To assess kidney function, we calculated the estimated glomerular filtration rate (eGFR) by referring to the abbreviated modification of diet in renal disease formula [29]. CKD stages were defined by eGFR according to the KDIGO: stage 1–2: >60; stage 3a: 45–59, stage 3b: 30–44; stages 4–5 < 30 ml/min/1.73 m2. Serum and urinary albumin and creatinine and serum c-reactive protein (CRP) were determined by standard methods. Albuminuria was assessed by albumin-to-creatinine ratio (ACR) and classified as mild, moderate, or severe when ACR < 300 (ACR1), 300–3000 (ACR2), and >3000 mg/g (ACR3). Automated chemiluminescence immunoassay systems (ADVIA Centaur, Bayer Vital, Fernwald, Germany) were used to determine thyrotropin (TSH), T4, free T4 (fT4), triiodothyronine (T3), fT3, and TBG concentrations in serum. rT3 was measured by RIA (Fa. Adaltis, Freiburg, Germany; with rT3 antiserum raised in rabbit; normal range: 0.14–0.54 nmol/l; sensitivity range: 0.03–3.1 nmol/l, % cross reactivity: T4: 0.029; T3: 0.0086). Anti-TPO Abs were measured by the Immulite 2000 Anti-TPO Ab assay; a solid-phase enzyme-labeled chemiluminescent sequential immunometric assay. The cut off level < 35 U/ml was provided by the manufacturer.

All laboratory testing was done at the University Hospital Essen Division of Laboratory Research by technicians blinded to patients’ clinical data.