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Subclinical thyroid disease and heart failure

Subclinical hypothyroidism (SCH), defined as the finding of an elevated serum thyroid-stimulating hormone (TSH) level with a normal free T4 level, is frequently observed in the general population. A recent publication, in data obtained from the LifeLines Cohort Study, showed a SCH prevalence of 10% in a relatively young group of individuals with mean age of 46 years.1 Earlier studies have shown SCH prevalences of 4–20% in the adult population, mainly depending on age, sex, dietary iodine intake, and the cut-off concentrations of serum TSH used to define the condition.2 In a large meta-analysis, Rodondi et al.3 reported an association between SCH and the development of cardiovascular disease (CVD), but a significantly increased risk of both coronary heart disease (CHD) events and mortality was only seen in participants with TSH levels of 10 mU/L or higher. Subclinical hyperthyroidism, defined as the finding of a serum TSH level below the lower limit of the laboratory’s reference range with a normal free T4 level, is less common. Reported prevalences range from 0.6% to 1.8% in the adult population, again depending on age, sex, and iodine intake.2 Studies on the association of subclinical hyperthyroidism with mortality showed conflicting results: some studies demonstrated increased all-cause mortality,4 while others did not.5 However, recently an independent association has been shown between lower TSH levels and increased cardiovascular mortality during long-term follow-up in thyroid cancer patients on TSH-suppressive therapy.6 It is known that an increased left ventricular mass, impaired diastolic dysfunction and increased risk on atrial fibrillation can be found during TSH-suppressive therapy. There are also several reports on the relationship between TSH and ageing, and both a progressive increase and decrease in TSH levels with advancing age have been reported.7,8 Investigators from the Leiden 85-plus study9 reported, in individuals aged 85 years and older, that SCH was associated with better survival than in subjects with normal TSH levels. However, such observational results should be interpreted with caution as other alterations in the oldest age group also are associated with better survival, such as high blood pressure and high cholesterol. In this issue, Chen et al.10 report a very elegant prospective follow-up study on the relationship between TSH levels and outcome in patients with heart failure (HF). They followed a total of 5599 patients with a diagnosis of HF at a health maintenance organization and assessed both cardiac-related hospitalizations and