J. Endocrinol. Invest. 25: 779-784, 2002 779 ABSTRACT. Autoimmune thyroiditis is often as- sociated with Type 1 diabetes mellitus (T1DM). In non-obese adult-onset diabetes diagnosed initially as Type 2 diabetes mellitus (T2DM), there is a proportion of cases with so far undi- agnosed T1DM. The objective of this study was to estimate the frequency of autoimmune thy- roiditis (AT) among non-obese (BMI <30.0 kg/m 2 ) patients with T2DM and to compare the frequency of AT in subgroups of patients ac- cording to the presence of glutamic acid de- carboxylase antibodies (GADA), insulin re- quirement, and post-breakfast C-peptide lev- els. The study included 118 adult patients (55 men and 63 women) with the initial diagnosis of T2DM and age at the onset of diabetes >35 yr. Median of age was 66 yr (range 39-82), and median duration of diabetes was 9 (range 1- 27) yr. AT was diagnosed using thyroid perox- idase antibodies, TG-antibodies, US and TSH levels. Nineteen per cent of the subjects were found to have AT, and the frequency of AT did not significantly differ between the groups of GADA+ and GADA- subjects. There was no dif- ference in the frequency of AT between the group treated with hypoglycemic agents and/or diet and the group requiring insulin. The frequency of AT was higher in the group with post-breakfast C-peptide levels ≤0.8 nmol/l compared to the group with post-breakfast C- peptide levels >0.8 nmol/l (37% vs 16%), how- ever the group with post-breakfast C-peptide levels ≤0.8 nmol/l had longer duration of dia- betes. (J. Endocrinol. Invest. 25: 779-784, 2002) © 2002, Editrice Kurtis INTRODUCTION Two major types of diabetes mellitus (DM) are identified: Type 1 diabetes mellitus (T1DM), with beta-cell destruction leading to absolute insulin deficiency and Type 2 diabetes mellitus (T2DM), with insulin resistance and usually relative (rather than absolute) insulin deficiency (1). Regarding adult-onset diabetes, a particular problem area still remains in the classification of DM. Here, the major problem is to distinguish the T2DM from a slowly progressing form of the T1DM that was first characterized in the early 1980’s (2) and called latent autoimmune diabetes (AT) in adults (LADA) later (3). The typical patient with LADA is 25-yr old or older, non-obese and presents with what clinically appears to be T2DM (4). Subse- quent clinical course, characterized by a slowly progressing development of insulin deficiency and the presence of serological markers of au- toimmunity (mainly the glutamic acid decarboxy- lase antibodies – GADA), suggests that it is a form of T1DM (5, 6). AT is often associated with typical T1DM. This has been reported in children (7-10) as well as in younger adults (11-13). AT has been document- ed using different diagnostic criteria that are a combination of the presence of thyroid peroxi- dase antibodies (TPOAb) or TG-antibodies (TGAb), assessment of thyroid function, US imag- ing of the thyroid and fine needle biopsy in some studies. Though the association of AT with T1DM is clear, little is known about the risk of autoimmune thy- roiditis in LADA. Here, the main problem still re- Key-words: Autoimmune thyroiditis, T2DM, glutamic acid decarboxy- lase antibodies, thyroid peroxidase antibodies, TG-antibodies, C-pep- tide. Correspondence: Dr. Magdalena Mate ˇjková-Be ˇ hanová, Institute of Endocrinology, Národní 8, 116 94 Prague, Czech Republic. E-mail: mbehanova@endo.cz Accepted May 23, 2002. Autoimmune thyroiditis in non-obese subjects with initial diagnosis of Type 2 diabetes mellitus M. Mate ˇ jková-Be ˇhanová*, V. Zamrazil*, K. Vondra*, J. Vrbíková*, P. Kuc ˇ era**, M. Hill*, and M. Ande ˇl*** *Institute of Endocrinology, **Department of Allergology and Immunology, and ***Diabetes Centre, 3 rd Faculty of Medicine, Charles University and University Hospital Kralovske Vinohrady, Prague, Czech Republic