Immunoglobulins of Untreated Graves' Patients with or without Thyrotropin Receptor Antibody (Determined by Porcine Thyrocytes) Universally Elicit Potent Thyroid Hormone-Releasing Activity in Cultured Human Thyroid Follicles

Thyrotropin receptor antibody (TRAb), comprising thyrotropin binding inhibitor immunoglobulin (TBII) and thyroid-stimulating antibody (TSAb), both of which are conventionally determined using porcine thyrocytes in Japan, is not always positive in patients with untreated Graves' disease. To elucidate whether immunoglobulin G (IgG) obtained from TBII/TSAb-positive ( + ) or negative ( -) Graves' disease patients are responsible for hyperthyroidism, we investigated the thyroid hormone-releasing activity (THRA) of these IgGs in human thyroid follicles in suspension culture, in which bovine thyrotropin (bTSH) is detectable even at 0.1 μU/mL. Human thyroid follicles, obtained from Graves' disease patients by subtotal thyroidectomy, were cultured in serum-free F-12/RPMI-1640 medium supplemented with bTSH or purified Graves' IgGs. After preculturing for 3 days, ¹²⁵I was added, and after an additional 3 days of culture, ¹²⁵I incorporated into the thyroid follicles and organic ^12SI released into the culture medium (mainly ¹²⁵I -T₄ -I- ¹²⁵I-T₃) were counted.

Seventy TBII( + )/TSAb( + )-, 3 TBII( + )/TSAb( - )-, and 3 TBII( - )/TSAb( + )- patients with untreated Graves' disease were all positive for THRA, which became undetectable in spontaneous remission obtained after several years of medical treatment. The THRA was equivalent to 0.8-230 μU/mL bTSH. Furthermore, 2 TBII(-)/TSAb(-) patients were significantly positive for THRA. This TBII( - )/TSAb( - )IgG stimulated human thyrocytes to produce cyclic adenosine monophosphate (cAMP), and this was partially inhibited by antihuman IgG antibody. The THRA induced by TBII( + )/TSAb( + ) IgGs as well as TBII( - )/TSAb( - ) IgG was inhibited by blocking-type TRAb obtained from TBII( + ) patients with myxedema. There was a significant correlation between THRA and TSAb. These in vitro findings suggest that all IgGs obtained from untreated Graves' patients (n = 78) elicit potent THRA in human thyroid follicles in suspension culture. Because the TBII( -)/TSAb( -) IgGs can stimulate cAMP production in human but not in porcine thyrocytes, they probably recognize epitope(s) of TSHbinding sites specific to the human thyrotropin (hTSH) receptor. Furthermore, we have demonstrated that the thyroid gland of hyperthyroid Graves' patients is stimulated by IgG(s) equivalent to at least 0.8 μU/mL bTSH (about 5 μU/mL hTSH) in vitro.