**10. Thyroid hormone effects on lipid metabolism**

Increased serum lipid levels in hypothyroidism is well-known. Hypothyroidism causes hypercholesterolemia especially augments low-density lipoproteins (LDL) and apolipoprotein B. Although, estimated prevalence of overt hypothyroidism in patients with hypercholesterolemia is between 1.3% to 2.8%, hypercholesterolemia is observed in 90% of the patients with hypothyroidism [104]. Altered lipid profile levels manifest even in subclinical hypothyroidism. Some authors revealed reversible increased LDL levels in subclinical hypothyroidism after thyroid hormone replacement. On the contrary, some other studies have determined no changes in LDL levels despite increased total cholesterol levels in subclinical hypothyroidism [105]. The underlying mechanisms of the hypercholesterolemia in hypothyroidism involves impaired fractional clearance of LDL by decreased LDL receptors and receptor activity in the liver [106]. Cholesterol catabolism is regulated by cholesterol 7α-hydroxylase [107]. This liver-origin enzyme has a negative correlation with T3 and may reduce catabolism and cause elevated serum cholesterol levels associated with hypothyroidism [103]. The increased serum lipid levels in subclinical hypothyroidism are strongly associated with increased cardiovascular risk [108]. This high risk may be reversed by thyroid hormone replacement therapy to establish euthyroid condition [106]. If left untreated, the dyslipidemia accompanying with the diastolic hypertension associated with hypothyroidism lead the patient prone to atherosclerosis [53, 86]**.**
