Introduction: Considering the remarkable increase in life expectancy among older people and lack of data on thyroid diseases in elderly Iranians, this study was designed to determine thyroid dysfunction in people, aged over 55 years. Materials and Method: This study was conducted on 819 Tehranian participants, aged over 55 years from the Tehran Thyroid Study, who were assessed every 3 years for serum free T4 (FT4) and TSH levels and were followed for 10.6 years. Data were analyzed by t test’ chi-square test, sig test and cox regression analysis. Results: Mean age of participants was 62.5±5.5 years. Of these 712 (86.9%) were euthyroid, 41(5%) had subclinical hypothyroidism (SCH), 19(2.4%) had overt hypothyroidism (OH), 34(4.1%) had subclinical hyperthyroidism and 13(1.5%) overt hyperthyroidism. Overall 13% of participants had thyroid dysfunction. The annual incidence rate of SCH and OH were 8.8 and 7.1 cases per 1000 people respectively. Multivariate regression analysis showed the risk of hypothyroidism in euthyroid people to be affected by TPO positivity [HR=1.9 (CI 95% 1.04-3.5)] and TSH>1.88 mlu/l [HR=8.1 (CI 95% 3.4-19)]. Concolusion: More than one-tenth of aged people have thyroid dysfunction. High TSH and TPO positivity are the most important risk factors of hypothyroidism in elderly people.