Thyroid antibodies in northern Norway: prevalence, persistence and relevance.

Objectives. To investigate the prevalence and persistence of thyroid autoantibodies in a population sample and to assess the development of biochemical hypothyroidism (defined as an elevated serum thyrotropin [TSH] concentration) in relation to their presence. Design and setting. A cross-sectional and longitudinal study based on the Tromso Study in 1979–80 and 1986–87. Subjects and main outcome measures. From 2551 random participants in 1979–80 aged 34±8.4 (mean±SD) years, sera were available in 2513 and 2504 persons for determination by passive haemagglutination of the antibody to thyroid microsomal antigen (anti-Tm) and of the antibody to thyroglobulin (anti-Tg). Total thyroxine (TT4) and TSH were measured in 114 of 176 antibody-positive subjects and in 101 controls. After 7 years, anti-Tm and anti-Tg were remeasured in 1939 and 1931 subjects, and TT4 and TSH in 92 of the initially antibody-positive subjects and in 69 controls. Results. Anti-Tm occurred more frequently than anti-Tg (in 6.1 vs. 2.8%; P<0.001). Anti-Tm (P< 0.001) and anti-Tg (P=0.027) were both more common in women than in men. The prevalence of anti-Tm (P=0.025), but not of anti-Tg, increased with age. Changes in titre levels after 7 years were mostly small or moderate. Both in women (P= 0.005) and in men (P<0.001) the TSH concentrations increased with increasing levels of anti-Tm, whereas in men, the concentrations also increased with increasing anti-Tg levels (P<0.001). Biochemical hypothyroidism developed with a 2.7% yearly incidence only in antibody-positive subjects, all except one of whom had anti-Tm. Conclusions. The prevalences of thyroid antibodies were comparable to those found in similar studies in other areas. Their presence was associated with the development of biochemical hypothyroidism.