Title: Relationship Between Thyroid Stimulating Hormone And Atherosclerotic Lipid Profiles In Postmenopausal Women

Background:Menopause is one of the most important risk factor for atherosclerosis. In the last decades, most of the scholars mainly focused on the changing levels of estrogen and recommend replacement therapy of estrogen. However, so many years later it is proved that even the postmenopausal women were given active hormone replacement therapy, there is still exist a certain remnant incidence of cardiovascular disease, which suggests that there might be any other causes and risk factors in the population of the postmenopausal women.It has been already convinced that hypothyroidism showed a significant positive correlation with atherosclerosis. According to it’s severity, thyroid dysfunction can be divided into clinical hypothyroidism and subclinical hypothyroidism, both situations are all manifested with elevated thyroid stimulating hormone(TSH), the difference is that clinical hypothyroidism characterized by decreased level of serum thyroxine, while the serum thyroxine level is remain in the normal reference range in subclinical hypothyroidism. However, both of the two situations are always accompanied with elevated serum cholesterol. Another study showed that the incidence of hypothyroidism was much higher in postmenopausal women than that of premenopausal population, especially in the elderly aged beyond60rs, among which about90%cases were subclinical hypothyroidism. What’s more, about75%of the subclincal hypothyroidism cases were characterized by a mild elevated TSH level (TSH<10mIU/L), while the other25%cases were characterized by overt elevated TSH level (TSH>10mIU/L). Base on the traditional theory, the mainly cause of the risk of atherosclerosis in clinical hypothyroidism was resulted from decreased thyroid hormones. Therefore, the guidelines for the treatment of hypothyroidism, which was published in JAMA, recommended that clinical hypothyroidism should actively receive thyroxine replacement therapy. For the subclinical hypothyroidism cases also need thyroxine replacement, due to the risk of progression to overt hypothyroidism at a rate about5%annually, especially for those patients with positive thyroid peroxidase antibodies. However, for those75%cases, which only have a mild elevated TSH level, the guidelines recommend dynamic observation rather than hormone replacement. In a word, the majority of the subclinical cases haven’t received active and effective thyroid hormone supplement.Although some studies have reported that thyroid hormone replacement therapy might be beneficial for the subclinical hypothyroidism with a mildly elevated TSH level, of which could ameliorate the development of atherosclerosis significantly. But there are still some controversy, the main focus of the disputation is that the thyroid hormone levels are still in the normal reference range. Recently, Whickham survey updated that the incidence of myocardial infarction in the mildly subclinical hypothyroidism was significantly higher than that of the normal population after20years of follow-up, suggesting that we should pay much more attention to the population of subclinical hypothyroidism with a mildly elevated TSH level.A series of studies in our laboratory have been done previously and confirmed the presence thyroid stimulating hormone receptor (TSHR) on the liver cell surface, by binding of the TSHR, TSH can stimulate the expression of hepatic cholesterol synthesis rate-limiting enzyme (hydroxymethyl glutaryl coenzyme A reductase, HMGCR), then promote hepatic cholesterol synthesis, and eventually elevate the serum level cholesterol. These effects were independent of thyroid hormones. LDLC is the most important form of serum cholesterol in charge of lipids transportation, oxidized low-density lipoprotein (oxidized low density lipoprotein, oxLDL) is the oxidation state of LDLC, both of which can induce a series of reactions and result in the development of atherosclerosis. Especially, oxLDL can stimulate the infiltration of macrophage then transform into foam cells. In addition, oxLDL may further activate platelets and endothelial cells to recruit P-selectin to the cell surface, if the activation continues and aggravated during certain disease situation, which might induce the damage of endothelial and lead to the occurrence of atherosclerosis. Till Ittermann has reported that serum TSH was positively related with the serum levels of oxLDL in subclinical thyroid disease states, including both subclinical hypothyroidism and subclinical hyperthyroidism. However, there is still a lack of direct evidence of TSH on the pathogenesis of atherosclerosis.It is well known that menopausal women are at high risk of hypothyroidism. Based on the previous study the population of postmenopausal women have a significant increase in the risk of therosclerosis accompanied with a higher incidence of subclinical hypothyroidism. The coexistence of the two clinical phenomenon, is it only a coincidence, or there are some causal link? However, there are still lack of detailed reports about thyroid function and blood lipid abnormalities, both in our country and at abroad.Accordingly, this project will be the screen the population of postmenopausal women from our previous epidemiological survey, which has been conducted at the district of Yangguang-shuncheng in Jinan. Then, we will have a overlook of the thyroid abnormalities in this population, analysis the characteristics of lipid profile and it’s relationship with thyroid dysfunction. To explore the risk factors for atherosclerosis induced by the presence of this population. Furthermore, we have assumed that elevated TSH levels in subclinical hypothyroidism groups were involved in the occurrence and development of atherosclerosis. To answer this question, we would screen the newly onset cases of subclinical hypothyroidism for further study. To explore the relationship of thyrotropin, lipid profile, lipid oxidation (oxLDL) and endothelial injury bio-marker (P-selectin). This project would provide a certain theoretical basis and be beneficial for the intervention of subclinical hypothyroidism.Objective:1. To investigate the incidence of thyroid dysfunction and it’s distribution in postmenopausal women, and observe the characteristics of lipid profile according to different thyroid status. To explore the risk factors of dyslipidemia and the impact of thyroid-stimulating hormone on lipid profiles in this population.2To investigate the relationship among TSH, atherogenic lipid profiles and P-selectin in postmenopausal women with subclinical hypothyroidism.Methods:1The study subjects were selected from our epidemiological survey during 2009-2010at the district of Yangguang-shuncheng in Jinan. The postmenopausal women, aged44-80years old, were defined as the inclusion criteria. A doctoral questionnaire was used to record the patient’s general information, including age, height, weight, SBP, DBP and waist circumference. Automatic biochemical analyzer was used to detect the serum levels of TC, TG, LDLC, HDLC, FT3, FT4, TSH, insulin and glucose. According to the diagnostic criteria for thyroid dysfunction, the subjects were divided into three groups:hyperthyroidism, euthyroidism and hypothyroidism. Further stratified by different levels of serum TSH, the subjects were divided into four groups:1st, below0.27mIU/L;2nd,0.27-4.2mIU/L;3rd,4.2-10mIU/L and4th, above10mIU/L, to explore the characteristic of lipid profile among the four subgroups. Third, according to the presence of thyroid related antibodies the subjects were divided into positive and negative antibody groups. Fourth, all the subjects were divided into different types of dyslipidemia:hypercholesterolemia and eucholesterolemia; hypertriglyceridemia and eutriglyceridemia; high-LDLC-lipidemia group and normal-LDLC group. Multiple regression analysis was used to detect the risk factors of dyslipidemia and its relationship with thyroid stimulating hormone.2The second part of this study included a total of45cases of post-menopausal patients with newly onset of subclinical hypothyroidism and27cases as normal control matched by age and BMI. ELISA methods were used to detect the levels of serum oxLDL and P-selectin. Path-analysis was used to explore the relationship between serum TSH and lipid profiles.Results:1.1Thyroid dysfunction is a very common phenomenon with an incidence of16.78%in the population of postmenopausal women, The incidence of clinical hypothyroidism and subclinical hypothyroidism was significantly higher than that of hyperthyroidism (15.86%vs0.92%).1.2General data analysis suggests that the serum levels of FT3, FT4were gradually decreased and the serum level of TC, LDLC levels were gradually increased with the increasing level of serum TSH. Those with positive thyroid-associated antibodies (TPOAb and TGAb) were prone to be manifested with thyroid dysfunction, both hyper and hypothyroidism.1.3After the adjustment of effects of age, BMI, FT3and FT4, the serum level of TSH was positively associated with serum levels of TC and LDLC, through which we can find that the correlation is independent of effects of thyroid hormones.1.4Multiple regression analysis showed that, after excluding the effects of age, BMI, thyroid hormones and other confounding factors, serum cholesterol and LDLC levels were still increased linearly with elevated serum TSH, of which the serum levels of TC and LDLC will be increased by0.052mmol/L and0.042mmol/L individually if TSH increased by1mIU/L.1.5The population of postmenopausal women showed an high prevalence of dyslipidemia, including hypercholesterolemia by the incidence of23.07%, hyper-triglyceridemia of14.42%, and high-LDLC-lipidemia of13.63%Subgroup analysis showed that the incidence of hypercholesterolemia and high-LDLC-lipidemia was increased in a linear manner with increasing levels of TSH (p<0.05).2.1There weren’t significant difference among the indices of BMI, SBP, DBP between the post-menopausal patients with newly onset of subclinical hypothyroidism and healthy controls. Subclinical hypothyroidism group was characterized by elevated TSH levels, while the serum levels of FT3and FT4were in accordance with that of controls. Compared with healthy controls, the serum levels of TC, TG, LDLC, oxLDL and P-selectin in SCH were increased by approximately23.04,29.69,30.55,23.19and21.01%, respectively.2.2In the entire population, serum level of TSH was positively related with serum TC, LDLC, oxLDL and P-selectin (r=0.471,0.435,0.314and0.390, p<0.01). After adjusting for age and BMI, serum level of TSH, TG, LDLC, oxLDL and P-selectin still showed positive correlations, which suggested that this effects were independent of FT3and FT4.2.3The Pillai’s Trace test was used to assess the lipid profiles in each individual and revealed a statistically significant increase in the lipid profiles among different subgroups stratified by TSH levels (F value=4.980,p=0.01). With the increase of serum level of TSH, lipid profiles in each group showed a significant proatherogenic changes, including elevated serum levels of TC, LDLC and oxLDL. Each component of lipid profiles in subclinical hypothyroidism with mildly elevated TSH was about21.52,30.18and22.98%, respectively, while in the subclinical hypothyroidism with overtly elevated TSH level was about26.37,30.91and23.79%, respectively. With the increase of serum level of TSH, the serum P-selectin level was also increased significantly in patients with subclinical hypothyroidism women.2.4After adjustment of age and BMI, the serum level of TSH was positively correlated with TG, TC, LDLC, oxLDL and P-selectin in the population postmenopausal women with subclinical hypothyroidism (p<0.05). Multiple regression analysis showed that the population of serum P-selectin and TSH, TC, LDLC, oxLDL and TG levels were positively correlated (p<0.05).2.5Path analysis revealed that the total effects of TSH on TC (total effects TC, TSH=0.4323) included a significant direct effect (direct effect TC, TSH=0.4932) and an indirect effect via an intermediary variable (FT3, FT4). Furthermore, TC exhibited a direct effect on LDLC (0.9095, p<0.05), as did LDLC on oxLDL (0.5858,p<0.05).Conclusion:1. The incidence of thyroid dysfunction in postmenopausal women was positively associated with the thyroid-related antibody-TPOAb and TGAb, meanwhile accompanied by a significant increased incidence of dyslipidemia, in which the serum level of TSH was significantly correlated with serum levels of TC, LDLC, especially, this effect is independent with that of FT3and FT4.2. Postmenopausal women showed a higher incidence of hypercholesterolemia and high-LDLC-lipidemia in accordance with the elevated TSH level. Therefore, it is very important to maintain serum TSH at an appropriate level in postmenopausal women, which might be beneficial to control the lipid profiles and thus delay the occurrence and development of atherosclerosis. 3. TSH was positively correlated with TC, LDLC, oxLDL and P-selectin in postmenopausal women with newly diagnosed subclinical hypothyroidism. In particular, the positive correlation remained significant after adjusting for serum FT3and FT4.4. The effects of serum TSH on the lipid profiles component of TC consists two aspects:the direct effects and indirect effects through FT3and FT4.5. Even with a mild elevation of serum TSH, subclinical hypothyroidism is associated with atherogenic lipid profiles and elevated serum P-selectin in postmenopausal women independent of thyroid hormones...