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Thyroid and Lipid Metabolism International Journal of Obesity (2000) 24, Suppl 2, S109±S112 ß 2000 Macmillan Publishers Ltd All rights reserved 0307±0565/00 $15.00 www.nature.com/ijo Thyroid and lipid metabolism E Pucci1, L Chiovato1 and A Pinchera1* 1Dipartimento di Endocrinologia e Metabolismo, Ortopedia e Traumatologia, Medicina del lavoro, Sezione di endocrinologia, Universita'di Pisa, Pisa, Italy Thyroid hormones in¯uence 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 speci®c regard to lipid metabolism, thyroid hormones affect synthesis, mobilization and degradation of lipids, although degradation is in¯uenced 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 mobilization of triglycerides stored in adipose tissue; (c) increase in the concentration of non-esteri®ed fatty acids (NEFA); and (d) increase of lipoprotein-lipase activity. While severe hypothyroidism is usually associated with an increased serum concentration of total cholesterol and atherogenic lipoproteins, 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 levels 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 overt and subclinical hypothyroidism, with or without angina, to counteract the cardiovascular risk resulting from hyper-dyslipidemia. International Journal of Obesity (2000) 24, Suppl 2, S109±S112 Keywords: thyroid hormones; lipid metabolism; atherosclerosis; angina pectoris Introduction of thyroid hormone de®ciency. The metabolism of different apolipoproteins is also affected.1 The main Thyroid hormones in¯uence nearly all major meta- and best-known effects of thyroid hormones on lipid bolic pathways. Their most obvious and well-known metabolism are discussed in the present review. action is the increase in basal energy expenditure obtained acting on protein, carbohydrate and lipid metabolism. They do not intervene in the minute-to- minute regulation of metabolism, but rather they Effects of thyroid hormones on lipid modulate the activities of metabolic pathways on a metabolism medium-or long-term basis, either by a direct action or by modifying the activity of other regulatory Lipolysis hormones such as insulin, glucagon and catechol- Thyroid hormones stimulate the utilization of lipid amines. The effect of thyroid hormones on protein substrates owing to an increased mobilization of metabolism is characterized by a profound stimulatory triglycerids stored in adipose tissue. Increased con- effect on the synthesis of numerous cytosolic and centrations and appearance rates of plasma non-ester- mitochondrial proteins as well as of secretory proteins i®ed fatty acids (NEFA) and glycerol re¯ect this such as albumin and hormones. Tracer studies in action. The mechanisms of thyroid hormone action humans have clearly shown that endogenous glucose on lipolysis remain unclear. No direct lipolytic effect production and utilization rates are increased by has been found in vitro. The increased lipolytic rate thyroid hormones. All aspects of lipid metabolism produced by thyroid hormones in vivo might be rela- including synthesis, mobilization and degradation ted to the increased subcutaneous blood-¯ow or to a are in¯uenced by thyroid hormones, but degradation modi®cation of the lipolytic action of catecholamines. is affected more than synthesis. Thyrotoxicosis is associated with a depletion in the stores of most lipids and with a decrease of their concentration in Hepatic fatty acid metabolism plasma. This is true for triglycerides, phospholipids There is a great deal of evidence that, in addition to and cholesterol. Opposite changes occur in conditions stimulating fatty acids oxidation through an increased NEFA availability, thyroid hormones induce lipogen- esis in the liver. The simultaneous stimulation of fatty acid oxidation and synthesis may represent a futile *Correspondence: A Pinchera, Dipartimento di Endocrinologia, Via Paradisa Italy. 2, 56124 Pisa, Italy. cycle of metabolites contributing to the increased E-mail: [email protected] energy expenditure. Hepatic synthesis and output of Thyroid and lipid metabolism E Pucci et al S110 triglycerides are decreased by thyroid hormones. The sometimes with hypertension, which may predispose synthesis and secretion of very-low density lipopro- patients to develop and=or may accelerate CAD. The teins (VLDL) are stimulated. precise mechanism responsible for hypertension in hypothyroidism has not been fully elucidated. Fatty acids utilization by extra-hepatic tissues Changes in circulating catecholamines, their receptors NEFA oxidation is increased by thyroid hormones, as and the renin ± angiotensin ± aldosterone system have well as plasma triglycerides clearance. Lipoprotein ± all been implicated.3 Despite a high prevalence of lipase activity is elevated in hypothyroid subjects. severe atherosclerosis found at autopsy, hypothyroid patients have a relatively low frequency of angina Lipoprotein metabolism pectoris and myocardial infarction during life. The Hypothyroidism is a well-known cause of hyperlipide- question of whether or not hypothyroidism predis- mia in both humans and animals. In hypothyroid patients poses to the development of ischemic heart disease is the most frequent lipid abnormality is hypercholester- a controversial one.4,5 Both conditions are relatively olemia, mainly due to an increased concentration of low common among 40 ± 60 y-old adults in industrialized density lipoproteins (LDL). Elevation of very low nations. Epidemiological studies estimate at 0.5 ± 10% density lipoproteins (VLDL) and high density lipopro- the prevalence of hypothyroidism in the general teins (HDL cholesterol) have also been reported. Plasma adult ± elderly population screened on the basis of triglycerides are increased because of an enhanced elevated serum TSH. Thus, the coexistence of esteri®cation of fatty acids at hepatic level. Plasma hypothyroidism and ischemic heart disease might be concentrations of lipoprotein (a) increase in hypothyr- due only to a statistical coincidence,6 and a real oidism. Lp(a) has been shown to have a thrombogenic cause ± effect relationship between the two conditions and an atherogenic effect. Epidemiological studies indi- might not exist. If this assumption is true, why is it cate that a greater cardiovascular risk is related to that the atherosclerotic lesions observed at autopsy in increased plasma concentrations of Lp(a). This lipopro- hypothyroid patients do not result in clinical CAD? tein favors the deposition of cholesterol in endothelial This discrepancy is most likely explained by the lesions, thus resulting in an increased atherogenic risk. decreased oxygen demand placed on myocardium in While thyrotoxicosis has a profound in¯uence on hypothyroid patients. lipid metabolism, these changes do not produce long- A more direct association between hypothyroidism term sequelae. More relevant from the clinical point of and CAD has been postulated in other studies.7 Inter- view are lipid changes associated with hypothy- estingly, angina pectoris and myocardial infarction roidism. As previously discussed, hyperlipidemia display a rather atypical course in hypothyroid occurs in patients with overt hypothyroidism and patients. This might be explained by the hypometa- might be responsible for an increased risk of coronary bolic state associated with hypothyroidism which is artery disease (CAD). Plasma levels of total cholesterol probably bene®cial to the ischemic myocardium by and LDL cholesterol are increased in patients with lowering energy demands and requirements. Indeed, hypothyroidism. However, it is not usually appreciated hypothyroidism is not associated with an unfavorable that hypothyroid patients also have increased plasma course of acute myocardial infarction. On the other levels of HDL cholesterol. In some studies, both the hand, hypothyroid patients appear to have an total=HDL cholesterol ratio and the HDL=LDL choles- increased incidence of residual ischemia after an terol ratio have been reported to predict the progression acute myocardial infarction, which may be explained of CAD. In other studies only the total=HDL cholesterol by the occurrence of a more severe and diffuse CAD ratio was found to be predictive of CAD. Since both total in hypothyroidism.8 cholesterol and HDL cholesterol levels are increased in patients with hypothyroidism, there is no obvious reason for increased cardiovascular risk. It is interesting to Angina and overt hypothyroidism underline that, as compared with primary hypothy- Despite a large body of data from clinical, epidemio- roidism, the lipid pro®le is more atherogenic in second- logical and autopsy studies linking overt hypothyroid- ary hypothyroidism, because the plasma levels of HDL ism to the development of atherosclerotic heart cholesterol are lower in the latter condition. Further- disease, the precise relationship between the two more, secondary hypothyroidism may coexist with conditions and the pathogenetic
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