Eur J Endocrinol
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DOI: 10.1530/eje.1.01870
European Journal of Endocrinology, Vol 152, Issue 3, 347-353
Copyright © 2005 by European Society of Endocrinology
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CLINICAL STUDY

Increased levels of serum osteoprotegerin in hypothyroid patients and its normalization with restoration of normal thyroid function

Toshiki Nagasaki, Masaaki Inaba, Shuichi Jono, Yoshikazu Hiura, Hideki Tahara, Kumi Shirakawa, Naoyoshi Onoda1, Tetsuro Ishikawa1, Eiji Ishimura and Yoshiki Nishizawa

Department of Metabolism, Endocrinology and Molecular Medicine and 1 First Department of Surgery, Osaka City University Graduate School of Medicine, 1-4-3, Asahi-machi, Abeno-ku, Osaka 545-8585, Japan

(Correspondence should be addressed to Masaaki Inaba; Email: inaba-m{at}med.osaka-cu.ac.jp)


   Abstract
Top
 Abstract
Introduction
 Patients and methods
 Results
 Discussion
 References
 
Hypothyroidism is associated with increased morbidity from cardiovascular disease, and an increase in serum osteoprotegerin (OPG) has recently been reported to be associated with the severity of coronary heart disease and cardiovascular mortality. The present study was designed to examine whether hypothyroidism causes an increase in serum OPG, and to determine whether levothyroxine (L-T4) replacement therapy might suppress serum OPG levels in hypothyroid patients. Fifty-three hypothyroid patients with chronic thyroiditis and age- and sex-matched normal control subjects were examined for the levels of serum OPG and plasma von Willebrand factor (vWF), a vascular injury marker. Thirty-seven of the hypothyroid patients were further monitored for changes in these markers during 1 year in a euthyroid state induced by L-T4 replacement therapy. Baseline OPG was significantly higher in hypothyroid patients than in normal controls (4.51 ± 0.50 vs 3.72 ± 0.23 pmol/l (mean ± S.E.); P = 0.0182). In multivariate analysis, baseline OPG was significantly associated with baseline levels of TSH (r = 0.280, P = 0.0162) and vWF (r = 0.626, P < 0.0001). During one year of L-T4 replacement therapy, hypothyroid patients showed a significant decrease in OPG levels from 4.35 ± 0.51 to 3.48 ± 0.26 pmol/l (P = 0.0166), a level comparable to normal controls. The change in serum OPG levels during L-T4 replacement therapy was significantly and independently associated in a negative fashion with baseline vWF (r = –0.503, P = 0.0014). This study suggested that the severity of hypothyroidism and vascular injury might have important independent roles in increasing the serum OPG level in hypothyroid patients. Furthermore, it was demonstrated that a sustained euthyroid state might have the potential to decrease the serum OPG level in hypothyroid patients and that the degree of vascular injury in the hypothyroid state is independently associated with a decrease in serum OPG during a 1-year normalization of thyroid function.


   Introduction
Top
 Abstract
 Introduction
Patients and methods
 Results
 Discussion
 References
 
Osteoprotegerin (OPG), a member of the tumor necrosis factor receptor family, has been identified as a regulator of bone resorption (1). OPG is produced by a variety of tissues besides bone, such as the cardiovascular system (vascular cells including coronary smooth muscle cells and endothelial cells in vitro (2)), lung, kidney and immune cells (1, 3). It has been shown that OPG-deficient mice develop severe osteoporosis and medial calcification of the aorta and renal arteries (4), and that OPG immunoreactivity can be demonstrated in early atherosclerotic lesions in human tissues (5). A recent clinical study reported an increase in serum OPG as a risk factor for cardiovascular mortality (6), and our recent report (7) demonstrated a significant association between increased serum OPG and the presence and severity of coronary heart disease.

Hypothyroidism, even in its latent form, is associated with increased morbidity from cardiovascular disease (8, 9). We recently reported that intima-media thickness (IMT) in the common carotid artery, a good predictor for cardiovascular events (10, 11), is increased in hypothyroid patients, and that increased IMT normalized after one year of levothyroxine (L-T4) replacement therapy (12). Accelerated atherosclerosis in hypothyroidism is accounted for by multiple mechanisms, including hyperlipidemia (13), hypercoagulable state (14), impaired endothelium-dependent vasodilation (15) and obesity (16). However, since there is no data on serum OPG levels in hypothyroidism, it is of importance to assess the possible involvement of OPG in the increased arterial thickening in hypothyroidism.

This background scenario prompted us to examine (i) whether serum OPG might be increased in hypothyroid patients, (ii) whether serum OPG might decrease after normalization of thyroid function by L-T4 replacement therapy, and (iii) which of a number of clinical variables, including blood pressure, body weight, thyroid hormone level, and the level of von Willebrand factor (vWF), a marker of vascular injury (17–21), is most important in influencing serum OPG in hypothyroid patients and its changes during L-T4 replacement therapy.


   Patients and methods
Top
 Abstract
 Introduction
 Patients and methods
Results
 Discussion
 References
 
Patients and study design

The present study was designed as an open prospective study. Written informed consent was obtained from each patient, and the study was approved by the ethics committee of Osaka City University Hospital. Fifty-three hypothyroid patients (10 males, 43 females) were enrolled in the study, among whom 37 patients (6 males, 31 females) were studied before and after undergoing L-T4 replacement therapy. The diagnosis of hypothyroidism was established on the basis of suppression of serum free thyroxine (FT4) and free triiodothyronine (FT3) to below the normal lower limit, and elevation of serum thyroid-stimulating hormone (TSH) to above the normal upper limit. The etiology of all hypothyroid patients was chronic thyroiditis as evidenced by the presence of firm diffuse goiter with a positive thyroid test and/or microsome test. The dose of L-T4 administered was adapted individually to maintain serum FT4, FT3 and TSH levels within their respective normal ranges. Biochemical markers, including serum OPG and plasma vWF, were measured in all patients before and at least 1 year (12–14 months) after normalization of thyroid function. Fifty-three healthy control subjects were enrolled from the participants in a local health-check program conducted at the Osaka City University Hospital. They had neither a past history of thyroid disease nor goiter. To avoid complications from other factors well known to affect atherosclerosis, patients meeting either of the following criteria were excluded from the study: (i) patients taking medication known to affect atherosclerosis, such as anti-hypertensive agents, lipid-lowering drugs, and anti-platelet drugs, and (ii) patients receiving other hormone replacement therapy.

Serum parameters

Blood was withdrawn just after an overnight fast, and vWF levels in plasma were measured as previously described (20–22). Total cholesterol, triglyceride and high-density lipoprotein (HDL)-cholesterol levels were determined using an autoanalyzer. The level of low-density lipoprotein (LDL)-cholesterol was calculated according to the formula given by Friedewald (23). FT4, FT3 and TSH were measured using commercially available kits (Ortho-Clinical Diagnostics, Amersham, Bucks, UK), using a one-step nonradioactive immunoassay for FT4 in the dedicated Amerlite enhanced luminescence assay system (24). Briefly, a monoclonal antibody to T4, conjugated with horseradish peroxidase (HRP; EC 1.11.1.7 [EC] ), competes for binding with FT4 or with a conjugate of a protein and levotriiodothyronine (L-T3) immobilized onto the surface of the assay microwells. The signal is generated by antibody-HRP bound to the protein–L-T3 conjugate, with luminol as substrate.

OPG levels were determined in serum samples which had been kept at –80 °C, using a sandwich ELISA (Cosmo Bio, Tokyo, Japan) as previously described (25). A monoclonal antibody against human OPG, designated as clone OI-19, was used to separate OPG from serum. The OPG was then detected with a peroxidase-labeled antihuman OPG monoclonal antibody, designated as clone OI-4, and a tetramethylbenzidine substrate. All samples were measured in duplicate and the results were averaged. The detection limit of this assay system was 0.03 ng/ml, and the intra- and interassay coefficients of variation were <3.2% and <5.4% respectively (25).

Statistical analysis

Data are expressed as means ± S.E. unless otherwise indicated. Statistical analysis was performed with the StatView V system (Abacus Concepts, Berkeley, CA, USA) on an Apple computer. Differences in basal values between hypothyroid patients and normal controls and the difference in each parameter before and after treatment were examined using a Wilcoxon test for assessment of the mean value. Differences in basal values or change in variables between patients in whom OPG levels remained unaltered and those in whom OPG levels decreased, were examined using a Mann–Whitney’s U test for assessment of the mean value. Independent association between one dependent and more than two independent variables was assessed by multiple regression analysis. P values of less than 0.05 were considered to be statistically significant.


   Results
Top
 Abstract
 Introduction
 Patients and methods
 Results
Discussion
 References
 
Basal levels of serum OPG and clinical variables in the hypothyroid state

Clinical characteristics of hypothyroid patients and normal controls are shown in Table 1Go. As mentioned above, all hypothyroid patients exhibited suppression of serum FT4 and FT3 levels with an increase in serum TSH above the normal upper limit. Serum levels of metabolic markers of thyroid function, including serum total cholesterol and LDL-cholesterol, differed significantly between the two groups, but there were no significant differences in age, gender, height, body weight, body mass index (BMI), smoking index (daily number of cigarettes multiplied by the number of years of smoking), systolic, diastolic, and mean blood pressures, pulse pressure, pulse rate, or serum levels of HDL-cholesterol, triglyceride and creatinine. Serum OPG levels were significantly higher in hypothyroid patients than in normal controls. Mean plasma vWF levels were significantly higher in hypothyroid patients than in normal controls.


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  		Table 1 Baseline characteristics of hypothyroid subjects and control subjects. Data expressed as means ± S.E.
 
Association of basal levels of serum OPG and other parameters in hypothyroid patients before L-T4 replacement therapy

Table 2Go shows the results of multiple regression analysis of the basal levels of various clinical variables to evaluate their association with the basal level of serum OPG in hypothyroid patients. In model 1, which included vWF, mean blood pressure, BMI and TSH as independent variables, vWF and TSH were found as significant factors that were positively associated with serum OPG. When TSH was replaced by FT3 or FT4, these variables failed to emerge as significant independent determinants of OPG (models 2 and 3). Moreover, in models 4, 5 and 6, which used total cholesterol, LDL-cholesterol and HDL-cholesterol respectively in place of TSH, none of these parameters was found to be significantly associated with serum OPG.


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  		Table 2 Multiple regression analysis to evaluate association of basal levels of serum OPG and other parameters in hypothyroid patients before L-T4 replacement therapy.
 
Effect of L-T4 replacement therapy on serum OPG and clinical variables in hypothyroid patients

In hypothyroid patients, the mean levels of serum OPG decreased significantly during 1 year in a euthyroid state induced by L-T4 replacement therapy to a level comparable to that in normal controls (control value: 3.62 ± 0.22 (n = 37) vs normal control: P = 0.336) (Table 3Go). L-T4 replacement therapy restored the serum levels of FT4, FT3 and TSH to normal levels and also produced significant decreases in serum total cholesterol and in LDL- and HDL-cholesterol levels. Body weight and BMI were also reduced significantly. No significant change was observed in systolic, diastolic and mean blood pressures, pulse rate, or serum triglyceride and creatinine levels. Serum vWF increased significantly during the year of normalization of thyroid function.


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  		Table 3 Clinical variables in hypothyroid patients before and after one year of normalization of thyroid function by L-T4 replacement therapy. Data are expressed as means ± S.E.
 
Differences between patients in whom OPG levels remained unaltered and those in whom OPG levels decreased during L-T4 replacement therapy

We separated the patients into those whose serum OPG levels remained unaltered (35.1% of hypothyroid patients) and those whose serum OPG levels decreased (64.9% of the group) during L-T4 replacement therapy, and the various variables were compared between the two groups of patients. None of the clinical variables listed in Table 1Go differed between those patients (data not shown).

Association of baseline characteristics with changes in OPG during L-T4 replacement therapy

Table 4Go shows the results of multiple regression analysis of the basal levels of various clinical variables before L-T4 replacement therapy. These data were used to evaluate the association of the variables with changes in OPG during L-T4 replacement therapy in hypothyroid patients. In model 1, which included vWF, mean blood pressure, BMI and TSH as independent variables, vWF was the only factor that was significantly and negatively associated with a change in OPG during L-T4 replacement therapy. Models 2 and 3, which included FT3 or FT4 in place of TSH, showed no significant association between thyroid hormone levels and changes in OPG. In models 4–6, no association of total cholesterol, LDL-cholesterol or HDL-cholesterol was found with changes in OPG.


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  		Table 4 Multiple regression analysis to evaluate association of change in OPG with other baseline characteristics.
 

   Discussion
Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
References
 
In the present study, we demonstrated that the serum level of OPG was significantly higher in hypothyroid patients than in normal controls (Table 1Go) and that the increased serum OPG returned to a normal level once thyroid function had been normalized, and was sustained within the normal range for at least 1 year. We have recently reported that hypothyroid patients showed increased intima-media thickness in the common carotid artery, and that this arterial thickening was also normalized during sustained normalization of thyroid function by L-T4 replacement therapy (12). These data were in good agreement with a previous report demonstrating that serum osteoprotegerin levels correlate with intima-media thickness in the carotid arteries in healthy postmenopausal women (26). Furthermore, we have observed that the reduction of intima-media thickness in the common carotid artery might have the potential to attenuate thyroid hormone-induced vascular injury, as reflected by an attenuation of the increased response of plasma vWF levels during L-T4 replacement therapy in hypothyroid patients (21).

A recent clinical study suggests that an increase in serum OPG levels is significantly correlated with cardiovascular mortality (6). The increase in serum levels of OPG in hypothyroid patients in the present study was comparable to that seen in a high risk group for the incidence of cardiovascular disease and vascular mortality in the prospective Bruneck Study (6). Furthermore, we have recently reported that the serum OPG level is associated with the presence and severity of coronary artery disease (7), suggesting that serum OPG is a valuable indicator for atherosclerosis and thus a good predictor of cardiovascular events. Supportive of this hypothesis is the recent evidence that OPG is present not only in early atherosclerotic lesions (27) but also in advanced calcified lesions in human atherosclerotic plaques in the carotid artery (5). Taking these data into consideration, it was suggested that an increase in serum OPG might also be a good indicator for increased arterial thickening in hypothyroid patients.

OPG is known to exert an inhibitory effect on arterial calcification and osteoclastogenesis (1, 4), although the molecular mechanism underlying the role of OPG in the progression of atherosclerosis is not fully understood. Our findings showed a significant and positive association between baseline OPG and baseline vWF or TSH in hypothyroid patients (Table 2Go). Since vWF is a reliable parameter for endothelial vascular injury (17–21), particularly in hypothyroid patients (21), these findings support the notion that an increase in serum OPG levels is also a reliable marker for vascular injury in hypothyroidism (8, 9). However, the baseline level of vWF, but not that of TSH, was significantly and independently negatively associated with the change in serum OPG during 1 year of euthyroidism (Table 4Go). These findings suggest that hypothyroid patients in whom serum vWF levels had been higher before L-T4 replacement therapy exhibited a greater decrease in serum OPG levels during treatment, supporting the association of an increase in serum OPG levels in hypothyroidism with vascular injury, rather than with the hypothyroid state itself. The limitation of the present study is that the multiple regression models shown in Table 2Go explain only 46.2 to 57.1% of the baseline OPG levels, indicating the possible presence of other regulatory factors that lead to elevation of serum OPG levels in hypothyroid patients. Hence, we examined the possible involvement of other established risk factors for atherosclerosis in the increase in serum OPG levels in hypothyroidism. Unfortunately, no data are available for the decrease in serum OPG levels during treatment for hyperlipidemia or hypertension. Although hypothyroidism induces hyperlipidemia (13), no significant association was found between serum OPG levels and the baseline level of serum lipids or changes in the serum lipid level during L-T4 replacement therapy (Tables 2Go and 4Go), negating the possible involvement of hyperlipidemia-induced vascular injury in the increase in serum OPG levels. Although thyroid hormone causes vascular injury via multiple mechanisms (20, 28–30), including its effect on the hemodynamic state, no association was found between serum OPG and hemodynamic factors such as mean blood pressure (Tables 2Go and 4Go), systolic and diastolic blood pressure, and pulse rate in the present study. These findings clearly negated the possibility that alterations in serum lipid levels and the hemodynamic states induced by hypothyroidism might be involved in the regulation of OPG in hypothyroid patients.

The clearance of OPG or its metabolism was reported to be under the influence of renal function (31) and creatinine clearance could be influenced by thyroid hormone (32). Although we have no data about the creatinine clearance, serum creatinine did not change significantly before and after L-T4 replacement therapy (Table 3Go). These data may negate the possibility, although not completely, that decreased clearance of OPG from serum in the hypothyroid state might increase serum OPG levels.

In summary, this is the first report demonstrating that hypothyroid patients exhibit a significant increase in serum OPG levels compared with normal controls, and that normalization of thyroid function for 1 year by L-T4 replacement therapy significantly decreases serum OPG to levels comparable to those in normal controls. It was further suggested that the increase in serum OPG levels in hypothyroid patients is independently associated with vascular injury, as reflected by increased plasma vWF levels, but not with the severity of hypothyroidism.


   References
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 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 

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Received 7 October 2004
Accepted 14 December 2004




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