Recent meta-analyses of population-based studies with long-term follow-up have clarified the risk of cardiovascular disorders in patients with subclinical thyroid dysfunction. Subclinical hyperthyroidism is associated with an increased risk of atrial fibrillation and cardiovascular morbidity and mortality; the risk is higher at a thyroid stimulating hormone (TSH) level < 0.1 mU/l than at 0.1-0.4 mU/l. Subclinical hypothyroidism increases the risk of cardiovascular morbidity and mortality, especially at TSH levels >10 mU/l. The incidence of ischaemic heart disease is reduced upon treatment with levothyroxine sodium treatment in patients aged ≤ 70 years, but not in patients aged > 70 years. Despite the lack of randomised clinical trials providing evidence, treatment of subclinical hypothyroidism or hyperthyroidism is recommended when the TSH level is < 0.1 or > 10 mU/l. Treatment of patients with TSH levels between 0.1-0.4 mU/l or 4-10 mU/l should depend on other risk factors and patient age, with no treatment for persons with a TSH level of 4-10 mU/l who are older than 65 years.
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