The prevalence of thyroid dysfunction was investigated in a small, rural community located at the coast in Middle Norway. Two hundred persons (114 women and 86 men) of the total 802 persons over 70 years of age in the community were examined regarding thyroid dysfunction. Blood samples were drawn from 197 (113 women and 84 men). In women previously diagnosed hypothyroidism was found in 3.5% and previously diagnosed hyperthyroidism in 0.9%. In men no previously diagnosed thyroid disease was found. Undiagnosed primary hypothyroidism (TT4 less than 70 nmol/l and TSH greater than 6 mU/l) was found in 1.8% and 1.2% of women and men, respectively. Latent hypothyroidism (TT4 70-150 nmol/l and TSH greater than 6 mU/l) was found in 3.5% and 2.4%, and borderline hypothyroidism (TSH 4.5-6.0 mU/l) in 3.5% and 2.4%, respectively. Undiagnosed hyperthyroidism was not found in women but in 1.2% of men. Antibody to the thyroid microsomal antigen (TMA) greater than or equal to 400 was detected in 17.5% of women and 9.6% of men. Clearly elevated serum thyrotropin (TSH) concentrations or previously diagnosed thyroid disease were found in 21.7% and 37.5% of the TMA positive women and men, respectively.