Evaluation of endothelial function in exogenous subclinical hyperthyroidism and the effect of treatment.

BACKGROUND: Subclinical hyperthyroidism (SHy) is a widespread condition in which cardiovascular manifestations are frequently occur, but there is still a debate about the vascular responsiveness in it. Measuring flow-mediated dilation (FMD) and intimae-media thickness (IMT) are used to evaluate endothelial function in these patients.
MATERIALS AND METHODS: Twenty-five patients with a diagnosis of exogenous SHy and 25 full matched healthy subjects were enrolled. At first FMD of brachial artery and IMT of common carotid artery were obtained from all the participants. In the second phase, in the second phase of study, the dosage of levothyroxine was reduced at least 25% of prior dosage, and this was continued until thyroid stimulating hormone became normal range. Measuring FMD and IMT was repeated after this intervention in the case group.
RESULTS: The mean age of case and control groups were 38.48 ± 12.05 and 36.72 ± 11.15 years, respectively. The mean of FMD in healthy people was dramatically higher than the subclinical hyperthyroid patients (P < 0.001) but no statistically significant difference was found for IMT (P = 0.459). After intervention in the case group, FMD was meaningfully increased (P < 0.001) but IMT of common carotid artery was not considerably changed (P = 0.491).
CONCLUSIONS: This study demonstrated that FMD decreased in exogenous subclinical hyperthyroid patients which could be partially restored by treatment. These findings suggest that treatment of subclinical hyperthyroid state could improve endothelial dysfunction and at the end decreased the cardiovascular complications.

INTRODUCTION

Subclinical hyperthyroidism (SHy) is a widespread condition which is defined as suppressed thyroid stimulating hormone (TSH) concentration while triiodothyronine (T3) and free thyroxin (FT4) concentrations are within normal limits.[1] Etiology of SHy is divided into endogenous and exogenous causes that exogenous sources include 58% of all causes.[23] Although all symptoms of hyperthyroidism could be seen in SHy with less severity, but most of the patients do not show obvious clinical manifestations.[4] For this frequently occurring condition, exact therapeutic approaches have not yet been established.

Cardiovascular manifestations are well-known components of hyperthyroidism. Hyperthyroidism is associated with increased cardiac output and heart rate, systolic hypertension, reduction in peripheral vascular resistance,[56] and impaired vascular elasticity of large and small arteries,[7] but there is still a debate about the vascular responsiveness in SHy.

Recent studies indicate that hyperthyroidism is associated with endothelial dysfunction[8] but the effect of SHy needs to be determined.

Thyroid hormone targeting vascular endothelium may increase blood flow with resultant shear stress of the vessel and changes in endothelial function. T3 decreases systemic vascular resistance (SVR) by dilating the resistance arterioles of the peripheral circulation. Mechanisms are modulation of nitric oxide (NO) production,[9] and increase of sodium and potassium flux in smooth muscle cells that lead to a decrease in vascular tone.[10] A decrease in SVR then leads to a decrease in the effective arterial filling volume, causing an increase in renin release.[11] This leads to stimulation of renal sodium reabsorption, and plasma volume is increased. Erythropoietin is stimulated by T3. Because of these effects, blood volume, ventricular preload, and cardiac output increase.[512]

Flow-mediated endothelium-dependent vasodilation (flow-mediated dilation [FMD]) of the brachial artery is a functional test of the capacity of increased blood flow, following induced ischemia and, therefore, is a marker of endothelial function.[6] Reactive hyperemia induced by transient ischemia of the forearm may increase blood flow and shear stress of the vessel wall, resulting in the release of endothelial NO, and possibly other vasodilator mediators, and vascular dilation.[13] Measuring intimae-media thickness (IMT) of the carotid artery by ultrasound is another established method for detection of early atherosclerotic changes which has been proven to be directly related to coronary artery disease.[14]

The aim of this study is to compare the endothelial function and carotid artery IMT of SHy patients to euthyroid subjects and whether changes induced by hyperthyroidism can be restored to normal during the euthyroid state following treatment.

MATERIALS AND METHODS

Study populations

Twenty-five patients with diagnosis of exogenous SHy and 25 age and gender-matched healthy subjects[15] according to medical history, physical examination, and routine laboratory tests were enrolled in this study. SHy was defined as normal free and total T3 and T4 with suppressed TSH (TSH < 0.3 mIU/L). Euthyroidism was defined as a serum thyrotropin (TSH) level between 0.3 and 4 mIU/L. The normal range of T3 and T4 was 76.3–220.8 ng/dl and 4.5–12.6 µg/dl, respectively.

All patients were treated with levothyroxine because of previous hypothyroidism.

Patient who had hypertension, diabetes mellitus, coronary artery disease, infectious disease, known case of liver and kidney diseases, hyperlipidemia, menstrual disorder, polycystic ovary syndrome, morbid obesity (body mass index >30), current smoker of cigarette, alcohol abuser or drug abuser, anemia, and migraine were not included in this study, and this rule was performed for all patients and control subjects.

Exclusion criteria were using any drugs except levothyroxine, pregnancy, and lactation. Furthermore, all participants had not used hormonal contraceptives or vasoactive drugs since 3 months before beginning of this study.

The Ethics Committee of Isfahan University of Medical Sciences approved the study protocol, and all patients gave informed consent before study inclusion.

Study design

In this study, at first FMD and carotid IMT were obtained from all the participants. A high-resolution B-mode ultrasonographic system (ATL ultrasound, HDI 5000, and BOTHELL, Washington, USA) with a linear transducer mid-frequency of 7.5 mHz was used to determine FMD of the brachial artery. An experienced ultrasonographer blinded to case and control groups performed all FMDs. At first; patients lay at rest for 10 min. Then, the baseline brachial artery diameter was determined by locating probe on 4–5 cm above the antecubital fossa of the nondominant arm. After that, a pneumatic tourniquet of a sphygmomanometer was inflated on the most proximal portion of the forearm to a pressure of 300 mmHg for 5 min. The cuff of sphygmomanometer was released and second time of scan was taken during 90 s after deflation of cuff. Diameters of artery were determined at the end of diastolic period with ultrasonic calipers from the edge of the anterior wall to the edge of the posterior wall of the brachial artery.[16] Two other observers supervised the procedures. Changes in diameter were computed as percentage relative to the baseline diameter. FMD was not measured during menstrual phase in female patients. All the FMDs were performed between 10 am and 12 midday. Furthermore, IMT of common carotid artery was calculated with that ultrasonographic system. The bulb dilation served as a landmark to indicate the border between distal common carotid artery and the carotid bulb. Images were obtained from the distal portion of the common carotid artery, 1–2 cm proximal to the carotid bulb. Two bright echogenic lines were defined as intimae and media layers.[14] Three measurements were taken from each subject, and mean value was used for the analysis.

In the second phase, for SHy dosage of levothyroxine decreased 25% of prior dosage and this dose continued for 2 months then TSH was checked. If it was in normal range, rechecked for assurance and if it was not in normal range 25% of last dosage decreased again and go on for next 2 months and this protocol continue until TSH become normal. After this period, all patients were recalled, and complete examination was done, and forearm FMD and carotid IMT was performed with previous ultrasonographic system and the same ultrasonographer. All the laboratory tests were analyzed in Alzahra Biochemistry Laboratory, Isfahan Faculty of Medicine. All the biochemical parameters were analyzed by using immunochemiluminometric assay 2010 SIEMENS ADVIA-century Cp (American).

Statistical analysis

The results are expressed as mean ± standard error. Differences in FMD and IMT between case and control groups were analyzed using the independent-sample t-test and differences within the groups (before and after changing levothyroxine dosage) were determined by paired-sample t-test. P value of 0.05 was considered significant. A statistical software package, SPSS (version 16), was used to perform statistical analysis.

RESULTS

The demographic information

25 exogenous SHy were enrolled in this study as the case group, between October 2012 and July 2013. These patients were treated with levothyroxine for about 78.25 ± 8.89 months. The mean age of these patients was 38.48 ± 12.05 years with a range of 17–60 years and the mean age of control group was 36.72 ± 11.15 years. Both two groups include 20 women and 5 men. No significant difference was observed with respect to age and gender among the study groups. Serum T4 and T3 were within normal limits in all subjects, but they were significantly higher in case group (P < 0.001). Before reduction of dose of levothyroxine, the control group's mean T3 and T4 was 115.32 ± 13.90 ng/dL (minimum: 91; maximum: 150 ng/dL) and 7.73 ± 0.87 µg/dL (minimum: 5.80; maximum: 9.90 µg/dL), respectively. In case group, the mean of T3 was 142.12 ± 22.96 ng/dL (minimum: 86; maximum: 184 ng/dL) and the mean of T4 was 9.92 ± 1.22 µg/dL (minimum: 7; maximum: 11.5 µg/dL).

The demographic and metabolic information of these two groups is summarized in Table 1.

Table 1

The general patient's characteristics

Endothelial function markers

The vascular parameters of the both groups (baseline diameter of brachial artery, FMD, and IMT of common carotid artery) are documented in Table 2. As illustrated in Table 2, the mean of FMD in control group (healthy people) was dramatically higher than the subclinical hyperthyroid patients (P < 0.001) but no statistically significant difference was found for IMT (P = 0.459).

Table 2

Comparison of the flow-mediated dilation parameters and intima-media thickness between case and control groups through the study

As summarized in Table 3, after intervention in case group and treating SHy, FMD was meaningfully increased (P < 0.001) but IMT of common carotid artery was not considerably changed (P = 0.491).

Table 3

Comparison of the flow-mediated dilation parameters, intima-media thickness, and thyroid stimulating hormone in case group before and after intervention

DISCUSSION

In this study, we wanted to determine whether the endothelial dysfunction measured via FMD can be influenced in exogenous SHy, and if so, what happen when SHy restored to normal during the euthyroid state following treatment. Despite several studies, endothelial dysfunction is not clearly demonstrated in SHy.

In this study, FMD was significantly lower in exogenous SHy compared to demographically matched subjects. The exclusion of participants with concomitant endocrine, renal, hepatic, and other diseases that influence endothelial function allowed us to decrease the effect of confounding variables and yielded a homogenous population.

We examined endothelial function with FMD, which is a simple noninvasive, harmless, accurate, and reproducible method that enables assessment of the vascular response to flow increase. FMD is currently the main method used to assess endothelial function and is convenient for clinical practice.[17]

Cikim et al.[1] were the first showed FMD values decreased in SHy, and our study showed the same finding. In that study, the patients include both exogenous and endogenous SHy. Yavuz et al. illustrated that TSH suppression therapy with levothyroxine leading to exogenous SHy may cause impaired endothelial function and decreased FMD.[18] However, another study demonstrated that flow-mediated endothelium-dependent vasodilation, as assessed by a noninvasive determinant of brachial FMD, increased in hyperthyroid patients.[19] This differences may be explained by the difference between subclinical and overt hyperthyroid condition. All the patients in that study were endogenous hyperthyroidism, and this was another dissimilarity between two studies.

Using ultrasound to measure IMT of the carotid artery is one of accepted method for detection of early atherosclerotic changes, which has been proven to be directly related to coronary artery disease.[14] However, we could not find any significant association between SHy and IMT. Cikim et al.[1] reported no significant differences in mean carotid IMT in a group of SHy patients compared to a euthyroid group. Cabral et al.[20] reported the same finding in SHy. However, the results are controversial in measuring IMT in subclinical hypothyroidism.[212223] These results suggesting that subclinical hyperthyroidism and SHy is not associated with an increase in cardiovascular risk when assessed by carotid IMT.[23] There is not clear in thyroid dysfunction what the causative agent of endothelial damage is, but seems thyroid autoantibodies induce decreased endothelial NO synthesis or inflammation.[24]

This study also makes obvious that FMD values enhanced when hyperthyroid status was restored to euthyroid state; however, the FMD levels remained lower than those of controls. This finding is in accordance with previous studies showing that vascular reactivity in hyperthyroid state can be restored by either medical therapy or subtotal thyroidectomy.[2526] Partial restoration of FMD when treating hyperthyroid patients may be due to increase of endothelium-derived hyperpolarizing factor in hyperthyroid patients, which cannot be corrected by treatment.[27] In our study, we illustrate that FMD changes are not related to hypothyroidism background because after reduce dose of levothyroxine, FMD became better than past.

Limitation of our study was that we did not compare the anti-thyroid peroxidase level in SHy before and after treatment. Therefore, we could not clarify the possible effect of inflammation in these findings.

CONCLUSION

Our study demonstrated that flow-mediated endothelium-dependent vasodilation, as assessed by a noninvasive determinant of brachial FMD, decreased in subclinical hyperthyroid patients which could be partially restored by treatment. These findings suggest that treatment of subclinical hyperthyroid state could improve endothelial dysfunction and at the end decreased the cardiovascular complications.

Financial support and sponsorship

Isfahan Endocrine and Metabolism Research Centre.

Conflicts of interest

There are no conflicts of interest.