The clinical significance of SIGLEC1 mRNA levels derived from peripheral leukocytes in autoimmune thyroid diseases and type 1 diabetes.

Autoimmune thyroid diseases (AITD), including Graves' disease (GD) and Hashimoto's thyroiditis (HT), may coexist with Type 1 diabetes mellitus (T1DM) as autoimmune polyglandular syndrome type 3 (APS3). Sialic acid-binding immunoglobulin-like lectin (SIGLEC) 1 is an interferon-inducible molecule implicated in autoimmune disorders, but its clinical significance across AITD, T1DM, and APS3 remains unclear. In this cross-sectional study, 219 patients with AITD, T1DM, or APS3 were enrolled. SIGLEC1 mRNA levels derived from peripheral blood cells were quantified using quantitative reverse transcription PCR. In the primary analysis, 213 patients with SIGLEC1 mRNA levels were included (AITD-only, n = 127; T1DM-only, n = 59; APS3, n = 27). SIGLEC1 mRNA levels differed significantly among the three groups (p = 0.00086). Holm-adjusted post hoc comparisons showed significantly higher SIGLEC1 mRNA levels in the T1DM-only group and APS3 group than in the AITD-only group (p = 0.0099 and p = 0.0099, respectively), whereas the T1DM-only and APS3 groups did not differ significantly (p = 0.289). Within the AITD-only cohort, thyroid autoantibody-positive patients had higher SIGLEC1 mRNA levels than antibody-negative patients (median 130.1 vs. 74.4 copies, p = 0.034). SIGLEC1 mRNA levels showed a modest positive correlation with HbA1c (r = 0.210, p = 0.0060) but not with thyroid function parameters. In multivariable models, APS3 remained independently associated with higher SIGLEC1 mRNA levels after adjustment for age, sex, and HbA1c. Elevated SIGLEC1 mRNA levels were associated with coexisting endocrine autoimmunity and thyroid autoantibody positivity in this cohort. Longitudinal studies are required to determine whether SIGLEC1 mRNA levels have prognostic or predictive utility.