Thyroid hormone (fT4) reduces lipoprotein(a) plasma levels

Serum Adiponectin Levels and Changes in Glucose Metabolism before and after Treatment for Thyroid Dysfunction

OBJECTIVE

Adiponectin is an adipokine which is known to decrease in individuals associated with obesity and insulin resistance. In this study, we aimed to investigate the serum adiponectin levels and glucose metabolism in patients with thyroid dysfunction before and after treatment.

METHODS

Newly diagnosed overt hypothyroid (n=20) and thyrotoxic (n=23) patients and healthy controls (n=20) with a body mass index of <30 kg/m(2) were evaluated prospectively. Patients with a known state of insulin resistance, including prediabetes and overt diabetes, and individuals with chronic diseases were excluded. Thyroid function and fasting plasma glucose (FPG), insulin, homeostatic model assessment (HOMA) insulin resistance (HOMA-IR) and HOMA-beta cell function (HOMA-beta), lipid and adiponectin levels were investigated in the basal state and after the restoration of euthyroidism.

RESULTS

The basal fasting FPG levels were lower in the hypothyroid patients than the control subjects (p=0.02) and similar between the thyrotoxic patients and control subjects (p=0.127). The basal HOMA-beta levels were higher in the patients with hypothyroidism than in those with thyrotoxicosis (p=0.015). Following the restoration of euthyroidism, the FPG levels significantly increased in the hypothyroid patients (p=0.002) and decreased in the thyrotoxic (p=0.001) patients. The basal plasma adiponectin levels were 14.55±8.4 mcg/mL, 13.79±9.13 mcg/mL and 11.68±6.0 mcg/mL in the hypothyroid and thyrotoxic patients and healthy controls, respectively (p=0.503). The adiponectin levels decreased significantly in the patients with hypothyroidism (p=0.047), whereas they did not change in the patients with thyrotoxicosis (p=0.770) after achieving euthyroidism.

CONCLUSION

In this study, following the restoration of euthyroidism, the FPG levels increased in the hypothyroidism patients and decreased in the thyrotoxicosis patients, despite the lack of changes in the HOMA-IR and HOMA-beta levels. Meanwhile, the hypothyroid, thyrotoxic and euthyroid subjects had similar basal adiponectin levels, and a significant decrease in the adiponectin levels was observed after treatment for hypothyroidism, despite the absence of changes after treatment for thyrotoxicosis, indicating the need for further studies with a larger sample size.

Lipoprotein(a): resurrected by genetics

Plasma lipoprotein(a) [Lp(a)] is a quantitative genetic trait with a very broad and skewed distribution, which is largely controlled by genetic variants at the LPA locus on chromosome 6q27. Based on genetic evidence provided by studies conducted over the last two decades, Lp(a) is currently considered to be the strongest genetic risk factor for coronary heart disease (CHD). The copy number variation of kringle IV in the LPA gene has been strongly associated with both Lp(a) levels in plasma and risk of CHD, thereby fulfilling the main criterion for causality in a Mendelian randomization approach. Alleles with a low kringle IV copy number that together have a population frequency of 25-35% are associated with a doubling of the relative risk for outcomes, which is exceptional in the field of complex genetic phenotypes. The recently identified binding of oxidized phospholipids to Lp(a) is considered as one of the possible mechanisms that may explain the pathogenicity of Lp(a). Drugs that have been shown to lower Lp(a) have pleiotropic effects on other CHD risk factors, and an improvement of cardiovascular endpoints is up to now lacking. However, it has been established in a proof of principle study that lowering of very high Lp(a) by apheresis in high-risk patients with already maximally reduced low-density lipoprotein cholesterol levels can dramatically reduce major coronary events.

Lipid abnormalities and cardiometabolic risk in patients with overt and subclinical thyroid disease

Dyslipidemia is a common finding in patients with thyroid disease, explained by the adverse effects of thyroid hormones in almost all steps of lipid metabolism. Not only overt but also subclinical hypo- and hyperthyroidism, through different mechanisms, are associated with lipid alterations, mainly concerning total and LDL cholesterol and less often HDL cholesterol, triglycerides, lipoprotein (a), apolipoprotein A1, and apolipoprotein B. In addition to quantitative, qualitative alterations of lipids have been also reported, including atherogenic and oxidized LDL and HDL particles. In thyroid disease, dyslipidemia coexists with various metabolic abnormalities and induce insulin resistance and oxidative stress via a vice-vicious cycle. The above associations in combination with the thyroid hormone induced hemodynamic alterations, might explain the increased risk of coronary artery disease, cerebral ischemia risk, and angina pectoris in older, and possibly ischemic stroke in younger patients with overt or subclinical hyperthyroidism.

Subclinical hypothyroidism and lipid abnormalities in older women attending a vascular disease prevention clinic: effect of thyroid replacement therapy

The authors evaluated the frequency and type of lipid disorders associated with subclinical hypothyroidism (SH) in older women referred to their university vascular disease prevention clinic. They also assessed the results of thyroid replacement therapy. Fasting serum lipid profiles and thyroid function tests were measured in 333 apparently healthy women (mean age: 71.8 +/- 7 years). These women were divided into 3 groups: group I: 60-69 years old (n = 132); group II: 70-79 years old (n = 153); group III: 80-89 years old (n = 48). SH was defined as a serum thyrotropin concentration higher than 3.20 mlU/mL with a normal free thyroxine concentration. The prevalence of SH was 7.5%. Thyrotropin was higher than 3.20 mU/mL in 25 women; 7 (5.3%), 14 (9.2%), and 4 (8.3%) in groups I, II, and III, respectively. Low-density lipoprotein cholesterol (LDL-C) concentrations were higher in the women with SH (p = 0.037). The mean values of total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C), TC/HDL-C ratio, lipoprotein (a) (Lp[a]), apolipoprotein A-I (apo AI) apolipoprotein B100 (apo B) and apo B/apo A ratio were higher and triglycerides (TG) were lower, compared with those with normal levels of thyrotropin. However, none of these differences reached significance. Restoration of euthyroid status (thyroxine: 50-100 microg/day) in 17 SH women significantly improved TC (p = 0.017), LDL-C (p = 0.014), TC/HDL-C (p = 0.05), LDL-C/HDL-C (p = 0.03), apo B (p = 0.013), and Lp(a) (p = 0.0005) values. SH is relatively common in older women attending a vascular disease prevention clinic. Thyroid hormone replacement therapy significantly improved serum lipids. In particular, the reduction in LDL-C and Lp(a) concentrations may be of clinical benefit.

Lipoprotein(a) concentration increases during treatment with carbamazepine

PURPOSE

Treatment with carbamazepine (CBZ) is known to affect apolipoprotein B-containing lipoprotein concentrations in serum. However, little is known about the effects of anticonvulsant drugs (AEDs) on lipoprotein(a) [Lp(a)], although Lp(a) has been characterized as independent cardiovascular risk factor. We investigated prospectively the effect of CBZ on lipoprotein(a) concentration in normolipidemic healthy adults.

METHODS

Twenty male volunteers were included in the study. Lp(a) levels were determined before and 69 +/- 19 days after CBZ administration by using an enzyme-linked immunoassay.

RESULTS

CBZ (mean plasma concentration, 6.6 +/- 0.6 microg/ml) caused a significant increase in Lp(a) concentrations, with a median change of +19.5% (95% CI: +8.2, +53.3; p < 0.001). Total cholesterol, low density lipoprotein (LDL) cholesterol, and triglycerides also increased significantly.

CONCLUSIONS

Although the precise mechanism of action of CBZ on Lp(a) elevation remains uncertain, it might be related to its enzyme-inducing properties. During treatment with CBZ, special focus should be given to elevated LDL cholesterol and Lp(a) concentrations with regard to increased risk for atherosclerotic vascular diseases.

The apolipoprotein(a) size, pentanucleotide repeat, C/T(+93) polymorphisms of apolipoprotein(a) gene, serum lipoprotein(a) concentrations and their relationship in a Korean population

BACKGROUND

In addition to apolipoprotein(a) [apo(a)] kringle 4 variable number of tandem repeat (K4-VNTR), pentanucleotide repeat polymorphism (PNRP) and C/T(+93) polymorphism [C/T(+93)] of apo(a) gene have been suggested to be related to lipoprotein(a) [Lp(a)] concentration. We studied the distribution of these genetic polymorphisms and their relationship with Lp(a) concentrations in a Korean population.

METHODS

One hundred thirty-two Korean adults were examined. Lp(a) was measured with enzyme-linked immunosorbent assay (ELISA). Apo(a) K4-VNTR was measured by high-resolution SDS-agarose gel separation and ECL Western blotting method. PNRP was measured after DNA amplification. The C/T(+93) ratio was measured by a amplification refractory mutation system.

RESULTS

Lp(a) was inversely correlated with K4-VNTR (r=0.732, p<0.0001), but was associated neither with any PNRP haplotype nor with C/T(+93) by multiple regression analysis, although we found a significant decrease of Lp(a) in PNRP 9/9 individuals (p<0.01). There was a strong linkage disequilibrium between 9 haplotypes of PNRP and the T haplotype of C/T(+93).

CONCLUSIONS

Inverse relationship between serum Lp(a) and K4 number of apo(a) was confirmed in normal Korean adults. PNRP 9/9 genotype appeared to have a reducing effect on Lp(a), but neither 9 haplotype heterozygotes of PNRP nor the T haplotype C/T(+93) affected Lp(a) concentrations in Koreans.

Standardization and clinical management of lipoprotein(a) measurements

The present article proposes personal suggestions to improve determinations and clinical interpretation of results of lipoprotein(a) assays. Methods and procedures for sampling and quantification of the various isoforms of lipoprotein(a) in serum, plasma and urine are reviewed with the aim of improving the reliability and reproducibility of results and reinforcing the clinical utility of lipoprotein(a) measurements.

CI-1011 lowers lipoprotein(a) and plasma cholesterol concentrations in chow-fed cynomolgus monkeys

Lipoprotein(a) (Lp(a)), which is generated through the covalent association of apolipoprotein(a) (apo(a)) and apo B-100-LDL, is an independent risk factor for several vascular diseases. Therefore, there is interest in developing therapies for lowering Lp(a). This investigation was carried out to determine the effect of CI-1011, a potent lipid regulator in rodents, on Lp(a) and other lipid parameters in cynomolgus monkeys (Macaca fascicularis). Nine healthy male monkeys on a normal chow diet were orally treated with CI-1011 at 30 mg/kg per day for 3 weeks. Lp(a) and total cholesterol levels were significantly decreased after 1 week and maximally reduced to 68 and 73% of control levels, respectively, after 3 treatment weeks. The decreases in total cholesterol were mainly due to changes in low density lipoprotein (LDL). The LDL:HDL ratio decreased by 30%. Triglycerides were unaffected by treatment. Lp(a) and total cholesterol levels returned to pretreatment values after stopping treatment suggesting a direct effect of the compound on their inhibition. Further studies demonstrated that CI-1011 was effective at a low dose of 3 mg/kg per day after 1 week of administration. CI-1011 also decreased apo B-100 to 80% of control levels, but this change was not sufficient to account for the Lp(a) lowering. There was also no correlation between the changes in Lp(a) and apo B-100 levels. Treatment of cynomolgus monkey primary hepatocyte cultures with CI-1011 resulted in a dose-dependent inhibition of Lp(a) levels suggesting a direct hepatic effect of the compound. Western blot analysis of the samples showed that changes in Lp(a) were associated mainly with decreased apo(a) (47%), but not apo B-100 (17%). These results demonstrate that CI-1011 effectively decreases Lp(a) levels both in vivo and in vitro.

Effects of growth hormone on serum lipids and lipoproteins: possible significance of increased peripheral conversion of thyroxine to triiodothyronine

The role of growth hormone (GH) and thyroid hormone in the regulation of lipid and lipoprotein metabolism is not fully established. Furthermore, the possible linkage between the well-known GH-induced increase in peripheral thyroxine (T4) to triiodothyronine (T3) generation and the effects of GH on lipid and lipoprotein metabolism has not been elucidated. In this double-blind placebo-controlled study, we compared the effects of GH and T3 administration alone and in combination on lipid and lipoprotein metabolism in a group of healthy young adults. The dose of T3 was selected to mimic the T2 increase seen during exogenous GH exposure. Eight normal male subjects (aged 21 to 27 years; body mass index, 21.11 to 27.17 kg/m2) were randomly studied during four 10-day treatment periods with (1) daily subcutaneous placebo injections and placebo injections and placebo tablets, (2) daily subcutaneous GH injections (0.1 IU/kg.d) and placebo tablets, (3) daily T3 administration (40 micrograms on even dates or 20 micrograms on uneven dates) plus placebo injections, and (4) daily GH injections plus T3 administration. GH administration increased free T3 (FT3) to the same level as during T3 administration. GH caused decreased levels of total cholesterol (TC) and low-density lipoprotein (LDL) cholesterol and increased levels of triglycerides (TG) and lipoprotein(a) (Lp(a)), but no changes in high-density lipoprotein (HDL) cholesterol and apolipoprotein B (apo B). T3 administration caused no alteration in these parameters, except for decreased levels of TC comparable to those seen after GH administration. Combined GH and T3 administration caused changes identical to those seen after GH administration, in addition to decreased apo B levels and a further decrease of TC levels. We conclude that GH and iodothyronines in the physiologic range exert distinct but disparate effects on lipids and lipoproteins, and do not support the hypothesis that the effects observed during GH administration are exclusively secondary to changes in peripheral T3 levels.

Lipoprotein(a) in renal disease

Lipoprotein(a) [Lp(a)] is a genetically determined risk factor for atherosclerotic vascular disease. Several studies have described a correlation between high Lp(a) plasma levels and coronary heart disease, stroke, and peripheral atherosclerosis. In healthy individuals Lp(a) plasma concentrations are almost exclusively controlled by the apolipoprotein(a) [apo(a)] gene locus on chromosome 6q2.6-q2.7. More than 30 alleles at this highly polymorphic gene locus determine a size polymorphism of apo(a). There exists an inverse correlation between the size (molecular weight) of apo(a) isoforms and Lp(a) plasma concentrations. Average Lp(a) levels are high in individuals with low molecular weight isoforms and low in those with high molecular weight isoforms. Mean Lp(a) plasma levels are elevated over controls in patients with renal disease. Patients with nephrotic syndrome exhibit excessively high Lp(a) plasma concentrations, which can be reduced with antiproteinuric treatment. The mechanism underlying this elevation is unclear, but the general increase in protein synthesis caused by the liver due to high urinary protein loss is a likely explanation. Patients with end-stage renal disease (ESRD) also have elevated Lp(a) levels. These are even higher in patients treated by continuous ambulatory peritoneal dialysis than in those receiving hemodialysis. Lipoprotein(a) concentrations decrease to values observed in controls matched for apo(a) type following renal transplantation. This clearly demonstrates the nongenetic origin of Lp(a) elevation in ESRD. Both the increase in ESRD and the decrease following renal transplantation are apo(a) phenotype dependent. Only patients with high molecular weight phenotypes show the described changes in Lp(a) levels. In patients with low molecular weight types the Lp(a) concentrations remain unchanged during both phases of renal disease. As in the general population, Lp(a) is a risk factor for cardiovascular events in ESRD patients. In this patient group the apo(a) phenotype seems to be equally or better predictive of the degree of atherosclerosis than is Lp(a) concentration. Further prospective studies will be necessary to confirm these observations. Whether Lp(a) also plays a key role in the pathogenesis and progression of renal diseases needs further study. Controversial data on the role of the kidney in Lp(a) metabolism result from insufficient sample sizes of several studies. Due to the broad range and skewed distribution of Lp(a) plasma concentrations, large study groups must be investigated to obtain reliable results.

Lipoprotein(a) in health and disease

Lipoprotein(a) [Lp(a)] represents an LDL-like particle to which the Lp(a)-specific apolipoprotein(a) is linked via a disulfide bridge. It has gained considerable interest as a genetically determined risk factor for atherosclerotic vascular disease. Several studies have described a correlation between elevated Lp(a) plasma levels and coronary heart disease, stroke, and peripheral atherosclerosis. In healthy individuals, Lp(a) plasma concentrations are almost exclusively controlled by the apo(a) gene locus on chromosome 6q2.6-q2.7. More than 30 alleles at this highly polymorphic gene locus determine a size polymorphism of apo(a). There exists an inverse correlation between the size (molecular weight) of apo(a) isoforms and Lp(a) plasma concentrations. The standardization of Lp(a) quantification is still an unresolved task due to the large particle size of Lp(a), the presence of two different apoproteins [apoB and apo(a)], and the large size polymorphism of apo(a) and its homology with plasminogen. A working group sponsored by the IFCC is currently establishing a stable reference standard for Lp(a) as well as a reference method for quantitative analysis. Aside from genetic reasons, abnormal Lp(a) plasma concentrations are observed as secondary to various diseases. Lp(a) plasma levels are elevated over controls in patients with nephrotic syndrome and patients with end-stage renal disease. Following renal transplantation, Lp(a) concentrations decrease to values observed in controls matched for apo(a) type. Controversial data on Lp(a) in diabetes mellitus result mainly from insufficient sample sizes of numerous studies. Large studies and those including apo(a) phenotype analysis came to the conclusion that Lp(a) levels are not or only moderately elevated in insulin-dependent patients. In noninsulin-dependent diabetics, Lp(a) is not elevated. Conflicting data also exist from studies in patients with familial hypercholesterolemia. Several case-control studies reported elevated Lp(a) levels in those patients, suggesting a role of the LDL-receptor pathway for degradation of Lp(a). However, recent turnover studies rejected that concept. Moreover, family studies also revealed data arguing against an influence of the LDL receptor for Lp(a) concentrations. Several rare diseases or disorders, such as LCAT- and LPL-deficiency as well as liver diseases, are associated with low plasma levels or lack of Lp(a).