A Case of Systemic Lupus Erythematosus (SLE) and Sjögren's Syndrome Associated with Anti-T₃ Autoantibodies

Abstract

A case of systemic lupus erythematosus (SLE) associated with Sjögren's syndrome had extremely low serum triiodothyronine (T₃) with normal levels of serum thyroxine (T₄) measured by single antibody radioimmunoassays (RIAs) and thyroid stimulating hormone (TSH) during steroid treatment. Measurement of serum T₃ and T₄ with double antibody RIAs showed unusually high T₃ and normal T₄ concentrations. Examination of her serum revealed the presence of IgG class anti-T₃ autoantibodies whose Scatchard plot was analyzed in two components; one with a higher associate constant (8.6×10⁸M^-1) and a lower binding capacity (5.6×10^-7mol/ml serum); the other a lower associate constant (3.5×10⁷M^-1) and a higher binding capacity (2.1×10^-6mol/ml serum). Antithyroglobulin (Tg) autoantibody has been positive throughout the seven year observation period. A significant positive correlation between titers of anti-Tg autoantibodies indicated that the antigen of anti-T₃ antibodies in the patient could be T₃ containing antigenic site (s) on the Tg molecule.