system, using dextran‐coated charcoal separation, 82% binding of 125I‐labelled T3 occurred in the absence of specific first antibody, with 55% binding retained at 1: 7 dilution with T3‐free serum. Comparable results were obtained in the T4 (RIA) system using polyethylene glycol separation. Following ethanol extraction, low T4 (RIA) and low normal T3 (RIA) values were obtained, using a double antibody technique. There was ten‐fold greater binding by the patient's serum to rabbit anti‐human IgG in both the T3 and T4 radioassay systems as compared to controls. No preferential binding to rabbit anti‐human IgM was noted. Scatchard plot analyses for the antibodies against T3 and T4 showed high affinity constants for these hormones. With adequate l‐thyroxine therapy, an appropriate decline in serum TSH to normal was achieved. It is concluded that where RIA determinations of T3 and T4 are inconsistent with other laboratory and clinical indices, the presence of autoantibodies to thyroid hormones should be suspected and appropriate tests undertaken.

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