Low normal free T4 confers decreased high-density lipoprotein antioxidative functionality in the context of hyperglycaemia

Epidemiology of metabolic syndrome and its components in Chinese patients with a range of thyroid-stimulating hormone concentrations

Objective

To investigate the relationship between thyroid stimulating hormone (TSH) concentration and the risks of developing metabolic syndrome and its components.

Methods

A total of 10,140 residents of the Yunyan district of Guiyang (Guizhou, China) who were ≥40 years old were selected by cluster random sampling between May and August 2011, of whom 5692 were eligible. TSH concentration and indices of metabolic syndrome were documented at baseline and 3 years later. Participants were allocated to a euthyroid (TSH 0.55–4.78 mIU/L) or high TSH concentration (TSH >4.78 mIU/L) group. Patients with overt hypothyroidism or were undergoing treatment for hypothyroidism were excluded.

Results

The crude and adjusted prevalences of metabolic syndrome were 39.9% and 33.9% in the euthyroid group and 44.3% and 37.5% in the high TSH group, respectively. Binary logistic regression analysis revealed a positive correlation between a high TSH concentration at baseline and the cumulative incidence of metabolic syndrome during follow up.

Conclusions

High TSH is associated with a higher risk of developing metabolic syndrome or one of its components; therefore, people with a high TSH concentration should be screened regularly to permit the early identification of metabolic syndrome and followed up thoroughly.

HDL Cholesterol Efflux Capacity is Impaired in Severe Short-Term Hypothyroidism Despite Increased HDL Cholesterol

CONTEXT

Severe hypothyroidism has profound effects on lipoprotein metabolism including high-density lipoprotein (HDL) cholesterol elevations but effects on HDL function metrics are unknown.

OBJECTIVE

To determine the impact of severe short-term hypothyroidism on HDL particle characteristics, HDL cholesterol efflux capacity (CEC), and HDL antioxidative capacity.

DESIGN

Observational study with variables measured during severe short-term hypothyroidism (median TSH 81 mU/L) and after 20 weeks of thyroid hormone supplementation (median TSH 0.03 mU/L) (Netherlands Trial Registry ID 7228).

SETTING

University hospital setting in The Netherlands.

PATIENTS

Seventeen patients who had undergone a total thyroidectomy for differentiated thyroid carcinoma.

MAIN OUTCOME MEASURES

HDL particle characteristics (nuclear magnetic resonance spectrometry), CEC (human THP-1-derived macrophage foam cells and apolipoprotein B-depleted plasma), and HDL anti-oxidative capacity (inhibition of low-density lipoprotein oxidation).

RESULTS

During hypothyroidism plasma total cholesterol, HDL cholesterol and apolipoprotein A-I were increased (P ≤ 0.001). HDL particle concentration was unchanged, but there was a shift in HDL subclasses toward larger HDL particles (P < 0.001). CEC was decreased (P = 0.035), also when corrected for HDL cholesterol (P < 0.001) or HDL particle concentration (P = 0.011). HDL antioxidative capacity did not change.

CONCLUSION

During severe short-term hypothyroidism CEC, an important antiatherogenic metric of HDL function, is impaired. HDL cholesterol and larger HDL particles are increased but HDL particle concentration is unchanged. Combined, these findings suggest that HDL quality and quantity are not improved, reflecting dysfunctional HDL in hypothyroidism.

Serum paraoxonase-1 activity is inversely related to free thyroxine in euthyroid subjects: The PREVEND Cohort Study

BACKGROUND

Low-normal thyroid function within the euthyroid range has been suggested to enhance atherosclerosis susceptibility. Paraoxonase-1 (PON-1) may protect against atherosclerotic cardiovascular disease development by attenuating oxidative stress. We evaluated relationships of PON-1 with thyroid stimulating hormone (TSH), free T₄ , free T₃ , lipids and apolipoprotein (apo)A-I in euthyroid subjects, and assessed whether such relationships are modified in the context of the metabolic syndrome (MetS).

MATERIALS AND METHODS

Serum PON-1 activity (arylesterase activity), TSH, free T₄ , free T₃ , lipids and apoA-I was measured in 2206 euthyroid subjects (aged 28-75 years; 1138 men (age 49 ± 13 years) and 1068 women (age 46 ± 12 years), recruited from the general population (PREVEND cohort).

RESULTS

In age- and sex-adjusted analysis, PON-1 activity (divided into tertiles) was positively related to TSH (β = -0.045, P = .036) and inversely to free T₄ (β = -0.042, P = .050) but not to free T₃ (β = -0.027, P = .20). PON-1 activity was positively related to total cholesterol, non-HDL cholesterol and triglycerides, as well as to HDL cholesterol and apoA-I (P < .01 to <.001). The inverse relationship of PON-1 activity with free T₄ remained present after adjustment for lipids and other potential confounders (β = -0.066, P = .002), but the positive relationship with TSH lost significance (β = 0.034, P = .11). The inverse relationship of PON-1 activity with free T₄ was not different in subjects with vs without MetS (P = .94), nor modified by the presence of its individual components (P ≥ .22 for each).

CONCLUSIONS

Serum PON-1 activity is inversely associated with free T₄ in euthyroid subjects, suggesting that low-normal thyroid function may affect PON-1 regulation.

Increased leptin/adiponectin ratio relates to low-normal thyroid function in metabolic syndrome

Background

Low-normal thyroid function within the euthyroid range may contribute to increased atherosclerosis susceptibility. The leptin/adiponectin (L/A) ratio is associated with cardiovascular disease and reflects adipose tissue dysfunction. Relationships of the L/A ratio with low-normal thyroid function are unknown.

Methods

Relationships of thyroid stimulating hormone (TSH) and free thyroxine (free T₄) with leptin, adiponectin and the L/A ratio in euthyroid subjects were documented in 67 fasting subjects with metabolic syndrome (Mets) and 86 euthyroid subjects without MetS (TSH and free T₄ levels within the institutional reference range).

Results

Neither plasma leptin nor adiponectin was significantly correlated with TSH or free T₄ in subjects with and without MetS. In the whole group, high sensitivity C-reactive protein (hs-CRP) was positively correlated with the L/A ratio (r = 0.485, P < 0.001). Notably, the L/A ratio was positively correlated with TSH in subjects with MetS (r = 0.252, P = 0.040) but not in subjects without MetS (r = −0.068, P = 0.54; interaction term, P = 0.027). In MetS subjects, the L/A ratio remained positively related with TSH after adjustment for age, sex, diabetes status, hs-CRP and the use of antihypertensive and glucose lowering medication (β = 0.283, P = 0.018), as well as after adjustment for individual MetS components (β = 0.294, P = 0.020).

Conclusions

In the context of MetS, a higher TSH within the euthyroid range confers an increased L/A ratio, a proposed marker of atherosclerosis susceptibility and adipocyte dysfunction.

Impaired HDL cholesterol efflux in metabolic syndrome is unrelated to glucose tolerance status: the CODAM study

Type 2 diabetes mellitus (T2DM) and metabolic syndrome (MetS) increase atherosclerotic cardiovascular disease risk. Cholesterol efflux capacity (CEC) is a key metric of the anti-atherosclerotic functionality of high-density lipoproteins (HDL). The present study aimed to delineate if T2DM and MetS cross-sectionally associate with altered CEC in a large high cardiometabolic risk population. CEC was determined from THP-1 macrophage foam cells towards apolipoprotein B-depleted plasma from 552 subjects of the CODAM cohort (288 controls, 126 impaired glucose metabolism [IGM], 138 T2DM). MetS was present in 297 participants. CEC was not different between different glucose tolerance categories but was lower in MetS (P < 0.001), at least partly attributable to lower HDL cholesterol (HDL-C) and apoA-I levels (P < 0.001 for each). Low grade inflammation was increased in IGM, T2DM and MetS as determined by a score comprising 8 different biomarkers (P < 0.05-< 0.001; n = 547). CEC inversely associated with low-grade inflammation taking account of HDL-C or apoA-I in MetS (P < 0.02), but not in subjects without MetS (interaction: P = 0.015). This study demonstrates that IGM and T2DM do not impact the HDL CEC function, while efflux is lower in MetS, partly dependent on plasma HDL-C levels. Enhanced low-grade inflammation in MetS may conceivably impair CEC even independent of HDL-C and apoA-I.

Pre-β-HDL formation relates to high-normal free thyroxine in type 2 diabetes mellitus

OBJECTIVES

Low-normal thyroid function within the euthyroid range may influence plasma lipoprotein levels. Associations between variation in thyroid function and pre-β-high density lipoproteins (pre-β-HDL), i.e. lipid-poor or lipid free HDL particles that act as initial acceptor of cell-derived cholesterol, are unknown. We determined relationships of plasma pre-β-HDL with thyroid function in euthyroid subjects with and without type 2 diabetes mellitus (T2DM).

DESIGN AND SUBJECTS

TSH, free T4, plasma (apo)lipoproteins, pre-β-HDL, pre-β-HDL formation (pre-β-HDL generation during incubation with lecithin:cholesterol acyltransferase being inhibited) and phospholipid transfer protein (PLTP) activity were measured in fasting plasma from 72 T2DM and 82 non-diabetic subjects.

RESULTS

TSH was similar and free T4 was slightly higher (P < 0.05) in T2DM vs. non-diabetic subjects. HDL cholesterol and apoA-I were lower, whereas pre-β-HDL (expressed as % of apoA-I), triglycerides and PLTP activity were higher in T2DM (P < 0.05 to P < 0.001). In T2DM, pre-β-HDL formation (in apoA-I concentration and in % of apoA-I) was positively related to free T4, PLTP activity, total cholesterol and triglycerides (P < 0.05 for each). Multivariable linear regression analyses, adjusted for age, sex, PLTP activity, total cholesterol and triglycerides, demonstrated that pre-β-HDL formation was positively related to free T4 (in apoA-I concentration: β = 0.278, P = 0.014; in % of apoA-I: β = 0.343, P = 0.003) in T2DM, but not in non-diabetic subjects (both P > 0.30; interaction terms: both P < 0.05).

CONCLUSIONS

Variations in thyroid function within the euthyroid range may influence the metabolism of pre-β-HDL in T2DM.

Low-normal thyroid function and the pathogenesis of common cardio-metabolic disorders

BACKGROUND

Subclinical hypothyroidism may adversely affect the development of cardiovascular disease (CVD). Less is known about the role of low-normal thyroid function, that is higher thyroid-stimulating hormone and/or lower free thyroxine levels within the euthyroid reference range, in the development of cardio-metabolic disorders. This review is focused on the relationship of low-normal thyroid function with CVD, plasma lipids and lipoprotein function, as well as with metabolic syndrome (MetS), chronic kidney disease (CKD) and nonalcoholic fatty liver disease (NAFLD).

MATERIALS AND METHODS

This narrative review, which includes results from previously published systematic reviews and meta-analyses, is based on clinical and basic research papers, obtained via MEDLINE and PubMed up to November 2014.

RESULTS

Low-normal thyroid function could adversely affect the development of (subclinical) atherosclerotic manifestations. It is likely that low-normal thyroid function relates to modest increases in plasma total cholesterol, LDL cholesterol and triglycerides, and may convey pro-atherogenic changes in lipoprotein metabolism and in HDL function. Most available data support the concept that low-normal thyroid function is associated with MetS, insulin resistance and CKD, but not with high blood pressure. Inconsistent effects of low-normal thyroid function on NAFLD have been reported so far.

CONCLUSIONS

Observational studies suggest that low-normal thyroid function may be implicated in the pathogenesis of CVD. Low-normal thyroid function could also play a role in the development of MetS, insulin resistance and CKD, but the relationship with NAFLD is uncertain. The extent to which low-normal thyroid function prospectively predicts cardio-metabolic disorders has been insufficiently established so far.

Low-normal thyroid function and novel cardiometabolic biomarkers

The concept is emerging that low-normal thyroid function, i.e., either higher thyroid-stimulating hormone or lower free thyroxine levels within the euthyroid reference range, could contribute to the development of atherosclerotic cardiovascular disease. It is possible that adverse effects of low-normal thyroid function on cardiovascular outcome may be particularly relevant for specific populations, such as younger people and subjects with high cardiovascular risk. Low-normal thyroid function probably relates to modest increases in plasma total cholesterol, low density lipoprotein cholesterol, triglycerides and insulin resistance, but effects on high density lipoprotein (HDL) cholesterol and non-alcoholic fatty liver disease are inconsistent. Low-normal thyroid function may enhance plasma cholesteryl ester transfer, and contribute to an impaired ability of HDL to inhibit oxidative modification of LDL, reflecting pro-atherogenic alterations in lipoprotein metabolism and HDL function, respectively. Low-normal thyroid function also confers lower levels of bilirubin, a strong natural anti-oxidant. Remarkably, all these effects of low-normal thyroid functional status appear to be more outspoken in the context of chronic hyperglycemia and/or insulin resistance. Collectively, these data support the concept that low-normal thyroid function may adversely affect several processes which conceivably contribute to the pathogenesis of atherosclerotic cardiovascular disease, beyond effects on conventional lipoprotein measures.

The relationship of the anti-oxidant bilirubin with free thyroxine is modified by insulin resistance in euthyroid subjects

Background

The strong anti-oxidative properties of bilirubin largely explain its cardioprotective effects. Insulin resistance is featured by low circulating bilirubin. Thyroid hormone affects both bilirubin generation and its biliary transport, but it is unknown whether circulating bilirubin is associated with thyroid function in euthyroid subjects. Aim is to determine relationships of bilirubin with TSH, free T₄ and free T₃ in euthyroid subjects without type 2 diabetes mellitus (T2DM), and to assess whether such a relationship would be modified by the degree of insulin resistance.

Methods

Total bilirubin, TSH, free T₄, free T₃, glucose, insulin, lipids and transaminases were measured in 1854 fasting euthyroid subjects without T2DM, recruited from the general population (PREVEND cohort). Insulin resistance was assessed by homeostasis model assessment.

Results

Bilirubin was positively related to free T₄ (β = 0.116, P<0.001) and free T₃ (β = 0.078, P = 0.001), but bilirubin was unrelated to TSH. The relationship of bilirubin with free T₄ was modified by insulin resistance with a larger effect in more insulin resistant individuals (adjusted for age and sex: β = 0.043, P = 0.056 for interaction; additionally adjusted for smoking, alcohol intake, transaminases and total cholesterol (β = 0.044, P = 0.044 for interaction). The association of bilirubin with free T₄ was also modified by high density lipoprotein cholesterol (age- and sex-adjusted: β = 0.040, P = 0.072).

Conclusions

Low bilirubin relates to low free T₄ in euthyroid non-diabetic subjects. Low normal free T₄ may particularly confer low bilirubin in more insulin resistant individuals.

Body Mass Index Is Associated with Hypercholesterolemia following Thyroid Hormone Withdrawal in Thyroidectomized Patients

Thyroid hormone withdrawal (THW) for postoperative radioiodine adjuvant therapy or diagnostic radioiodine whole body scan in patients with differentiated thyroid cancers results in acute thyroid hormone deficiency and abnormal lipid profiles. To better clarify the clinical pattern of dyslipidemia occurring after THW, we retrospectively analyzed the association between serum total cholesterol level after THW and various clinical factors in a total of 61 patients who underwent total thyroidectomy due to papillary thyroid cancers from January 2010 to March 2012, in Severance Hospital, Seoul, Korea. Preoperative baseline total cholesterol was significantly correlated with post-THW total cholesterol level; however, age, gender, or elevated TSH level after THW itself was not correlated with post-THW total cholesterol level. A significant correlation between preoperative measured BMI and post-THW total cholesterol level was found (r = 0.263, P = 0.041). In multiple logistic analysis, BMI was an independent determining factor of post-THW total cholesterol level (P = 0.012).