J. Endocrinol. Invest. 35: 96-103, 2012 DOI: 10.3275/8190

SHORT REVIEW

Endothelial dysfunction and subclinical hypothyroidism: A brief review S. La Vignera, R. Condorelli, E. Vicari, and A.E. Calogero Section of Endocrinology, Andrology and Internal Medicine and Master in Andrological, Human Reproduction and Biotechnology Sciences, Department of Internal Medicine and Systemic Diseases, University of Catania, Catania, Italy

traditional effects on vasculature. In particular, SH is associated with increased of LDL-cholesterol, diastolic blood pressure, and markers of chronic inflammation (C reactive protein) and simultaneously reduces the bioavailability of nitric oxide to blood vessels and increases the expression of angiotensin receptor. Furthermore, replacement therapy seems to improve all these aspects. (J. Endocrinol. Invest. 35: 96-103, 2012) ©2012, Editrice Kurtis

ABSTRACT. Subclinical hypothyroidism (SH) is characterized by normal serum free T4 and free T3 levels and increased serum TSH levels. The relationship between SH and cardiovascular diseases has been one of the most popular topics recently. There is still some controversy concerning the cardiovascular impact of SH and management protocols. The vast majority of the studies published so far, suggests that SH accelerates endothelial dysfunction through traditional effects on risk factors that promote atherosclerosis and non-

INTRODUCTION

and T4 on the arterioles of hamster cheek pouch microcirculation in vivo (in this evaluation microvessels were visualized using a fluorescent microscopy technique). Topical application of T3 consistently induced dose-dependent dilation of arterioles within 2.0±0.5 min of administration. The application of T4 (150, 257, 514, and 5,140 nM/l) caused different dose-dependent effects: dilation at the 3 lower doses within 16±2 min and rhythmic diameter changes at the highest dose. In particular, T3-induced dilation was countered by the inhibition of NOS with N(G)-nitro-L-arginine-methyl ester or N(G)-nitro-L-arginine (5). In another evaluation, T3 (or placebo) was infused for 7 h into the brachial artery to raise local T3 to levels observed in moderate hyperthyroidism and vascular reactivity was tested by intra-arterial infusion of vasoactive agents. In this study, the authors assessed changes in forearm blood flow (FBF) measured by plethysmography. Results of this study showed that FBF response to the endothelium-dependent vasodilator acetylcholine (Ach) was enhanced by T3 (p=0.002 for the interaction between T3 and Ach). The slopes of the doseresponse curves were 0.41±0.06 and 0.23±0.04 ml/dl × min/μg in the T3 and placebo study, respectively (p=0.03). T3 infusion had no effect on the FBF response to sodium nitroprusside, in addition, 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 × min/mg in the placebo and T3 study, respectively; p<0.05). Finally, the increase in basal FBF induced by T3 was inhibited by NG-monomethyl-L-arginine (L-NMMA) (6). In another study, NOS activity in the heart (left and right ventricles), vessels (aorta and cava) and kidney (cortex and medulla) of euthyroid, hyperthyroid, and hypothyroid rats after 6 weeks of treatment, was measured and determined by measuring the conversion of L-[(3)H]arginine to L-[(3)H]-citruline. Results of this study showed that NOS activity was higher in all tissues from hyperthy-

Thyroid hormone has many effects on the heart and vascular system and it is clear from many invasive and noninvasive measurements in patients with thyroid disease that cardiac functions such as heart rate, cardiac output, and systemic vascular resistance are closely linked to thyroid status. T3 (the active cellular form of thyroid hormone) decreases systemic vascular resistance by dilating the resistant arterioles of the peripheral circulation (1). In particular, the endothelium and smooth muscle cells are a biological target of action for thyroid hormones with vasodilatory effect, even in coronary arteries (2). T3 in culture induces relaxation of smooth muscle cells with nongenomic effect and independently of nitric oxide (NO) production (3). Relatively to this aspect, Ojama et al. to determine the molecular mechanisms for the vasoactive properties of T3, studied primary cultures of aortic endothelial and vascular smooth muscle cells and results of this study showed that exposure to T3 resulted in cellular relaxation within 10 min and primary cultures of vascular endothelial cells exposed to T3 showed no NO production as measured by cellular cyclic GMP content and nitrite release, suggesting that T3 acted directly on the vascular smooth muscle cell to cause vascular relaxation (4). However, the response in vivo (vasodilation) is delayed and depends on the activation of NO synthase (NOS) (5). The aim of one of the main studies that showed this evidence was to assess the effects of topically applied T3

Key-words: Atherosclerosis, endothelial dysfunction, subclinical hypothyroidism. Correspondence: S. La Vignera, MD, Sezione di Endocrinologia, Andrologia e Medicina Interna, Dipartimento di Medicina Interna e Patologie Sistemiche, Università di Catania, Policlinico “G. Rodolico”, Via S. Sofia 78, Edificio 4, Stanza 2C19, 95123 Catania, Italy. E-mail: [email protected] Accepted November 24, 2011. First published online December 16, 2011.

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S. La Vignera, R. Condorelli, E. Vicari, et al. Table 1 - Different methods for assessing endothelial function in humans.

roid rats when compared with controls, except in the right ventricle. In the hypothyroid group, NOS activity showed a more heterogeneous pattern, with significant increases in both ventricles but significant reduction in the aorta, while in the vena cava, renal cortex, and medulla the enzyme activity also tended to be higher, but significance was not reached (7). Interestingly, T4 is converted into T3 at this level, through the action of deiodase type 2 constitutively expressed in vascular smooth muscle cells (8). This evidence suggests that the intracellular activation of thyroid hormones may be involved in the transcription of target genes coding for proteins that are crucial in maintaining vascular homeostasis, such as the gene for angiotensin II (9), or genes coding for growth factors responsible for the migration of smooth muscle cells with angiogenic effect, as observed in coronary arteries (2). Subclinical hypothyroidism (SH) is characterized by normal serum free T4 (FT4) and free T3 (FT3) levels and increased serum TSH levels. The relationship between SH and cardiovascular (CV) diseases has been one of the most popular topics recently and there is still some controversy concerning the CV impact of SH and management protocols (10). Endothelial dysfunction, which is an early step of atherosclerosis, has been reported in patients with SH (11). It is known that mild hypothyroidism accelerates atherogenesis through modification of atherosclerotic risk factors and direct effects on the blood vessels of thyroid hormone replacement therapy may help to prevent atherosclerosis in this group of patients (11). On the basis of these premises, this review summarizes the main evidence found in the literature through PubMed with the following keywords “endothelial dysfunction and subclinical hypothyroidism”. Table 1 shows the significance of the main methods used for the study of endothelial function.

Intracoronary infusions of vasoactive agents A doppler tipped guidewire is placed in the proximal segment of a coronary artery, through a 6-catheter, and the doppler flow velocity is continuously recorded. Acetylcholine (dilates normal coronary arteries in the presence of intact endothelium, through a receptor-mediated stimulation of nitric oxide (NO) production by endothelial cells) is infused in the coronary artery at increasing rates. Coronary blood flow velocity is measured by an on-line spectral analyzer and recorded on tape. Intrabrachial infusion of vasoactive agents A cannula is inserted into the brachial artery under local anesthesia and is connected through stopcocks to a pressure transducer for simultaneous monitoring of systemic mean blood pressure and heart rate. Forearm blood flow (FBF) is measured by gauge-strain plethysmography (A plethysmograph is an instrument for measuring changes in volume within an organ or whole body). After the determination of baseline FBF, acetylcholine is infused in the brachial artery with a gradually increasing infusion rate. FBF under acetylcholine infusion is measured as the average of at least 3 consecutive steady-state measurements at the end of each infusion period. High resolution ultrasound: assessment of endothelium dependent flow mediated vasodilation The vasodilatory response of the brachial artery to increased shear stress is called flow mediated dilation (FMD), and reflects the ability of vascular endothelium to produce NO. The brachial artery is imaged above the antecubital fossa in the longitudinal plane, by using a linear array transducer (with frequency 7-12 MHz) attached to a high quality mainframe ultrasound system. The diameter of the brachial artery is initially determined at rest, and blood flow is estimated by time averaging the pulsed doppler velocity signal obtained from a mid artery sample volume. The diameter of the brachial artery is determined manually with electronic calipers or automatically using edge detection software. After baseline brachial artery diameter determination, ischemia is produced by inflating a cuff placed at the distal forearm, at a pressure 50 mmHg greater than the systolic blood pressure. The release of the ischemia cuff after 5 min leads to an increase in FBF, resulting in a vasodilatory effect on the brachial artery. The maximum blood flow velocity is detected by analysing mid artery pulsed doppler signal immediately after or up to 15 sec after cuff release, while the maximum diameter of the brachial artery is determined approximately 60 sec after release or 45-60 sec after the peak hyperemic flow. Gauge-strain plethysmography (evaluation of reactive hyperemia) The technique evaluates the percentage change of flow from baseline to the maximum flow during reactive hyperemia following a 5-min ischemia of the distal forearm. Several studies suggest that endogenous NO plays only a minor role in vasodilation during reactive hyperemia, and that reactive hyperemia is largely caused by endothelium related mechanisms other than NO, such as adenosine, prostaglandins, and endothelium-derived hyperpolarising factor.

POSITIVE EVIDENCES Observational studies The following are the studies that show a worst endothelial function in relation to thyroid basal function and in particular to serum TSH levels. These studies are arbitrarily defined as “observational” in this review. In one of the first studies, conducted by Lekakis et al., 35 subjects with various TSH levels were investigated by high-resolution ultrasound imaging of the brachial artery to assess endothelial and smooth muscle responses. Results of this study showed that flow-mediated, endothelium-dependent vasodilation (EDV) was significantly higher in subjects with TSH 0.4-2 μIU/ml (11.8±2.7%), compared with subjects with TSH 2.01-4 μIU/ml (6.8±2.9%), 4.01-10 μIU/ml (5.2±6.3%) and >10 μIU/ml (4.0±4.4%), and TSH levels correlated inversely to endothelium-dependent dilatation (12). Also, Shavdatuashvili evaluated the endothelial function and in addition the lipoprotein profile in patients with overt hypothyroidism, SH, and euthyroid subjects and assessed the effects of these conditions on endothelial function, showing that overt hypothyroid patients have significantly higher total cholesterol, LDL cholesterol (LDL-C), and triglycerides levels

than controls, which correlated positively with TSH and inversely with FT4 levels. Moreover, SH patients had less marked changes, but significant positive relationships were found between serum TSH and total and LDL-C levels. Finally, flow-mediated, EDV was significantly higher in control group compared with patients with SH and/or overt hypothyroidism and TSH levels correlated inversely with endothelium-dependent dilatation (13). In another study, Xiang et al. investigated the alteration of endothelial function in Hashimoto’s thyroiditis patients with euthyroidism. Study subjects included 28 female Hashimoto’s thyroiditis patients with euthyroidism, 23 female Hashimoto’s thyroiditis patients with hypothyroidism, and 22 healthy women. High-resolution ultrasound was used to measure brachial artery diameter at rest, after reactive hyperemia and after sublingual glyc-

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eryltrinitrate (GTN). Results of this study showed that flowmediated arterial dilation in Hashimoto’s thyroiditis patients with euthyroidism was significantly lower (3.88%) than in controls (4.98%, p=0.000) and higher than in Hashimoto’s thyroiditis patients with hypothyroidism (3.26%, p<0.001). Moreover arterial FMD among Hashimoto’s thyroiditis patients with hypothyroidism was significantly lower than in controls (p=0.000). In addition GTN-induced arterial dilation, baseline vessel size, and baseline blood flow were not significantly different among the three groups (p>0.05). Finally, on multiple regression analysis, anti-thyroid peroxidase antibody (TPOAb), TSH, FT3, LDL-C, and lipoprotein (a) [Lp(a)] were found to be significant factors associated with endothelium-dependent arterial dilation (14). Then, in another clinical model, Biondi et al. evaluated the endothelial response of coronary flow in young and middle-aged patients with SH, without associated CV risk factors compared with healthy control (HC) subjects; in this study, the authors evaluated 20 women with newly diagnosed, untreated, and persistent SH due to Hashimoto’s thyroiditis and 15 volunteers served as controls. The results of this study showed that coronary diastolic peak velocities at rest did not differ between the 2 groups but were significantly lower after cold pressure test in patients with SH, thereby resulting in a lower coronary flow reserve and the difference did not vary significantly after adjusting resting and cold pressure test velocities for the respective mean blood pressures and TSH was inversely correlated with coronary flow reserve in the pooled population (15). In another clinical study on 37 patients with SH (29 women, 8 men) and 23 healthy volunteers (19 women, 4 men), endothelial dysfunction was measured by examining brachial artery responses to endotheliumdependent and endothelium-independent stimulation with sublingual nitroglycerin (NTG), in addition serum tumor necrosis factor α (TNFα), interleukin-6 (IL-6), high sensitivity C-reactive protein (hs-CRP), and homeostasis model assessment of insulin resistance score were measured. Results of this study showed that there was a statistically significant difference in endothelium-dependent and endothelium-independent vascular responses (NTG) between the patients with SH and the normal HC (10); moreover, TSH, LDL, IL-6, TNFα, and hs-CRP levels in the patient group were significantly higher than those in control group. In addition, a positive correlation was found only between EDV and TNFα, hs-CRP, IL-6, TSH, total cholesterol, LDL, and triglycerides. Then, endothelium-independent vascular response was not correlated with any of the metabolic or hormonal parameters and neither of the groups were insulin resistant and there was not any difference either in fasting insulin or in glucose levels. In addition, endothelium-dependent and endothelium-independent vascular responses (NTG) were lower in patient group (10). Finally, in a series of 217 non diabetic patients with chronic kidney disease at stage 34, a condition frequently associated with low T3 serum levels and endothelial dysfunction, the plasma concentration of FT3 was closely associated with flow mediated dilation (FMD) (r=0.38; p<0.001) and FT3 was also inversely associated with plasma concentration of the endogenous inhibitor of NOS, asymmetric dimethylarginine

(r=–0.18; p=0.007) (16). Table 2 summarizes the main observational studies conducted. Interventional studies with levothyroxine The following studies showed an improvement of endothelial function after pharmacological normalization of thyroid function in SH patients. These studies are arbitrarily defined as “interventional” in this review. To test the hypothesis that patients with SH are characterized by endothelial dysfunction and impaired NO availability, the FBF (strain-gauge plethysmography) response to intrabrachial Ach, an endothelium-dependent vasodilator, at baseline and during infusion of L-NMMA, a NOS inhibitor, was studied in 14 patients (serum cholesterol 218±41 mg/dl) and 28 euthyroid subjects, subdivided into groups A and B (serum cholesterol, 170±19 mg/dl and 217±21 mg/dl, respectively). The response to sodium nitroprusside and minimal forearm vascular resistances were also evaluated. In SH patients, vasodilation to Ach was significantly reduced, compared with groups A and B. L-NMMA blunted significantly the vasodilation to Ach in groups A and B, whereas it was ineffective in SH patients. The response to sodium nitroprusside and minimal vascular resistances were similar. In SH patients, 6 months of euthyroidism by T4 administration increased Ach-vasodilation and restored L-NMMA inhibition (17). In another clinical evaluation, carotid artery intima-media thickness (IMT) and lipoprotein profile were evaluated in 45 SH patients at baseline and after 6 months of randomized, placebo-controlled LT4 replacement and treatment significantly reduced both total and LDL-C (p<0.0001 for both) and mean-IMT (by 11%, p<0.0001). The decrement in IMT was directly related to the decrements of both total cholesterol and TSH (p=0.02 and p=0.0001, respectively) (18). Another study investigated the alteration of plasma osteoprotegerin (OPG) concentrations before and after T4 replacement therapy, and its association with endothelium-dependent arterial dilation in patients with SH. The study included 20 women with overt hypothyroidism, 20 women with SH, and 20 healthy women. All patients were then given T4 to maintain FT3, FT4, and TSH serum levels near or within the respective normal ranges. Plasma OPG levels in overt hypothyroidism and SH patients before treatment were significantly higher than that in controls. In multivariate analysis, OPG was significantly associated with TSH (r=0.306, p<0.05) and endothelium-dependent arterial dilation (r=–0.675, p<0.01) at baseline. After normalization of thyroid function, OPG levels in both groups decreased significantly reaching values very close to that in controls (19). Also Xiang et al., evaluated the alteration of plasma OPG concentration before and after T4 replacement therapy and its association with endotheliumdependent arterial dilation in patients with overt hypothyroidism or SH. In this study, T4 therapy was given to 20 patients with overt hypothyroidism and to 20 SH patients, all female, till FT3, FT4, and TSH were near or within the respective normal ranges. Plasma OPG levels before treatment were 3.13±0.27 ng/l and 2.95±0.24 ng/l in patients with overt hypothyroidism or SH, respectively. Both concentrations were significantly higher than that found in controls (2.42±0.26 ng/l). Multivariate analysis

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S. La Vignera, R. Condorelli, E. Vicari, et al. Table 2 - Endothelial dysfunction and subclinical hypothyroidism (SH): main positive evidence. Author

Assessment of endothelial function

Main finding

Observational studies 1

Lekakis et al, 1987

Flow mediated dilation (FMD)

TSH levels correlated inversely with FMD

2

Quesada et al, 2002

Nitric oxide sinthase (NOS) activity

NOS activity showed a more heterogeneous pattern in SH patients

3

Shavdatuashvili et al, 2005

FMD

TSH levels correlated inversely with FMD

4

Kosar et al, 2005

Doppler echocardiography (TDE) technique

TDE showed that the SH patients had lower early diastolic mitral and tricuspid annular velocity and early/late diastolic mitral and tricuspid annular velocity ratio, and significantly longer isovolumetric relaxation time

5

Owen et al, 2006

Left ventricular echocardiographic function

Baseline augmentation gradient was elevated in patients with SH compared with controls

6

Taddei et al, 2006

Forearm blood flow (FBF) (strain-gauge plethysmography) response to intrabrachial acetylcholine (Ach)

In SH, the response to Ach was reduced in comparison with controls

7

Xiang et al, 2006

Brachial artery diameter at rest, after reactive hyperemia and after sublingual glyceryltrinitrate

FMD among Hashimoto's thyroiditis patients with hypothyroidism was significantly lower

8

Oflaz et al, 2007

TDE

Coronary flow reserve in patients with SH as well as in patients with overt hypothyroidism was lower than that of euthyroid subject

9

Biondi et al, 2009

Coronary flow reserve

TSH was inversely correlated with coronary flow reserve

10

Turemen et al, 2011

Brachial artery responses to endothelium-dependent and endothelium-independent stimulation

FMD and endothelium-independent vascular responses were lower in SH patients

11

Yilmaz et al, 2011

FMD

Free T3 was inversely associated with plasma concentration of the endogenous inhibitor of NO synthase, asymmetric dimethylarginine

Interventional studies 1

Ojama et al, 2002

Primary cultures of aortic endothelial and vascular smooth muscle cells

Exposure to T3 showed no nitric oxide production

2

Taddei et al, 2003

FBF response to intrabrachial acetylcholine

In SH patients, 6 months of euthyroidism by T4 administration increased acetylcholine-vasodilation

3

Monzani et al, 2004

Carotid artery intima-media thickness (IMT)

LT4 treatment significantly reduced mean-IMT

4

Colantuoni et al, 2005

Arterioles of hamster cheek pouch microcirculation in vivo

T3-induced dilation was countered by the inhibition of nitric oxide synthase

5

Guang-Da X et al, 2005

FMD and osteoprotegerin (OPG) levels

OPG was significantly associated with FMD and after normalization of thyroid function, OPG levels decreased significantly

6

Napoli et al, 2007

FBF

FBF response to the endothelium-dependent vasodilator acetylcholine was enhanced by T3

7

Akinci et al, 2007

Serum sCD40L and high sensitivity C reactive protein (hs-CRP)

SH patients showed a significant reduction in hs-CRP and serum sCD40L levels after T4 administration

8

Razvi et al, 2007

FMD

L-T4 treatment improved FMD

9

Xiang et al, 2007

FMD and osteoprotegerin levels

After the normalization of thyroid function, OPG levels decreased respectively in overt hypothyroidism and SH

10

Dagre et al, 2007

FBF response during reactive hyperemia utilizing venous occlusion strain-gauge plethysmography

The duration of reactive hyperemia in treated group with LT4 did not differ significantly from controls

11

Adrees et al, 2009

Carotid IMT and brachial artery ultrasound

After T4 treatment, carotid artery baseline diameter increased and carotid IMT decreased, while brachial artery diameter increased basally and following endothelium-dependent vasodilatation

12

Xiang GD et al, 2009

FMD

After 6 months of regular aerobic exercise, there was a remarkable increase in FMD

13

GDX et al, 2010

FMD

FMD improved markedly in α-lipoic acid treated patients

14

Shakoor et al, 2010

Endothelial progenitor cells (EPC)

EPC are reduced in SH and after treatment with thyroid hormone there is an significant increase in EPC

15

Alibaz et al, 2011

Baseline and nitroglycerin-induced diameter of brachial artery

Baseline and nitroglycerin induced diameter values as well as FMD, increased after T4

markedly reaching 2.53±0.28 ng/l and 2.54±0.21 ng/l, respectively in overt hypothyroidism and SH, reaching values very close to those found in controls. The absolute changes of OPG correlated positively with the

showed that OPG was significantly associated with TSH (r=0.306, p<0.05) and endothelium-dependent arterial dilation (r=–0.675, p<0.01) at baseline. After the normalization of thyroid function, OPG levels decreased

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study, 56 women with SH were evaluated before and after T4 replacement for 18 months. Carotid IMT, brachial artery ultrasound (for endothelial evaluation) blood pressure, plasma lipids, and homocysteine were evaluated before and after T4 adiministration. The results of this study showed that systolic and diastolic blood pressure, total cholesterol, triglyceride, LDL-C, Lp(a), and homocysteine were greater in SH. Following T4 replacement, these parameters decreased to levels that no longer differed from normal subjects. Moreover after T4 treatment, carotid artery baseline diameter increased by 7.1% and carotid IMT decreased by 13%, while brachial artery diameter increased basally by 12.5% and following EDV by 17.5% (24). Recently in another study conducted in 27 patients with SH and 22 HC, the endothelial function was evaluated using brachial artery doppler ultrasonography and after restorating euthyroidism, measurements were repeated. In this evaluation baseline and NTG-induced diameter (NID) of brachial artery were similar in patients with SH and the control group. In addition, compared with the control group, the patients with SH showed significantly reduced flow-mediated diameter. Finally, baseline and NID values, as well as flow-mediated diameter, increased significantly after T4 therapy in SH group (11). Table 2 summarizes the main interventional studies conducted.

changes of TSH, negatively with the changes of endothelium-dependent arterial dilation. No significant correlation was found with other parameters in hypothyroid patients during treatment (20). Akinci et al. assessed the lipid profile, serum sCD40L (a protein expressed mainly by activated platelets which have been found to be associated with CV events), and hs-CRP levels in 21 overt and 22 SH age-matched female patients with chronic autoimmune thyroiditis at baseline and 1 month after achieving euthyroidism by T4 replacement, and compared them with the data from 22, age-matched, HC. Overt and subclinical hypothyroid patients showed decreased sCD40L levels compared to age-matched controls. The patients with SH showed slightly increased hsCRP levels, but the result was not statistically significant. In multiple regression analysis, FT3 and FT4 were found to be independent predictors of sCD40L levels. After T4 replacement, serum sCD40L levels increased significantly in patients with overt hypothyroidism. Although an increase was also observed in patients with SH, it was not statistically significant. T4 replacement showed no significant effect on hs-CRP levels in patients with overt hypothyroidism. However, SH patients showed a significant reduction in hs-CRP levels after T4 administration (21). Razvi et al. performed another clinical study of intervention, on 100 patients with SH (intervention consisted of 100 μg LT4 or placebo daily for 12 weeks each). In this study L-T4 treatment reduced total cholesterol (vs placebo) from 231.6 to 220 mg/dl, p<0.001; LDL-C from 142.9 to 131.3 mg/dl, p<0.05; waist to hip ratio from 0.83 to 0.81, p<0.006; and improved FMD from 4.2 to 5.9%, p<0.001. Moreover multivariate analysis showed that increased serum FT4 level was the most significant variable predicting reduction in total cholesterol or improvement in FMD (22). Dagre et al. assessed non-invasively NO-dependent endothelial function of resistance arteries in subjects with hypothyroidism of varying severity. Ninety-six female subjects were divided into 5 groups based on TSH levels at presentation: Group 0 (no.=23) with TSH: 0.32.0 μU/ml, Group 1 (no.=22) with TSH: 2.1-4.0 μU/ml (upper normal), Group 2 (no.=18) with TSH: 4.1-10 μU/ml (SH), Group 3 (no.=22) with TSH >10 μU/ml (overt hypothyroidism). One additional group with well-controlled hypothyroidism on T4 replacement therapy (Group 4, no.=11, TSH: 0.3-2.0 μU/ml) was also studied. Endothelial function of resistance arteries was assessed by measuring FBF response during reactive hyperemia utilizing venous occlusion strain-gauge plethysmography. Duration of reactive hyperemia was significantly different among groups of subjects with varying hypothyroidism. It was significantly shorter in subjects with upper normal TSH values (Group 1) compared with controls, while it was comparable to that of patients with SH (Group 2). However, the duration of reactive hyperemia in Group 1 was significantly longer compared to Group 3 (overt hypothyroidism). Similarly, the duration of reactive hyperemia in patients with SH was significantly longer compared to subjects with overt hypothyroidism. The duration of reactive hyperemia in group 4 did not differ significantly from controls. There was a highly significant linear correlation between duration of reactive hyperemia and TSH (r=–0.383, p<0.001) (23). In another clinical

Particular conditions Articles on the other methods (echocardiography) or novel markers of CV risk [endothelial progenitor cells (EPC)] and less usual clinical models [effects of cyclooxygenase (COX) inhibitors, the role of oxidative stress, the benefits of physical activity, e.g.] are presented in this section. Endothelial dysfunction, subclinical hypothyroidism, and ecocardiography Kosar et al. investigated the effect of SH on the right ventricular (RV) and left ventricular (LV) function using tissue doppler echocardiography (TDE) technique. The study included 36 newly diagnosed SH patients and 28 HC. 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, and significantly longer isovolumetric relaxation time (IRT) of RV and LV. However, Aa, Sa, and isovolumetric contraction time and ejection time of RV and LV did not differ significantly. 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 RV were observed (25). In another study, indices of vascular stiffness and LV echocardiographic function were measured. In particular, the authors evaluated 19 female SH patients without CV disease and 10 female controls. The results of this study showed that baseline augmentation gradient was elevated in patients with SH compared with controls; when the euthyroid state was achieved, it decreased significantly. Heart ratecorrected augmentation index was 26.7±9.9 vs 18.8±9.9% (p<0.02) and decreased significantly to 19.7±9.6% after treatment. Time of travel of the reflected wave was significantly higher (139.3±11.7 msec) compared with controls (141.5±8.8 msec) and increased sig-

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Endothelial dysfunction, subclinical hypothyroidism, and oxidative stress Oxidative stress may be partially responsible for endothelial dysfunction in patients with SH and a recent study evaluated whether the antioxidant α-lipoic acid could improve endothelial dysfunction in these patients (30). In this study, 40 women with newly diagnosed SH and 18 healthy women with euthyroid status were enrolled and randomized into 2 groups to receive no treatment (no.=20), α-lipoic acid (no.=20) for 3 weeks. FMD was measured at baseline and after 3 weeks and FMD in αlipoic acid and no-treatment group were 3.92% and 4.02%, respectively, which resulted significantly lower than that in controls (5.64%). After 3-week treatment, plasma thiobarbituric acid reactive substances (TBARS) levels decreased significantly in α-lipoic acid group compared with before-treatment levels in SH patients, and remained unchanged in no-treatment group; in addition FMD improved markedly (4.82%) in α-lipoic acid-treated patients and did not change in no-treatment group; finally, the absolute changes in FMD showed significant negative correlation with the changes in TBARS (r=–0.773, p<0.001) (30).

nificantly (144.9±11.9 msec) (26). Oflaz et al. 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 SH. In this study, coronary flow reserve in patients with SH as well as in patients with overt hypothyroidism was lower than that of euthyroid subject (27). Endothelial dysfunction, subclinical hypothyroidism, and cyclooxygenase inhibitors Taddei et al. evaluated the role of low-grade systemic inflammation in the pathogenesis of endothelial dysfunction in patients with SH and autoimmune thyroiditis. The authors evaluated 53 patients with SH and 45 healthy subjects and studied the FBF (strain-gauge plethysmography) response to intrabrachial Ach (0.1515 μg/min.dl) with and without local vascular COX inhibition by intrabrachial indomethacin (50 μg/min.dl) or NOS blockade by L-NMMA (100 μg/min.dl) or the antioxidant vitamin C (8 mg/min.dl). The protocol was repeated 2 h after systemic non-selective COX inhibition (100 mg indomethacin) or selective COX-2 blockade (200 mg celecoxib) oral administrations. SH patients showed higher CRP and IL-6 values. In controls, vasodilation induced by Ach was blunted by L-NMMA and unchanged by vitamin C. In contrast, in SH, the response to Ach, reduced in comparison with controls, was resistant to L-NMMA and normalized by vitamin C. In these patients, systemic but not local indomethacin normalized vasodilation to Ach and the inhibition of LNMMA on Ach. Similar results were obtained with celecoxib. When retested after indomethacin administration, vitamin C no longer succeeded in improving vasodilation to Ach in SH patients. Response to sodium nitroprusside was unchanged by indomethacin or celecoxib (28).

Endothelial dysfunction, subclinical hypothyroidism, and endothelial progenitor cells EPC, expressing both endothelial and stem cell markers, are known to offer a novel CV risk marker. Shakoor et al. evaluated whether EPC count or function is reduced in SCH and whether it improves with T4 therapy (31). In this study EPC were studied in peripheral blood before and after T4 together with CV risk factors in 20 SH and HC and results showed that EPC count was significantly reduced in SH compared to HC: median (range) of phenotype CD133+/VEGFR-2+ was 0.09 (0.02-0.44) vs 0.47 (0.17-2.12), p<0.001; and median (range) of other phenotype examined CD34+/VEGFR-2+ was 0.10 (0.04-0.46) vs 0.39 (0.11-2.13), p<0.001. In addition, there was a significant positive correlation between phenotype CD133+/VEGFR-2+ with FT4 levels (r=0.38; p=0.02); HDL cholesterol (HDL-C) levels (r=0.51; p=0.001); and negative correlation with TSH concentrations (r=–0.64; p<0.001). After adjustment for conventional CV risk factors, SH predicted lower EPC count. In SH participants, EPC count increased and was similar to HC after T4, in particular median (range) of the phenotype CD133+/VEGFR-2+ was 0.32 (0.03-0.94) vs 0.09 (0.020.44), p<0.001; and of phenotype CD34+/VEGFR-2+ was 0.26 (0.06-0.88) vs 0.10 (0.04-0.46), p<0.001 (31).

Endothelial dysfunction, subclinical hypothyroidism, and aerobic exercise Regular aerobic exercise improves FMD in SH patients and changes in lipids and inflammation during periods of physical activity may partially contribute to the improvement of endothelial function. In fact, Xiang et al. evaluated 30 sedentary women with SH and 27 sedentary healthy euthyroid women and all individuals participated to exercise training of 6 months (29). Before and after exercise training, high-resolution ultrasound was used to measure FMD. At baseline, FMD among subjects with SH was 3.87%, which was significantly lower than that in controls (5.98%) and after 6 months of exercise, there was a remarkable increase in FMD (31.3%) and VO2 max (36.7%), in addition a significant decreases in total cholesterol (20%), LDL-C (29%), triglycerides (47.6%), and CRP (61.5%) were observed in patients with SH. Moreover, the absolute changes in FMD showed significant correlation with changes in LDL (r=–0.596), triglycerides (r=–0.532), and CRP (r=–0.511), and multiple regression analysis showed that changes of these compounds were significant determinants of FMD changes in SH patients during physical activity (29).

Endothelial dysfunction, subclinical hypothyroidism, and elderly patients Similar (although mild) CV abnormalities such as atherosclerosis resulting from hypertension associated to atherogenic lipid profile, increased circulating CRP and homocysteine, increased arterial stiffness, endothelial dysfunction, and altered coagulation parameters present in overt hypothyroidism are present also in SH (32, 33). The CV benefits of replacement therapy in overt hypothyroidism are not questionable, independently from the patient’s age (33); on the contrary, no consensus has

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blood pressure and markers of chronic inflammation (CRP) and simultaneously reduces the bioavailability of NO to blood vessels and increases the expression of angiotensin receptor. Furthermore, replacement therapy seems to improve all these aspects.

been reached so far about the actual CV and/or general health impact of SH (32). In SH there is a slight but significant increase of coronary heart disease (32, 33); moreover, L-T4 therapy is discouraged in aged subjects, because the increased oxygen consumption consequent to thyroid hormone administration could be dangerous, especially in the presence of coexisting ischemic heart disease (32). In keeping with this concept are recent data showing reduced mortality risk in untreated mild hypothyroid subjects aged >85 yr, suggesting that some degree of decreased thyroid activity at the tissue level might have favorable effects in the oldest-old (32).

REFERENCES 1. 2.

3.

NEGATIVE EVIDENCES

4.

Some studies showed that minimal thyroid dysfunction has no adverse effects on endothelial function in the population studied. The study of Duman et al. compared the effects of simvastatin vs T4 administration on lipid profile and endothelial function in patients with SH (34). In detail, 59 patients with newly diagnosed SH were enrolled and randomized into 3 groups to receive no treatment (no.=19), T4 (no.=20), or simvastatin (no.=20). EDV and endothelium-independent vasodilation were measured at baseline and after 8 months. Results of this study showed that, serum total cholesterol, triglycerides, and LDL-C were significantly lower after simvastatin administration and EDV increased significantly in the simvastatintreated group; moreover, the improvement of EDV correlated with the percent decrease of LDL-C (r=0.68, p<0.01). Although T4 administration caused a trend towards an increase in EDV compared to baseline, statistical significance was not achieved. EDV remained unchanged in all 3 groups. Therefore, the results of this study showed that simvastatin but not T4 treatment significantly improves EDV of the brachial artery and dyslipidemia in patients with SH (34). Cabral et al. evaluated the endothelial function measured by the FMD of the brachial artery and the carotid artery IMT in a group of women with SH compared with euthyroid subjects. In detail, 21 patients with SH and 21 euthyroid controls matched for body mass index, age, and atherosclerotic risk factors participated to the study. In this evaluation, triglycerides, total cholesterol, HDL-C, LDL-C, apoprotein A (apo A), apo B, and Lp(a) were also determined. Results of this study showed that lipid parameters (except HDL-C and apo A, which were lower) and IMT values were higher in the common carotid and carotid bifurcation of SH patients with positive serum TPO-Ab when compared with the negative TPO-Ab group, but the difference was not statistically significant compared to euthyroid subjects (35).

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CONCLUSION The vast majority of the studies so far published suggests that SH accelerates endothelial dysfunction through traditional effects on risk factors that promote atherosclerosis and non-traditional effects on vasculature. In particular SH is associated with increased of LDL-C, diastolic

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