Subclinical hypothyroidism and the development of heart failure: an overview of risk and effects on cardiac function.

The prevalence of subclinical hypothyroidism (SCH) ranges from 5 to 15% of the general population. However, it remains controversial if SCH warrants life-long thyroxine replacement therapy. Patients with a thyroid-stimulating hormone (TSH) level > 10 mIU/L have a higher risk of developing heart failure with reduced ejection fraction as compared to subjects with normal thyroid function. However, abnormally high TSH levels could also be connected with an overall lower metabolic rate and better survival in elderly subjects. The potential mechanisms responsible for diastolic dysfunction of the left ventricle (LV) in SCH are connected with endothelial dysfunction and arterial stiffness, inflammatory state and are driven by TSH apoptosis-derived microparticles. The impact of SCH on LV systolic function is more controversial, and it is connected not only with cardiac remodelling but also with predisposition of patients with SCH to the conditions leading to heart failure. This review presents an overview of processes in the context of potential benefits of thyroxine supplementation therapy.