Evaluation of endothelial function in subclinical hypothyroidism and subclinical hyperthyroidism

Noninvasive indicators of atherosclerosis in subclinical hypothyroidism

INTRODUCTION

Cardiovascular system is rich in thyroid hormone receptors and is one of the major sites of action for thyroid hormones. However, the effect of subclinical hypothyroidism (SCH) on atherosclerosis has not been cleared yet.

MATERIALS AND METHODS

SCH is defined as high thyroid-stimulating hormone (TSH) levels in the presence of normal serum T4 and T3 levels. A total of 32 patients with SCH and 29 controls were included in the study. Carotid intima-media thickness, flow-mediated dilatation, and aortic distensibility were compared between the groups.

RESULTS

FMD was lower in patients with SCH than in controls. GTN-induced vasodilatation was similar in the patients with SCH and controls. There was no statistically significant difference between the patients with SCH and controls with respect to CIMT and aortic distensibility.

CONCLUSION

SCH is associated with endothelial dysfunction as established by FMD. Inconsistent results of CIMT and aortic stiffness can be explained by these parameters being measures of structural changes whereas FMD is a dynamic measure that reflects the impact of both acute and chronic influences on endothelial function.

Preservation of renal function by thyroid hormone replacement therapy in chronic kidney disease patients with subclinical hypothyroidism

CONTEXT

Subclinical hypothyroidism is not a rare condition, but the use of thyroid hormone to treat subclinical hypothyroidism is an issue of debate.

OBJECTIVE

This study was undertaken to investigate the impact of thyroid hormone therapy on the changes in estimated glomerular filtration rate (eGFR) in subclinical hypothyroidism patients with stage 2-4 chronic kidney disease.

PATIENTS

A total of 309 patients were included in the final analysis.

MAIN OUTCOME MEASURE

The changes in eGFR over time were compared between patients with and without thyroid hormone replacement therapy using a linear mixed model. Kaplan-Meier curves were constructed to determine the effect of thyroid hormone on renal outcome, a reduction of eGFR by 50%, or end-stage renal disease. The independent prognostic value of subclinical hypothyroidism treatment for renal outcome was ascertained by multivariate Cox regression analysis.

RESULTS

Among the 309 patients, 180 (58.3%) took thyroid hormone (treatment group), whereas 129 (41.7%) did not (nontreatment group). During the mean follow-up duration of 34.8 ± 24.3 months, the overall rate of decline in eGFR was significantly greater in the nontreatment group compared to the treatment group (-5.93 ± 1.65 vs. -2.11 ± 1.12 ml/min/yr/1.73 m(2); P = 0.04). Moreover, a linear mixed model revealed that there was a significant difference in the rates of eGFR decline over time between the two groups (P < 0.01). Kaplan-Meier analysis also showed that renal event-free survival was significantly lower in the nontreatment group (P < 0.01). In multivariate Cox regression analysis, thyroid hormone replacement therapy was found to be an independent predictor of renal outcome (hazard ratio, 0.28; 95% CI, 0.12-0.68; P = 0.01).

CONCLUSION

Thyroid hormone therapy not only preserved renal function better, but was also an independent predictor of renal outcome in chronic kidney disease patients with subclinical hypothyroidism.

Blood pressure, thyroid-stimulating hormone, and thyroid disease prevalence in primary aldosteronism and essential hypertension

BACKGROUND

A positive correlation between thyroid-stimulating hormone (TSH) and blood pressure (BP) has been identified in normotensives and in patients with essential hypertension (EH). This study was designed to evaluate, in primary aldosteronism (PA) and in EH, potential association of BP, TSH, and ultrasonographic changes of the thyroid.

METHODS

We studied 188 patients: 92 with PA and 96 matched essential hypertensives. Clinical and ambulatory BP (ABP), and thyroid function were evaluated in all patients. In PA and in a subgroup of EH patients (n = 65) thyroid ultrasonography was performed.

RESULTS

In PA patients, diastolic office and diastolic ABP increased across TSH quartiles and multivariate analysis confirmed a positive significant correlation between TSH and diastolic BP, independently of aldosterone levels, body mass index (BMI), duration of hypertension, and age. In EH patients, we found a significant linear increase in systolic and diastolic ABP with increasing TSH. The prevalence of thyroid dysfunctions was similar in PA and EH (15% and 19%, respectively). In PA patients, we found a higher prevalence of ultrasonographic alterations than in EH (66% vs. 46%, P < 0.05). PA patients presenting morphological abnormalities had higher homeostasis model assessment-insulin resistance levels than patients with normal gland at ultrasonography (4.2 ± 1.8 vs. 3.1 ± 0.8 P < 0.05).

CONCLUSIONS

We found a positive correlation between TSH and BP both in PA and EH patients. Moreover, in PA patients we observed a high prevalence of thyroid morphological alterations.

Evaluation of the effect of L-thyroxin therapy on endothelial functions in patients with subclinical hypothyroidism

Subclinical hypothyroidism (SH) is characterized by normal serum free T4 (FT4), free T3 (FT3) levels and increased serum thyroid-stimulating hormone (TSH) levels. Endothelial dysfunction, which is an early step of atherosclerosis, has been reported in patients with subclinical hypothyroidism. The aim of this study is to evaluate endothelial functions and the effect of L-thyroxin (L-T4) therapy on endothelial functions in SH. Twenty-seven patients with SH and 22 healthy controls were evaluated in terms of endothelial functions, using brachial artery Doppler ultrasonography. After restorating euthyroidism, measurements were repeated. Baseline and nitroglycerin induced diameter (NID) of brachial artery were similar in patients with SH and the control group. Compared to the control group, the patients with SH showed significantly reduced flow-mediated diameter (FMD). Baseline and NID values were significantly higher after LT4 therapy in SH group. FMD also significantly increased after LT4 therapy. Hypothyroidism accelerates atherogenesis through modification of atherosclerotic risk factors and direct effects on the blood vessels. In this study, we observed marked improvement in endothelial functions after L-T4 therapy in SH patients. We suggest that thyroid hormone replacement therapy may help to prevent atherosclerosis in this group of patients.

Relation between thyroid-stimulating hormone and the occurrence of cardiovascular events and mortality in patients with manifest vascular diseases

Aims: To investigate whether levels of thyroid-stimulating hormone (TSH) within the normal range are associated with an increased risk of new vascular events and mortality in patients with clinical manifest vascular diseases and whether this relation is influenced by adiposity.

Methods and results: Prospective cohort study in 2443 patients (1790 men and 653 women) with clinical manifest vascular disease and TSH levels in the normal range. Median follow up was 2.7 (interquartile range 1.4–3.9) years. Clinical endpoints of interest were: myocardial infarction, stroke, vascular death, and all-cause mortality. In patients with manifest vascular disease, the prevalence of (subclinical) hypothyroidism was 5.7%, while 3.6% had (subclinical) hyperthyroidism. An increase in 1 unit of TSH was associated with a 33% higher risk (HR 1.33; 95% CI 1.03–1.73) for the occurrence of myocardial infarction, adjusted for age, gender, renal function, and smoking. In patients with a body mass index (BMI) below the median of 26.7 kg/m2 the HR per unit TSH for myocardial infarction was 1.55 (95% CI 1.08–2.21) compared to 1.18 (95% CI 0.81–1.71) in patients with a BMI ≥26.7 kg/m2. Visceral adipose tissue thickness below the median (≤8.8 cm) was associated with higher HR per unit TSH for myocardial infarction (HR 1.69; 95% CI 1.21–2.35) compared to visceral adipose tissue thickness >8.9 cm (HR 1.00; 95% CI 0.66–1.49). There was no relation between TSH and risk of stroke, vascular death, the combined endpoint, or all-cause mortality.

Conclusion: Higher TSH levels within the normal range are associated with an increased risk of myocardial infarction, in patients with clinical manifest vascular disease. This relation is most prominent in patients without visceral obesity.

Association between subclinical hypothyroidism and severe diabetic retinopathy in Korean patients with type 2 diabetes

The association between subclinical hypothyroidism (SCH) and microvascular complications of type 2 diabetes is unclear. We examined whether SCH is associated with diabetic retinopathy or nephropathy in Korean patients with type 2 diabetes. Data from 489 patients who visited the diabetes clinic at a university hospital between 2001 and 2007 were analyzed retrospectively. Participants were evaluated for glycemic control, thyroid function, and diabetic retinopathy and nephropathy. Diabetic retinopathy was classified into five grades. Diabetic nephropathy was assessed by the presence of albuminuria. Patients in the SCH group had a higher proportion of women, older age, longer duration of diabetes, higher systolic and diastolic blood pressure, and higher insulin resistance index compared with the euthyroid group. No significant difference in family history of diabetes or body mass index was found between groups. The prevalence of severe diabetic retinopathy (severe nonproliferative diabetic retinopathy or proliferative diabetic retinopathy) was significantly higher in the SCH group than the euthyroid group (32.8% vs. 19.6%, P = 0.036), whereas no between-group difference was found in the prevalence of diabetic nephropathy. After adjustment for potential confounding factors (HbA1c, BMI, duration of diabetes, diabetic nephropathy, and hypertension) by multivariate logistic regression analysis, SCH remained significantly associated with severe diabetic retinopathy (odds ratio 2.086 (95% CI, 1.010-4.307), P = 0.047). These results suggest that SCH was independently associated with severe diabetic retinopathy in patients with type 2 diabetes. Further prospective studies are required to confirm the association between SCH and diabetic retinopathy.

Effects of thyroxine replacement on endothelial function and carotid artery intima-media thickness in female patients with mild subclinical hypothyroidism

BACKGROUND

Previous studies have suggested an association between subclinical hypothyroidism and coronary artery disease that could be related to changes in serum lipids or endothelial dysfunction.

METHODS

Thirty-two female subclinical hypothyroidism patients were randomly assigned to 12 months of L-thyroxine replacement or no treatment. Endothelial function was measured by the flow-mediated vasodilatation of the brachial artery, as well as mean carotid artery intima-media thickness, and lipid profiles were studied at baseline and after 12 months of follow-up.

RESULTS

The mean ( ± SD) serum thyroid-stimulating hormone levels in the L-thyroxine replacement and control groups were 6.09 ± 1.32 and 6.27 ± 1.39 μUI/ml, respectively. No relationship between carotid artery intima-media thickness or brachial flow-mediated vasodilatation and free T4 and serum thyroid-stimulating hormone was found. The median L-T4 dose was 44.23 ± 18.13 μg/day. After 12 months, there was a significant decrease in the flow-mediated vasodilatation in the subclinical hypothyroidism control group (before: 17.33 ± 7.88 to after: 13.1 ± 4.75%, p =0.03), but there were no significant differences in flow-mediated vasodilatation in the L-thyroxine treated group (before: 16.81 ± 7.0 to after: 18.52 ± 7.44%, p = 0.39). We did not find any significant change in mean carotid intimamedia thickness after 12 months of L-thyroxine treatment.

CONCLUSION

Replacement therapy prevents a decline in flow-mediated vasodilatation with continuation of the subclinical hypothyroidism state. Large prospective multicenter placebo-controlled trials are necessary to investigate endothelial physiology further in subclinical hypothyroidism patients and to define the role of L-thyroxine therapy in improving endothelial function in these patients.

[Markers of endothelial function in hypothyroidism]

The role of the endothelium in human disease has become the focus of scientific investigation and recently noninvasive and less expensive measures of endothelial function have become available. The endothelium modulates the vascular tonus and participates in inflammatory processes, platelet aggregation and thrombosis. Consequently, endothelial dysfunction has been implicated as an important event in the pathogenesis of atherosclerosis. Hypothyroidism is associated with an increased cardiovascular risk, and the assessment of endothelial function holds a great deal of promise as an assessment tool for the detection of preclinical cardiovascular alterations associated with thyroid dysfunction. Some recent studies have demonstrated a relationship between thyroid status and endothelial function, but large multicenter, placebo-controlled prospective trials are necessary to address this issue and the effect of levothyroxine replacement treatment in endothelial function. The objective of this work is to discuss the perspective picture in endothelium and thyroid function relationship.

Normal flow-mediated vasodilatation of the brachial artery and carotid artery intima-media thickness in subclinical hypothyroidism

Subclinical hypothyroidism (SHT) is a disease for which exact therapeutic approaches have not yet been established. Previous studies have suggested an association between SHT and coronary heart disease. Whether this association is related to SHT-induced changes in serum lipid levels or to endothelial dysfunction is unclear. The aim of this study was to determine endothelial function measured by the flow-mediated vasodilatation of the brachial artery and the carotid artery intima-media thickness (IMT) in a group of women with SHT compared with euthyroid subjects. Triglycerides, total cholesterol, HDL-C, LDL-C, apoprotein A (apo A), apo B, and lipoprotein(a) were also determined. Twenty-one patients with SHT (mean age: 42.4 +/- 10.8 years and mean thyroid-stimulating hormone (TSH) levels: 8.2 +/- 2.7 microIU/mL) and 21 euthyroid controls matched for body mass index, age and atherosclerotic risk factors (mean age: 44.2 +/- 8.5 years and mean TSH levels: 1.4 +/- 0.6 microIU/mL) participated in the study. Lipid parameters (except HDL-C and apo A, which were lower) and IMT values were higher in the common carotid and carotid bifurcation of SHT patients with positive serum thyroid peroxidase antibodies (TPO-Ab) (0.62 +/- 0.2 and 0.62 +/- 0.16 mm for the common carotid and carotid bifurcation, respectively) when compared with the negative TPO-Ab group (0.55 +/- 0.24 and 0.58 +/- 0.13 mm, for common carotid and carotid bifurcation, respectively). The difference was not statistically significant. We conclude that minimal thyroid dysfunction had no adverse effects on endothelial function in the population studied. Further investigation is warranted to assess whether subclinical hypothyroidism, with and without TPO-Ab-positive serology, has any effect on endothelial function.

Transport of thyroxine and 3,3',5-triiodothyronine in human umbilical vein endothelial cells

The prerequisite for the uptake of thyroid hormone (TH) in peripheral tissues is the exit of TH from the bloodstream. The first step in this process is transport across the endothelium. Little is known about this important step in TH physiology. Therefore, we aimed to characterize the TH transport processes across the endothelium using human umbilical vein endothelial cells as a model. Transport studies showed rapid uptake of 1 nm [(125)I]T(3) and [(125)I]T(4) in these cells. The apparent Michaelis constant value for [(125)I]T(3) uptake was about 1 microm, and the IC(50) for T(4) inhibition of T(3) uptake was about 3 microm. The aromatic amino acids phenylalanine, tyrosine, and tryptophan and the L-type amino acid transporter-specific ligand 2-aminobicyclo-(2, 2, 1)-heptane-2-carboxylic acid did not inhibit [(125)I]T(3) or [(125)I]T(4) uptake. Verapamil was capable of reversibly reducing transport of [(125)I]T(3) and [(125)I]T(4). Human umbilical vein endothelial cells incubated with the affinity label BrAcT(3) resulted in a labeling of multiple proteins, which are probably protein disulfide isomerase related. Extrapolating our findings to the endothelial lining of blood vessels suggests that T(3) and T(4) uptake is mediated by the same transport system. Because TH transport characteristics do not correspond to known TH transporters, further studies are required to identify the TH transporter protein(s) at the molecular level. Possible candidates may be widely expressed Na(+)-independent transporter proteins.

The effect of levothyroxine on arterial stiffness and lipid profile in patients with subclinical hypothyroidism

BACKGROUND

The influence of treatment for subclinical hypothyroidism (SCH) on cardiovascular morbidity and mortality, arterial stiffness, and lipid profile has not been elucidated yet. The aim of this study was to evaluate the effect of levothyroxine on arterial stiffness, lipid profile, and inflammation.

METHODS

The study included 30 patients with SCH. Patients were treated with levothyroxine and were assessed at baseline and at 1, 4, and 7 months. Blood samples were taken for lipid profile and highly sensitive C-reactive protein (hs-CRP). Arterial stiffness was evaluated by augmentation index (AIx). In conditions that cause arterial stiffness, the pulse wave traveling from the periphery to the heart reaches the heart during systole, resulting in augmentation of the central pressure. This increase, calculated as the AIx, is a good expression of central aortic pressure.

RESULTS

After accomplishing euthyroidism, the AIx decreased from 17.2 +/- 8.3 to 14.3 +/- 6.5 (p < 0.01) and AIx percentage decreased from 36.2 +/- 11.5 to 33.2 +/- 9.1 (p = 0.03). Systolic blood pressure (SBP) decreased from 134.7 +/- 20 to 127.6 +/- 13.7 mmHg (p < 0.01). In those patients whose AIx decreased, low-density lipoprotein (LDL) levels decreased by 0.4 +/- 0.96 mmol/L compared to the patients whose AIx did not decrease and LDL increased by 0.62 +/- 1.48 mmol/L (p = 0.057). Total cholesterol decreased by 0.72 +/- 1.64 mmol/L in the patients whose AIx decreased and increased by 1 +/- 2.53 mg/dL in the patients whose AIx did not improve (p = 0.06).

CONCLUSIONS

In patients with SCH, treatment with levothyroxine had a significant beneficial effect on arterial stiffness and SBP, and no effect on lipid profile or hs-CRP.

Exogenous subclinical hyperthyroidism impairs endothelial function in nodular goiter patients

BACKGROUND

Exogenous subclinical hyperthyroidism is associated with cardiovascular and metabolic changes. The aim of this study was to evaluate the effect of levothyroxine (LT4) suppression on endothelial function and insulin sensitivity in euthyroid nodular goiter patients.

METHODS

Twenty-two euthyroid patients with multinodular goiter (MNG) and 22 matched healthy controls were studied. LT4 was administered in doses ranging from 50 to 150 microg/day to reach target serum thyroid-stimulating hormone (TSH) levels <0.5 mIU/L. Patients were studied before and after 8 weeks after the target TSH level <0.5 mIU/L. The control group was studied twice, 16 weeks apart. Flow mediated vasodilatation (FMD), insulin sensitivity index (ISI), lipid peroxidation, and high-sensitivity C-reactive protein (hsCRP) were the outcome measures.

RESULTS

LT4 treatment significantly suppressed TSH levels to 0.2 +/- 0.1 mIU/L (minimum and maximum range was 0.05-0.3 mIU/L). FMD decreased from 10.7 +/- 2.7% to 5.4 +/- 1.7% (p < 0.001) and mean ISI decreased from 2.56 +/- 1.10 to 1.41 +/- 0.50 (p < 0.001) with LT4 treatment in the MNG group. Lipid peroxidation measured as thiobarbituric acid reactive substances (Tbars) (p < 0.05), and hsCRP (p < 0.001) levels significantly increased compared to the baseline in the MNG group. FMD measurement inversely correlated with free T4 (p = 0.008) and Tbars (p = 0.004), and positively correlated with ISI (p = 0.004). Serum Tbars and hsCRP were independent predictors of FMD (p = 0.004) in multivariate analysis. All results expressed as mean +/- SD.

CONCLUSIONS

TSH suppression therapy with LT4 leading to subclinical hyperthyroidism may cause impaired endothelial function, increased oxidative stress, and decreased insulin sensitivity in euthyroid nodular goiter patients.

The clinical significance of subclinical thyroid dysfunction

Subclinical thyroid disease (SCTD) is defined as serum free T(4) and free T(3) levels within their respective reference ranges in the presence of abnormal serum TSH levels. SCTD is being diagnosed more frequently in clinical practice in young and middle-aged people as well as in the elderly. However, the clinical significance of subclinical thyroid dysfunction is much debated. Subclinical hyper- and hypothyroidism can have repercussions on the cardiovascular system and bone, as well as on other organs and systems. However, the treatment and management of SCTD and population screening are controversial despite the potential risk of progression to overt disease, and there is no consensus on the thyroid hormone and thyrotropin cutoff values at which treatment should be contemplated. Opinions differ regarding tissue effects, symptoms, signs, and cardiovascular risk. Here, we critically review the data on the prevalence and progression of SCTD, its tissue effects, and its prognostic implications. We also examine the mechanisms underlying tissue alterations in SCTD and the effects of replacement therapy on progression and tissue parameters. Lastly, we address the issue of the need to treat slight thyroid hormone deficiency or excess in relation to the patient's age.

Coronary flow reserve is impaired in subclinical hypothyroidism

OBJECTIVE

Although the cardiovascular system is highly sensitive to thyroid hormones, the cardiovascular effects of subtle thyroid dysfunction such as subclinical hypothyroidism (SHT) remain unclear. Therefore, we investigated coronary flow reserve (CFR) reflecting coronary microvascular function in patients with SHT.

METHODS

Fifty subjects with SHT and 30 control subjects with normal serum thyroid hormones and TSH levels were included in this study. Coronary diastolic peak flow velocities were measured at baseline and after dipyridamole infusion. CFR was calculated as the ratio of hyperemic to baseline diastolic peak velocity.

RESULTS

Age, gender, diastolic and systolic blood pressure, body mass index (BMI), serum lipid parameters, and thyroid hormone levels were similar between the groups. Heart rate was significantly lower in the SHT group. Left ventricular diastolic filling parameters were significantly different in the SHT group while other echocardiographic parameters were similar. CFR values were significantly lower in subjects with SHT than in the control group (2.38 +/- 0.44 vs. 2.98 +/- 0.47, p < 0.0001).

CONCLUSIONS

These findings suggest that CFR, which reflects coronary microvascular function, is impaired in patients with SHT.

Propylthiouracil, independent of its antithyroid effect, produces endothelium-dependent vasodilatation through induction of nitric oxide bioactivity

OBJECTIVE

Propylthiouracil (PTU), independent of its antithyroid effect, is recently found to have a potent antiatherosclerotic effect. The aim of this study is to investigate whether PTU has a beneficial effect on endothelial function.

METHODS AND RESULTS

Ninety patients with a history of hyperthyroidism receiving either PTU (n=45) or methimazole (MMI) (n=45) during the euthyroid status were enrolled in this study. Brachial artery endothelium-dependent (flow-mediated dilatation [FMD]) and endothelium-independent (nitroglycerin-mediated dilatation) responses were assessed by high-resolution ultrasound image. Data for these two groups were compared with those of 41 healthy control subjects. The FMD values were significantly increased in patients maintained on PTU versus those in the MMI and control groups (9.3+/-4.4%, 3.4+/-2.5%, and 3.6+/-3.4%, respectively; P<0.01). Nitroglycerin-mediated dilatation had no significant difference between the PTU, MMI, and control groups (17.4+/-7.5%, 15.9+/-6.1%, and 17.5+/-6.8%, respectively; P=0.455). On multivariate analysis, no significant relationship was found between the FMD and thyroid hormone index levels. To further elucidate whether PTU has a direct effect on endothelial function, the effect of PTU on isolated segments of Sprague-Dawley rat aorta was studied. Vasodilatation induced by PTU was endothelium-dependent and could be blocked by pretreatment with nitric oxide (NO) inhibitors. PTU also increased NO formation in aortic segments.

CONCLUSIONS

This study demonstrated that PTU produced endothelium-dependent vasodilatation through thyroid-independent and NO-mediated mechanisms that may contribute to its beneficial effect on atherosclerosis.

Carotid artery intima media thickness is inversely related to serum free thyroxine in euthyroid subjects

OBJECTIVE

The effect of thyroid function on cardiovascular risk may extend to the euthyroid range. In euthyroid subjects, we determined whether carotid artery intima media thickness (IMT), a measure of subclinical atherosclerosis, is related to thyroid function.

DESIGN AND SUBJECTS

Cross-sectional study in a cohort of 78 nonsmoking, predominantly middle-aged, euthyroid subjects (44 men and 34 women, mean age 56 years, TSH between 0.5 mU/l and 4.0 mU/l and FT4 between 11.0 pmol/l and 19.5 pmol/l).

MEASUREMENTS

IMT (mean of three segments in both carotid arteries by ultrasonography), clinical factors, insulin resistance (HOMA(ir)), plasma lipids, C-reactive protein (CRP), serum FT4, TSH and thyroid autoantibodies.

RESULTS

In several multiple linear regression models, age- and sex-adjusted IMT was found to be independently related to either pulse pressure and body mass index (BMI), to high density lipoprotein (HDL) cholesterol or to FT4, but not to TSH, thyroid autoantibodies, HOMA(ir), CRP, non-HDL cholesterol and triglycerides. In a subsequent model which included age, sex, pulse pressure, body mass index (BMI), HDL cholesterol and FT4, IMT was independently and positively related to age (beta = 0.43, P < 0.001), male sex (beta = 0.34, P = 0.014), pulse pressure (beta = 0.29, P = 0.002), BMI (beta = 0.24, P = 0.007) and inversely related to FT4 (beta = -0.19, P = 0.046). IMT was also inversely related to FT4 in a model which included HDL cholesterol, non-HDL cholesterol and triglycerides.

CONCLUSIONS

In euthyroid subjects, IMT is associated with FT(4), after controlling for clinical factors, lipid levels and thyroid autoantibodies. These findings raise the possibility that, even within the euthyroid range, low normal thyroid function may adversely affect cardiovascular risk.

Enhancement of endothelium-dependent flow-mediated vasodilation in hyperthyroidism

OBJECTIVE

Vascular responsiveness changes in hyperthyroid patients remains controversial. This study attempts to determine whether the vasomotor activity can be influenced by hyperthyroid conditions, and, if so, whether changes induced by hyperthyroidism may be restored to normal during the euthyroid state after treatment.

DESIGN

A case-control clinical study.

PATIENTS AND MEASUREMENTS

Forty-five pretreated hyperthyroid patients (mean age 36.62 +/- 10.12 years, 36 female) were compared with 45 gender- and age-matched control subjects (mean age 38.98 +/- 11.17 years, 40 female). Brachial artery endothelium-dependent flow-mediated vasodilation (FMD) and endothelium-independent nitroglycerin-mediated vasodilation (NMD) responses were assessed noninvasively by high-resolution ultrasound imaging. Among the 45 hyperthyroid patients, 27 patients underwent the same procedures prospectively in the post-treatment euthyroid state.

RESULTS

The FMD values were significantly increased in hyperthyroid patients vs. those of controls (8.94 +/- 5.65%vs. 3.77 +/- 3.42%, P < 0.001), whereas NMD levels were not significantly different (18.17 +/- 7.76%vs. 17.28 +/- 6.63%, P = 0.560). Multiple regression analysis revealed that the presence of hyperthyroidism was the only significant factor associated with FMD. In the follow-up study of 27 hyperthyroid patients, the FMD values were significantly decreased in the post-treatment euthyroid state compared with those in the pretreated hyperthyroid state (6.40 +/- 4.27%vs. 8.83 +/- 4.61%, P = 0.021), although these values were still higher than those of controls.

CONCLUSIONS

This study demonstrated that endothelium-dependent FMD was increased in the hyperthyroid patients, and could be partially restored by treatment with antithyroid agents.

Coronary flow reserve is also impaired in patients with subclinical hypothyroidism

The diagnosis of subclinical hypothyroidism and detection of its effects on cardiovascular system is important. Also, the patients with subclinical hypothyroidism even at the very early stage are at increased risk for developing atherosclerosis. We evaluated coronary microvascular circulation and endothelial dysfunction of epicardial coronary arteries by the measurement of coronary flow velocity reserve via a non invasive technique, transthoracic Doppler echocardiography in subclinical hypothyroidism. Coronary flow reserve in patients with subclinical hypothyroidism such as in overt hypothyroidism was lower than that of euthyroid subject. As a conclusion, endothelial and microvascular dysfunction, which are early harbingers of atherosclerosis, are shown in overt and subclinical hypothyroidism.

Effects of thyroxine replacement on lipid profile and endothelial function after thyroidectomy

Background

Hypothyroidism is a risk factor for atherosclerotic cardiovascular disease. This study investigated the effects of l-thyroxine replacement therapy on lipid profile and endothelial function after thyroidectomy in patients with overt transient non-autoimmune hypothyroidism.

Methods

Twenty-two patients with non-toxic multinodular goitre treated by total or near-total thyroidectomy and 22 healthy individuals matched for age, sex and body mass index were studied. Lipid profile and endothelial function were determined in each patient at the euthyroid phase before thyroidectomy (stage 1), the hypothyroid phase 3 weeks after surgery (stage 2), and the euthyroid phase 3 months (stage 3) and 6 months (stage 4) after the start of thyroxine treatment.

Results

The lipid profile and endothelial function deteriorated between stage 1 and stages 2 and 3. Findings at stage 4 were similar to those at stage 1. There was a positive correlation between serum thyroid-stimulating hormone (TSH) and total cholesterol (rs = 0·588, P &lt; 0·001), and a negative correlation between serum TSH and flow-mediated arterial dilatation (rs = 0·506, P &lt; 0·001). Total cholesterol and TSH were independent determinants of endothelial function.

Conclusion

A 3-week hypothyroid period after thyroidectomy led to a more atherogenic lipid profile and impaired endothelial function that persisted for at least 3 months.

Thyroid disease and vascular function

Altered cardiac function in thyroid disease is well recognized and has been extensively investigated, vascular function has however been less well studied in those with thyroid dysfunction. Thyroid hormones, thyroxine (T(4)) and triiodothyronine (T(3)) are important regulators of cardiac function and cardiovascular hemodynamics. The cardiovascular system responds to minimal but persistent changes in circulating thyroid hormone levels producing changes in vascular reactivity and endothelial function. The detection of endothelial dysfunction and/or arterial stiffness allows early identification of individuals at risk as these occur in both patients with risk factors for coronary artery disease and in those with established disease. This may allow treatment to be targeted at high risk individuals with the aim of slowing the progression of vascular disease. The various methods used to assess arterial function are reviewed and the changes demonstrated in human and animal models of thyroid dysfunction.

Subclinical hypothyroidism

PURPOSE OF REVIEW

Mild or subclinical hypothyroidism is characterized by normal serum free thyroxine concentrations with elevated serum thyroid-stimulating hormone concentrations. Subclinical hypothyroidism is relatively prevalent in the general population, especially among women and the elderly. The main cause of subclinical hypothyroidism is autoimmune chronic thyroiditis. The present report reviews the most important and recent studies on subclinical hypothyroidism, and discusses the most controversial aspects of this topic.

RECENT FINDINGS

Several studies have demonstrated that subclinical hypothyroidism may affect both diastolic and systolic cardiac function. It may also worsen many risk factors for cardiovascular disease, including hypertension, abnormal endothelial function, and elevated low-density lipoprotein cholesterol concentrations. Furthermore, a growing body of evidence suggests that subclinical hypothyroidism may cause symptoms or progress to symptomatic overt hypothyroidism.

SUMMARY

Prompt treatment of subclinical hypothyroidism in pregnant women is mandatory to decrease risks for pregnancy complications and impaired cognitive development in offspring. Children with subclinical hypothyroidism should be treated to prevent growth retardation. Whether nonpregnant adult patients with subclinical hypothyroidism should be treated, and at what thyroid-stimulating hormone values, is debatable.

Acute effects of triiodothyronine on endothelial function in human subjects

CONTEXT

Thyroid hormone regulates several cardiovascular functions, and low T(3) levels are frequently associated with cardiovascular diseases. Whether T(3) exerts any acute and direct effect on endothelial function in humans is unknown.

OBJECTIVE

Our objective was to clarify whether acute changes in serum T3 concentration affect endothelial function.

DESIGN, SETTING, AND SUBJECTS

Ten healthy subjects (age, 24 +/- 1 yr) participated in a double-blind, placebo-controlled trial at a university hospital.

INTERVENTIONS

T3 (or placebo) was infused for 7 h into the brachial artery to raise local T3 to levels observed in moderate hyperthyroidism. Vascular reactivity was tested by intraarterial infusion of vasoactive agents.

MAIN OUTCOME MEASURES

We assessed changes in forearm blood flow (FBF) measured by plethysmography.

RESULTS

FBF response to the endothelium-dependent vasodilator acetylcholine was enhanced by T3 (P = 0.002 for the interaction between T3 and acetylcholine). The slopes of the dose-response curves were 0.41 +/- 0.06 and 0.23 +/- 0.04 ml/dl x min/microg in the T3 and placebo study, respectively (P = 0.03). T3 infusion had no effect on the FBF response to sodium nitroprusside. T3 potentiated the vasoconstrictor response to norepinephrine (P = 0.006 for the interaction). Also, the slopes of the dose-response curves were affected by T3 (1.95 +/- 0.77 and 3.83 +/- 0.35 ml/dl x min/mg in the placebo and T3 study, respectively; P < 0.05). The increase in basal FBF induced by T3 was inhibited by NG-monomethyl-L-arginine.

CONCLUSIONS

T3 exerts direct and acute effects on the resistance vessels by enhancing endothelial function and norepinephrine-induced vasoconstriction. The data may help clarify the vascular impact of the low T3 syndrome and point to potential therapeutic strategies.

Usefulness of Pulsed-Wave Tissue Doppler Echocardiography for the Assessment of the Left and Right Ventricular Function in Patients with Clinical Hypothyroidism

Clinical hypothyroidism (HT) is often associated with cardiovascular disorders, such as endothelial and myocardial dysfunction. Previous studies have explored left ventricular (LV) function using pulsed-wave tissue Doppler echocardiography (TDE) in HT. However, no study has utilized this technique in the assessment of right ventricular (RV) function in HT. Accordingly, we investigated the effects of clinical HT on LV and RV function by TDE. The study subjects included 35 newly diagnosed HT patients and 32 healthy normal controls. For each subject, serum FT3, FT4, TT3, TT4, and thyroid stimulating hormone (TSH) levels were measured, and standard echocardiography and TDE were performed. No statistically significant difference was found between patients and controls with regard to age, gender, body mass index, heart rate, and blood pressure. Compared to controls, TSH levels were significantly higher, and TT4 and FT4 levels were significantly lower. TDE showed that patients had significantly lower early diastolic tricuspid annular velocity (Ea) and early/late (Ea/Aa) diastolic tricuspid annular velocity ratio (P < 0.001 and P < 0.001, respectively), and significantly longer isovolumetric relaxation time (P < 0.001) than those of the controls. Aa, Sa, isovolumetric contraction time, and ejection time did not significantly differ. In addition, a significant relationship between some TDE indexes, and thyroid hormones (TT4 and FT4) and TSH was observed. We showed that patients with clinical HT are associated with impaired RV diastolic function, in addition to impaired LV diastolic function using TDE.

Endothelial dysfunction in subjects with subclinical hyperthyroidism

Numerous clinical and experimental reports suggest that a high von Willebrand factor (vWf) level reflects endothelial damage or endothelial dysfunction. The present study was designed to evaluate vWf in subclinical hyperthyroidic subjects compared with euthyroidic subjects. We selected 20 subclinical hyperthyroidic subjects and 20 euthyroidic control subjects matched for age, gender and body mass index. The level of vWf was significantly higher in the subclinical hyperthyroidic group than in the euthyroidic group (42.9+/-9.6 vs 37.6+/-6.4%, p=0.026). In conclusion, our results suggest that subjects with subclinical hyperthyroidism tend to have an endothelial dysfunction. Endothelial dysfunction could contribute to increasing the cardiovascular risk in subclinical hyperthyroidism.

Does Treating Subclinical Hypothyroidism Improve Markers of Cardiovascular Risk?

Subclinical hypothyroidism is defined as an elevated serum thyroid-stimulating hormone (TSH) level in the face of normal free thyroid hormone values. The overall prevalence of subclinical hypothyroidism is 4-10% in the general population and up to 20% in women aged >60 years. The potential benefits and risks of therapy for subclinical hypothyroidism have been debated for 2 decades, and a consensus is still lacking. Besides avoiding the progression to overt hypothyroidism, the decision to treat patients with subclinical hypothyroidism relies mainly on the risk of metabolic and cardiovascular alterations. Subclinical hypothyroidism causes changes in cardiovascular function similar to, but less marked than, those occurring in patients with overt hypothyroidism. Diastolic dysfunction both at rest and upon effort is the most consistent cardiac abnormality in patients with subclinical hypothyroidism, and also in those with slightly elevated TSH levels (>6 mIU/L). Moreover, mild thyroid failure may increase diastolic blood pressure as a result of increased systemic vascular resistance. Restoration of euthyroidism by levothyroxine replacement is generally able to improve all these abnormalities. Early clinical and autopsy studies had suggested an association between subclinical hypothyroidism and coronary heart disease, which has been subsequently confirmed by some, but not all, large cross-sectional and prospective studies. Altered coagulation parameters, elevated lipoprotein (a) levels, and low-grade chronic inflammation are regarded to coalesce with the hypercholesterolemia of untreated patients with subclinical hypothyroidism to enhance the ischemic cardiovascular risk. Although a consensus is still lacking, the strongest evidence for a beneficial effect of levothyroxine replacement on markers of cardiovascular risk is the substantial demonstration that restoration of euthyroidism can lower both total and low-density lipoprotein-cholesterol levels in most patients with subclinical hypothyroidism. However, the actual effectiveness of thyroid hormone substitution in reducing the risk of cardiovascular events remains to be elucidated. In conclusion, the multiplicity and the possible reversibility of subclinical hypothyroidism-associated cardiovascular abnormalities suggest that the decision to treat a patient should depend on the presence of risk factors, rather than on a TSH threshold. On the other hand, levothyroxine replacement therapy can always be discontinued if there is no apparent benefit. Levothyroxine replacement therapy is usually safe providing that excessive administration is avoided by monitoring serum TSH levels. However, the possibility that restoring euthyroidism may be harmful in the oldest of the elderly population of hypothyroid patients has been recently raised, and should be taken into account in making the decision to treat patients with subclinical hypothyroidism who are aged >85 years.

Evaluation of right and left ventricular function using pulsed-wave tissue Doppler echocardiography in patients with subclinical hypothyroidism

Previous studies showed that subclinical hypothyroidism (SH) was associated with cardiovascular disorders, such as endothelial dysfunction, atherosclerosis and myocardial dysfunction. Only one study investigated left ventricular (LV) function using pulsed tissue Doppler echocardiography (TDE) in patients with SH. However, no study has used this technique in the identification of right ventricular (RV) function in these patients. We aimed to investigate the effect of SH on RV and LV function using TDE technique. The present study included 36 newly diagnosed SH patients and 28 healthy controls. For each subjects, serum free T3 (FT3), free T4 (FT4), total T3 (TT3), total T4 (TT4), TSH, peroxidase antibody (TPOab) and thyroglobulin antibody (TGab) levels were measured, and standard echocardiography and TDE were performed. In patients with SH, TSH levels were significantly higher, and TPOab and TGab levels were significantly higher when compared to healthy controls. TDE showed that the patients had significantly lower early diastolic mitral and tricuspid annular velocity (Ea) and early/late (Ea/Aa) diastolic mitral and tricuspid annular velocity ratio (p<0.05, p<0.05 and p<0.001, p<0.001, respectively), and significantly longer isovolumetric relaxation time (IRT) of left and right ventricles (p<0.001 and p<0.001, respectively). However, Aa, Sa, and isovolumetric contraction time (ICT) and ET (ejection time) of left and right ventricle did not significantly differ (p=ns for all). In addition, a negative correlation between TSH and TD-derived tricuspid Ea velocity and Ea/Aa ratio, and a positive correlation between TSH and IRT of right ventricle were observed. Our findings demonstrated that SH is associated with impaired RV diastolic function in addition to impaired LV diastolic function.