Correlation Between Serum Levels of Cholesterol and Homocysteine with Oxidative Stress in Hypothyroid Patients
Introduction: A bnormaJities in cholesterol and homo cysteine metabolism have been reported in thyroid diseases. Since elevated lev els of both parameters are involved in atherogenesis, and thyroid hormones are modu lators of oxidative stress. In this study, the corre lation between serum levels of cholesterol, and homocysteine, and oxidative stress was assessed in patients with thyroid dysfunction.
Materials and Methods: A total of 60 patients with thyroid dysfunction (30 with hypothyroid ism and 30 with hyperthyroidism) were included in this study. Thirty apparently healthy sex and age-matched individuals were :;elected as control group. The mean age of hypothyroid, hyperthy roid and control groups were 43±7.7, 39±12 and 40±7.9 years, respectively. Serum levels of homo cysteine were measured by HPLC and those of thyroid hormones (T3, T", T3 R. uptake and TSH) by radioimmunoassay techniques. Levels of total antioxidant capacity, lipid profiles and creatinine were determined by standard methods in Cobas Mira Autoanalyzer.
Results: The mean±SD levels of homocysteine in hyperthyroid, hypothyroid and control groups were 7.79±1.44, 17.09±6.93 and 8.08±1.92 pmol/L, respectively. Comparing with control group sig nificant elevation was noted in hypothyroid pa tients (p=O.OOOl). Significant correlation between serum levels of creatinine and that of homocysteine was observed (r=0.86, p= 0.0001). Singnifi cant elevation in the levels of total cholesterol and LDL-C were observed in hypothyroid pa tients (p<0.05). Significant reduction in the se rum antioxidant capacity was found in patients suffering from hypothyroidism (p=O.Ol). But not in hyperthyroid subjects. Significant inverse cor relation was observed between serum levels of antioxidant capacity and those of homocysteine (r=-O.79, p=0.02), total cholesterol (r=-O.93, p= 0.02) and LDL-C (r=-0.83, p=O.OO1) in hypothyroid pa tients. This correlations were not significant in the hyperthyroid and control groups (p > 0.05).
Conclusions: The correlation between serum lev els of homocysteine, total cholesterol and LDL-C with total antioxidant capacity in hypothyroid ism suggests that there is an overproduction of free radicals in these patients. It is concluded that the enhanced production of free radicals might be an important contributing factor in ab normalities seen in homocysteine and choles terol metabolism.
Keywords: Hyperhomocysteinemia; Oxidative stress; Thyroid dysfunction; Serum lipids; Anti-oxidant capacity
High plasma homocysteine concentration induces pathologic changes in the arterial wall and thus is strongly associated with an increased risk of atherosclerosis, manifested as cardiovascular, cerebrovascular and ripheral vascular events. I There are consistent reports that patients with hypothyroidism have elevated total homocysteine in plasma and that homocysteine level is reduced following therapy with thyroxine.2 High serum cholesterol in hypothyroidism and its low concentration in hyperthyroidism are common findings. A significant correlation between serum cholesterol and total homocysteine has been demonstrated.3 The mechanism behind this correlation has not been clarified, but increases in both cholesterol and total homocysteine levels in hypothyroidism may have an interactive effect, which may contribute to the high prevalence of arterial occlusive disease in hypothyroidism.4
Thyroid hormones are physiologic modulators ofboth tissue oxidative stress and protein degradation.s The mechanism linking hypothyroidism with oxidative stress is unknown. Oxidative stress increases the concentration of oxidized LDL, a risk factor for atherosclerosis. Homocysteine is also an inducer of LDL oxidation. A strong covariation between total plasma homocysteine and cholesterol in hypothyroidism may have important medical implications. Detennination of TSH in subjects with unexplained hyperhomocysteinemia and high plasma cholesterol has been recommended.6•7 The aim of the present study is to investigate the correlation between serum levels of cholesterol and homocysteine with that of oxidative stress in patients with thyroid dysfunction.
Materials and Methods
Thirty hypothyroid patients (13 males and 17 females), 30 hyperthyroid subjects (11 males and 19 females) and 30 healthy adults were enrolled in this study. Subjects in hypo and hyperthyroid groups were sampled from patients who were seen in endocrine clinics. Individuals in the control group were from patients with solitary thyroid nodule, who were euthyroid clinically, had normal TFTs, and were not on thyroid hormone preparations. Hyperthyroidism was diagnosed by high free T4 index and suppressed serum TSH leveL Subjects with low free T4 index and elevated serum TSH were diagnosed as having hypothyroidism.
EDT A-blood samples for plasma homocysteine and total antioxidant capacity were provided after an overnight fasting. Samples were placed on ice, centrifuged within 1 hour, and separated plasma was stored at 70°C before assay. Additional fasting samples were collected for total cholesterol, high density lipoprotein-cholesterol (HDL-C), low density lipoprotein-cholesterol (LDL-C), creatinine, thyroid hormones, T3R uptake and TSH.
Plasma concentration of homocysteine was measured by high performance liquid chromatography after reduction of plasma disulfides with tris (2-carboxyethyl) phosphine, precipitation of proteins with trichloroacetic acid, derivatization with 7 -floro-2, 1, 3benzoxadiazol-4-sulfonate (SBD-F), and fluorescent detection.8 Thyroid hormones, TSH and T3 uptake were measured by standard radioimmunoassay techniques. Serum total cholesterol, HDL-C and LDL-C were measured with standard enzymatic methods in the Cobas Mira Autoanalyzer. The concentrations of creatinine in serum were determined in the same Auto analyzer using Jaffe reaction. Total antioxidant capacities in plasma samples were assessed using the Randox total antioxidant status kit.
SPSS 11 for Windows computer program was used to perform statistical analysis. Paired student's t-test was employed to determine the significance of differences between the measured parameters of hypothyroid, hyperthyroid and control groups. Compari sons between different groups were made using ANOV A and all correlations were evaluated by linear regression. Results are expressed as mean±SD and statistical significance was set at p<0.05.
The mean age of the hypothyroid, hyperthyroid, and control groups were 43±7.7, 39± 12, and 40±7.9 years, respectively. Clinical and laboratory characteristics of hypothyroid, hyperthyroid and control groups are shown in Table 1. Significant differences were noticed between the levels of T), T4, TSH and Tr uptake in the three groups (p<0.05). The mean±SD levels of measured parameters, including serum total homocysteine, creatinine, total cholesterol, LDL-C, HDL-C and total antioxidant capacity in hypothyroid and hyperthyroid subjects, were compared with those of control group (Tables 2 and 3, respectively). Significant elevation in the serum levels of total homocysteine, creatinine, total cholesterol and LDL-C and marked reduction in plasma concentrations of total antioxidant capacity were found in the hypothyroid group (p<0.05), but the changes in the serum levels of HDL-C were not significant. In the hyperthyroid group, the reductions in the levels of total cholesterol, creatinine and total antioxidant capacity were not significant and no marked changes were noticed in the levels of the other parameters. The correlations between the biochemical factors in hypothyroid, hyperthyroid and control groups are summarized in Table 4. Inverse significant correlation between the levels of total homocysteine and total antioxidant capacity was noticed in the hypothyroid group. The correlation between the levels of creatinine and those of total homocysteine was only marked in the case of the hypothyroid group. An inverse correlation between increased levels of total cholesterol and LDL-C and those of total antioxidant capacity was detected in the hypothyroid group but it was not significant in hyperthyroid and control groups. No meaningful significant correlation was noticed between total antioxidant capacity and HDL-C in subjects of the patients and control groups.
The mean age of the hypothyroid, hyperthyroid, and control groups were 43±7.7, 39± 12, and 40±7.9 years, respectively. Clinical and laboratory characteristics of hypothyroid, hyperthyroid and control groups are shown in Table 1. Significant differences were noticed between the levels of T), T4, TSH and Tr uptake in the three groups (p
Elevated plasma levels of the amino acid homocysteine has been identified as an independent risk factor for atherosclerosis.9 The mechanisms by which hyperhomocysteinemia causes atherosclerosis are not completely understood. Animal models of hyperhomocysteinemia have shown altered vascular function, including the promotion of smooth . muscle cell growth and the development of atherosclerosis. Diffuse arterial damage and an increased propensity to thrombus formation are commonly noted in people with homocysteinuria. Concomitant lipoprotein abnormalities, including increased oxidation and binding, may also be mechanisms by normalities, including increased oxidation and binding, may also be mechanisms by which hyperhomocysteinemia promotes atherosclerotic process. 10
There are consistent reports that patients with hypothyroidism have elevated total homocysteine in plasma and that total homocysteine is reduced following therapy with thyroid hormones. 11.12 Hypothyroidism is associated with increased cardiovascular morbidity, which cannot be fully explained by the atherogenic lipid profile observed. Other abnormalities have also been suggested to be responsible for the increased cardiovascular morbidity in hypothyroid patients.13,14 Homocysteine causes LDL oxidation probably through superoxide anion formation in the auto-oxidation process of the amino acid.15 Homocysteine thiolactone also acetylates extracellular proteins such as apoB and LDL.16 Since substantial evidence indicates that oxidized low-density lipoprotein contributes to atherogenesis through a number of mechanisms,17 the relationship between oxidative stress and the levels of cholesterol and homocysteine was evaluated in this study.
In the present work, we observed significantly higher levels of homocysteine in hypothyroid patients in comparison with thyrotoxic individuals which is in agreement with previously reported data. '8 Another important finding was the association of changes in serum levels of creatinine and cholesterol with concentrations of homocysteine in hypothyroidism. Renal function is a well-known determinant of plasma homocysteine level. Altered thyroid function not only leads to changes in glomerular filtration rate but also to changes in body weight and body composition. An alterative explanation for the concurrent elevation of plasma homocysteine and serum creatinine in hypothyroidism is the formation of homocysteine in conjunction with creatine synthesis, which is related with creatine synthesis, which is related to muscle mass. 19
We observed high serum cholesterol and LDL-C in hypothyroid and low concentration in hyperthyroid patients, which is in agreement with other reports.20,21 Significant correlation was noticed between the levels of homocysteine and those of cholesterol and LDL-C in the hypothyroid group. A marked correlation between serum cholesterol and homocysteine has also been demonstrated in some epidemiological studies.22 Although the mechanism behind this covariation has not been clarified,23 increases in both cholesterol and homocysteine in hypothyroid subjects may have an interactive effect,24 which may contribute to the high prevalence of arterial occlusive diseases in hypothyroid patients.25 On the other hand, low levels of both factors in hyperthyroid patients may be protective.26
Oxidative stress induced by homocysteine is reflected by an increase in malondialdehyde (MDA), a measure of membrane lipid peroxidation, and a decrease in plasma antioxidant capacity eight hours after methionine loading in healthy subjects?? Homocysteine may induce intracellular inactivation of the glutathione (GSH) antioxidant defense system, and possibly reduces homocysteine derived GSH synthesis.28 Excess or deficiency of the thyroid hormones cause alteration in MDA levels and glutathione peroxidase activities of tissues.29 In the present study, the level of total antioxidant capacity and correlation with the levels of cholesterol and the other measured parameters were evaluated in thyroid dysfunction. A reverse and significant correlation was observed between the level of total antioxidant capacity and the concentration of homocysteine in the hypothyroid group but the correlation was not significant in the case of the hyperthyroid group. Similar results have been reported in other studies.30
In conclusion, homocysteine levels were increased and plasma levels of total antioxidant capacity were decreased significantly in hypothyroidism. A strong covariation between homocysteine and cholesterol and their correlation with the levels of total antioxidant capacity in hypothyroidism may increase cardiovascular risk. Determination of serum levels of thyroid hormones is recommended in subjects with unexplained hyperhomocysteinemia and hypercholesterolemia.
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