E tsh kit

Blood samples were processed and transported to the central certified laboratory, and analyzed within 24 h. For thyroid function tests, approximately 15 mL of blood was collected from each participant. After separating the serum within 30 min, the samples were sent to a certified central laboratory and analyzed. Serum TSH and fT4 levels were measured using an electrochemiluminescence immunoassay (Roche Diagnostics, Mannheim, Germany). Serum TSH levels were assessed using an E-TSH kit (Roche Diagnostics). This study used the TSH reference range of 0.62–6.68 mIU/L for the Korean population [17 (link)]. Serum fT4 levels were assessed using an E-Free T4 kit (Roche Diagnostics), with a laboratory reference range of 0.89–1.76 ng/dL.

Data on demographic characteristics, personal medical history, and family history of thyroid diseases were collected by interview during the survey. Life style risk factors (i.e., smoking status) were based on self-reporting. Serum TSH, FT4, and TPOAb concentrations were measured by an electrochemiluminescence immunoassay. Serum TSH was measured using an E-TSH kit (Roche Diagnostics, Mannheim, Germany). Serum FT4 was measured by E-Free T4 kit (Roche Diagnostics, Mannheim, Germany). Serum TPOAb was measured by E-Anti-TPO kit (Roche Diagnostics, Mannheim, Germany). The results of TSH, FT4, and TPOAb met the specifications regarding accuracy, general chemistry, special immunology, and ligand of the quality control and quality assurance program of the College of American Pathologists. UI was measured using an inductively coupled plasma mass spectrometry device (ICP-MS; Perkin Elmer ICP-MS, Waltham, MA, USA). The laboratory that measured UI is enrolled in the “Ensuring the Quality of Urinary Iodine Procedures (EQUIP)” quality assurance program run by the Centers for Disease Control of the United States of America [17 ].

The levels of serum TSH in a subsample of 2,400 individuals aged over 10 years were analyzed. Approximately 15 mL of blood was collected, and within 30 minutes the serum was separated and then transferred to the testing facility. Serum TSH levels were measured with an electrochemiluminescence immunoassay (Cobas8000 E-602; Roche, Germany) at the central laboratory (NeoDin Medical Institute, Seoul, Korea) within 24 hours after sampling. An E-TSH kit (Roche Diagnostics) was used for measuring the levels of TSH, and the reference range was 0.35–5.50 mIU/L. The results reported met specifications for accuracy, general chemistry, special immunology, and ligand established by the quality control and assurance program of the College of American Pathologists.

Blood samples were obtained from each participant’s antecubital vein in the morning after fasting for at least eight hours. As previously reported, serum TSH, free T4, and thyroid peroxidase antibody (TPOAb) were measured with an electrochemiluminescence immunoassay (Cobas8000 E-602/Roche Diagnostics, Mannheim, Germany)^{21 (link)}. Briefly, TSH was measured with an E-TSH kit (Roche Diagnostics), and the TSH reference interval was determined to be between the 2.5th and 97.5th percentile of the serum TSH levels of the reference population, as previously reported^{21 (link)}. Serum free T4 was measured using an E-Free T4 kit (Roche Diagnostics), and the reference range was 0.89–1.76 ng/mL. TPOAb was measured using an E-Anti-TPO kit (Roche Diagnos-tics); the normal range for TPOAb in humans is < 34.0 IU/mL.
Lipid profiles were measured with a Hitachi Automatic Analyzer 7600 (Hitachi, Tokyo, Japan) using commercially available kits (Sekisui, Osaka, Japan). Serum total cholesterol and triglycerides were measured by enzymatic methods. Insulin was measured by electrochemiluminescence immunoassay (Cobas 8000/Roche/Germany) using Elecys/insulin (Roche/Germany).

Serum TSH and fT4 levels were measured with an electrochemiluminescence immunoassay (Roche Diagnostics, Mannheim, Germany). TSH was measured using an E-TSH kit (Roche Diagnostics), and its reference interval was considered to be 0.62–6.86 mIU/L, which was the reference interval for serum TSH in Korea based on KNHANES VI data.[1 (link)] The fT4 was measured using an E-Free T4 kit (Roche Diagnostics), and the reference interval was 0.89–1.76 ng/mL. UI concentrations were measured with an inductively coupled plasma mass spectrometry device (ICP-MS; Perkin Elmer ICP-MS, Waltham, MA, USA), using an iodine standard (Inorganic Venture, Christiansburg, VA, USA). Urinary creatinine concentration was determined by kinetic colorimetric assay on a Hitachi 7600 automatic analyzer (Hitachi Co., Tokyo, Japan), using CREA reagent (Roche Diagnostics). Urinary iodine (μg):creatinine (g) ratio (UICR) was calculated by dividing urinary iodine by urinary creatinine and then multiplying the result by 100, for adjustment of water excretion rates at the time of spot urine specimen collection.

Thyroid function tests were conducted in one-third of all KNHANES VI participants, (approximately 2400 persons aged ≥ 10 years annually). The sum of 3 years of testing represents the entire population of Korea. Serum TSH was measured using an electrochemiluminescence immunoassay (E-TSH kit, Roche Diagnostics, Mannheim, Germany), and the TSH reference interval was determined to be between the 2.5th and 97.5th percentile of serum TSH levels measured in the reference population (0.62–6.86 mIU/L)^{58 (link)}. TSH levels were logarithmically transformed to normalize the distribution. In this study, we used the TSH levels to represent the thyroid function as TSH is the most sensitive marker of thyroid function, influenced by minute changes in free T4 concentrations^{59 (link)}. Serum free T4 was also measured through an electrochemiluminescence immunoassay (E-Free T4 kit, Roche Diagnostics, Mannheim, Germany), with a reference range of 0.89–1.76 ng/mL. Anti-thyroid peroxidase antibody (TPOAb) levels were measured using an E-Anti-TPO kit (Cobas e 801; Roche Diagnostics) with a reference range of 0–34 IU/mL. Urine iodine concentrations were measured using an inductively coupled plasma mass spectrometer (ICP-MS; Perkin Elmer ICP-MS, Waltham, MA, USA) with an iodine standard (Inorganic Venture, Christiansburg, VA, USA).

The definition of SCH based on a normal serum FT4 concentration with an elevated serum TSH concentration has been widely used in previous literature. However, the reference range used for TSH has been inconsistent, and the upper limit has varied in previous literature. In our clinical practice, TSH was measured using an E-TSH kit (Roche Diagnostics), for which the reference range was 0.35–5.00 μIU/mL. Therefore, in our study, the concentration of TSH > 5.0 μIU/mL was considered to exceed the upper limit. The concentration > 10.0 μIU/mL often indicated overt hypothyroidism and was often accompanied by alterations in lipid and carbohydrate metabolism. Therefore, the concentration of TSH within the range from 5.0 to 10.0 μIU/mL, and with a normal FT4 concentration were used to define SCH in our study.
The definition of metabolic risk factors was adopted as well-accepted criteria. Either or both of the blood pressures, systolic blood pressure (SBP) > 130 mmHg and diastolic blood pressure (DBP) > 85 mmHg, were categorized as hypertension; and a concentration of TG > 150 mg/dL was categorized as hypertriglyceridemia. Obesity was defined as BMI ≥ 25Kg/m2; hyperglycemia was defined as a concentration of AC > 100 mg/dL; and low HDL was defined as a concentration of HDL < 50 mg/dL for men and 40 mg/dL for women.