Thyroid and lipid metabolism

Thyroid hormones influence all major metabolic pathways. Their most obvious and well-known action is an increase in basal energy expenditure obtained acting on protein, carbohydrate and lipid metabolism. With specific regard to lipid metabolism, thyroid hormones affect synthesis, mobilization and de gradation of lipids, although degradation is influenced more than synthesis . The main and best-known effects on lipid metabolism include: (a) enhanced utilization of lipid substrates; (b) increase in the synthesis and mobiliz ation of triglycerides stored in adipose tissue; (c) increase in the concen tration of non-esterified fatty acids (NEFA); and (d) increase of lipoprote in-lipase activity. While severe hypothyroidism is usually associated with an increased serum concentration of total cholesterol and atherogenic lipop roteins, the occurrence of acute myocardial infarction (AMI) in hypothyroid patients is not frequent, However, hypothyroid patients appear to have an increased incidence of residual myocardial ischemia following AMI. Even in subclinical hypothyroidism, which is characterized by raised serum TSH leve ls with normal serum thyroid hormone concentrations, mild hyperlipidemia is present and may contribute to an increased risk of atherogenesis. Prudent substitution therapy with L-thyroxine is indicated in patients with both ov ert and subclinical hypothyroidism, with or without angina, to counteract t he cardiovascular risk resulting from hyper-dyslipidemia.