Five-year incidence and progression of thyroid dysfunction in an older population

Background: Very few studies have assessed both the incidence and progression of thyroid dysfunction in a single older population-based cohort. In this study, we aimed to assess the 5-year incidence, progression and risk factors for development of thyroid dysfunction in an older Australian population. Methods: The Blue Mountains Eye Study is a longitudinal population-based cohort study. During 1997-1999, 1768 participants (>= 55 years) had thyroid function assessed. After excluding participants reporting any form of treatment for their thyroid condition at baseline, 951 participants (91.4%) without thyroid dysfunction and 54 (5.4%) with thyroid dysfunction were re-examined 5 years later. Thyroid dysfunction was defined using serum thyrotropin (thyroid stimulating hormone (TSH)) screen, followed by serum free T4 assessment. Results: The overall 5-year incidence of thyroid dysfunction was 4.7% (95% confidence interval (CI) 3.4-6.1). Obesity (body mass index >= 30 kg/m2) and serum TSH > 2 mIU/L at baseline predicted incident overt hypothyroidism (odds ratio (OR) 4.05, CI 1.74-9.41) and (OR 5.46, CI 1.16-25.67) respectively. The 5-year incidence of subclinical hypothyroidism was significantly higher in women than in men, 2.5% versus 0.7% (P = 0.03). Progression to overt hypothyroidism was observed in 17.9% of subjects with subclinical hypothyroidism over 5 years. Conclusions: The 5-year incidence of thyroid dysfunction in this older population was relatively low, and was associated with obesity and serum TSH level > 2 mIU/L at baseline. Over one in six persons with subclinical hypothyroidism progressed to overt thyroid dysfunction over the 5-year period. Our findings highlight the need for appropriate management of subclinical hypothyroidism among older people.