Archived serum samples that had been stored at −70°C were available for 2108 participants in the 1981 survey, who formed the cohort. Samples had been securely stored in airtight polypropylene tubes, which were filled to capacity and had not been thawed during storage. Although the stability of free thyroxine concentrations in long-term storage has not been formally demonstrated, the stability of other analytes, including thyrotropin, antibodies, and steroid hormones, under such conditions has been shown.11 The proportion of men was higher among subjects for whom serum samples were available compared with those for whom serum samples were not available (50% vs 40%, P<.001), but within each sex group, the age and prevalence of previously diagnosed thyroid disease did not differ significantly between subjects with and without available serum samples. Serum thyrotropin, free thyroxine, thyroid peroxidase antibody, and thyroglobulin antibody concentrations were measured using an Immulite 2000 chemiluminescent analyzer (Diagnostic Products Corporation, Los Angeles, Calif) in 2001. For the thyrotropin assay, functional sensitivity was 0.02 mIU/L. Interassay imprecision (expressed as coefficient of variation) for each analyte was as follows: thyrotropin, 7.6%; free thyroxine, 9.6%; thyroid peroxidase antibodies, 7.2%; and thyroglobulin antibodies, 8.5%. Reference ranges (based on 95% confidence intervals [CIs] after excluding gross outliers and subjects with self-reported thyroid disease or goiter) were as follows: thyrotropin, 0.4 to 4.0 mIU/L; free thyroxine, 0.7 to 1.8 ng/dL (9-23 pmol/L); thyroid peroxidase antibodies, less than 35 kIU/L; and thyroglobulin antibodies, less than 55 kIU/L. Euthyroidism was defined as a serum thyrotropin level of 0.4 to 4.0 mIU/L (regardless of free thyroxine concentration), subclinical hyperthyroidism as a thyrotropin level less than 0.4 mIU/L with normal free thyroxine, and subclinical hypothyroidism as a thyrotropin level greater than 4.0 mIU/L with normal free thyroxine.6 The subclinical hyperthyroid group was further divided into subjects with a thyrotropin level less than 0.1 mIU/L and those with a thyrotropin level between 0.1 and less than 0.4 mIU/L. The subclinical hypothyroid group was further divided into subjects with a serum thyrotropin level of 10.0 mIU/L or less and those with a thyrotropin level greater than 10.0 mIU/L. Positive thyroid antibodies was defined as an increased concentration of thyroid peroxidase antibodies or thyroglobulin antibodies.