Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Thyroid hormone (TH) abnormalities are common in patients with diabetes mellitus (DM). These thyroid hormone abnormalities have been associated with inflammatory activity in several conditions but this link remains unclear in DM. We assessed the influence of subclinical inflammation in TH metabolism in euthyroid diabetic patients. Cross-sectional study involving 258 subjects divided in 4 groups: 70 patients with T2DM and 55 patients with T1DM and two control groups of 70 and 63 non-diabetic individuals, respectively. Groups were paired by age, sex, and body mass index (BMI). We evaluated the association between clinical and hormonal variables [thyrotropin, reverse T3 (rT3), total and free thyroxine (T4), and triiodothyronine (T3)] with the inflammation markers C-reactive protein (hs-CRP), serum amyloid A (SAA), and interleukin-6 (IL-6). Serum T3 and free T3 were lower in patients with diabetes (all P < 0.001) compared to the control groups. Interleukin-6 showed positive correlations with rT3 in both groups (P < 0.05). IL-6 was independently associated to FT3/rT3 (B = −0.193; 95 % CI −0.31; −0.076; P = 0.002) and FT4/rT3 (B = −0.107; 95 % CI −0.207; −0.006; P = 0.039) in the T1DM group. In the T2DM group, SAA (B = 0.18; 95 % CI 0.089; 0.271; P < 0.001) and hs-CRP (B = −0.069; 95 % CI −0.132; –0.007; P = 0.03) predicted FT3 levels. SAA (B = −0.16; 95 % CI −0.26; −0.061; P = 0.002) and IL6 (B = 0.123; 95 % CI 0.005; 0.241; P = 0.041) were related to FT4/FT3. In DM, differences in TH levels compared to non-diabetic individuals were related to increased subclinical inflammatory activity and BMI. Altered deiodinase activity was probably involved. 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