**11. Subclinical hyperthyroidism and heart**

The impact of thyroid hormones especially the subclinical hyperthyroidism on cardiovascular system faces an ever-increasing interest in last decades. Subclinical hyperthyroidism represents low serum TSH levels accompanying with normal serum free T4 and T3 concentration and its prevalence varies from 0.6–16% [109]. Persistent abnormal TSH measurements have to be re-examined within 2–3 months from initial values [109, 110]. There are two main categories regarding subclinical hyperthyroidism: Grade 1 defines a mildly low serum TSH (0.1–0.45 mIU/L) levels whereas Grade 2 represent lower levels of serum TSH (< 0.1 mIU/L). Subclinical hyperthyroidism may have exogenous or endogenous etiology. Exogenous origin mainly occurs as a result of a TSH suppressive therapy (or excessive use of levothyroxine) for thyroid carcinoma. Endogenous reasons resembles overt hyperthyroid state causes including mild Graves' Disease, multinodular goiter, and autonomous functioning thyroid nodule. Various authors have evaluated the effects of subclinical hyperthyroidism on cardiovascular and skeletal systems, particularly in older populations. Nanchen et al. analyzed conducted a study with a large cohort composed of patients with subclinical hyperthyroidism and observed a significantly higher hospitalizations rate due to heart failure in older patients, especially in those with grade 2 subclinical hyperthyroidism [111]. A distinct inverse proportion between TSH level and the risk of AF have been concluded in previous studies [112]. Furthermore, current retrospective studies with large cohorts have obviously determined a link between subclinical hyperthyroidism and cardiovascular events especially heart failure. More importantly, all-cause mortality observed higher in this group [113]. The European Thyroid Association recommends full treatment in patients older than 65 years with grade 2 subclinical hyperthyroidism. Beyond that, same guideline suggests to treat milder grade if any additional heart disease or other significant comorbidities or risk factors present [110].
