Interference of anti-T3 autoantibodies in the measurement of total and free T3 in serum.

A case of anti-trilodothyronine autoantibodies is presented in this report. The thyroid hormone profile of a hypertensive patient, with multinodular goiter and history of exogenous thyroid hormone therapy, was found to be highly ambiguous. The total as well as free T3 levels were consistently high (out of range) whereas the T4 (total and free) values were always within normal limits. The thyrotropin was found to be partially suppressed. Very low T3-Uptake indicated some kind of interference in the immunoassays. We incubated the sera with the radio-iodine labelled T3 and observed that the patient's serum bound about ten times more radioactivity than a control run in parallel. On further resolving the serum proteins on cellulose acetate electrophoresis, the radioactivity was detected in the γ-globulin band. Therefore it was established that the patient's serum carried the antibodies reactive with T3 which were interfering in the immunoassays. Elevated anti-thyroid microsomal antibodies were also present in the patient's serum. The anti-T3 antibodies were highly specific for T3 and did not show any cross reactivity with the T4 or its analogues used in the estimation of free T4.