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.