Autoantibodies to Glutamic Acid Decarboxylase in Patients with IDDM and Autoimmune Thyroid Disease

Autoantibodies to glutamic acid decarboxylase (GAD), previously reported to be the 64,000-M_r (64K) islet cell protein, were measured by a radioimmunoassay using purified pig brain GAD in 29 insulin-dependent diabetes mellitus (IDDM) patients with autoimmune thyroid disease (AITD) and in 29 sex- and disease duration-matched IDDM patients without AITD. Islet cell antibodies (ICAs) and 64K antibodies were also determined. In IDDM patients with short-duration diabetes (<1 year), the prevalence and levels of GAD antibodies were 100% (8 of 8) and 609 ± 166 U (means ± SE), respectively, in IDDM patients with AITD and 81.8% (9 of 11) and 90 ± 51 U, respectively, in patients without AITD. In patients with long-standing IDDM (3–22 years), the prevalence and levels of GAD antibodies were 76.2% (16 of 21) and 193 ± 66 U, respectively, in patients with AITD and 50.0% (9 of 18) and 36 ± 14 U, respectively, in patients without AITD. For up to 6 years after the onset of IDDM, the levels of GAD antibodies in IDDM patients with AITD were significantly higher than in IDDM patients without AITD. A close and significant correlation was found between GAD antibodies and ICA or 64K antibodies in IDDM patients with AITD. Our results demonstrate that high levels of GAD antibodies were present in IDDM patients with AITD. The observed differences in GAD immunoreactivity between IDDM patients with and without AITD might help evaluate the role of GAD antibodies in IDDM.