Introduction

In 1878, Greenfield found diffuse atherosclerosis in a 58-year-old woman with myxedema at autopsy1, 2. Soon after, Kocher reported that arteriosclerosis commonly occurs after thyroid extirpation and raised the hypothesis of a causal relationship between hypothyroidism and atherosclerosis3. This link between the hemostatic system and thyroid disease was finally described in 1913, when an episode of cerebral vein thrombosis in a hyperthyroid patient was reported4.

It is now well-known that thyroid dysfunction and autoimmunity may modify the physiological processes of primary and secondary hemostasis and lead to bleeding or thrombosis5. Following levothyroxine treatment, patients with overt hypothyroidism display decreased bleeding time, prothrombin time (PT), activated partial thromboplastin time (APTT), and clotting time as well as increased factor VIII activity, von Willebrand factor, and platelet count6. The occurrence of myocardial infarction (MI) shortly after the initiation of thyroid hormone substitution treatment could reflect an acutely increased risk of thrombosis7, 8.

The association between the thyroid hormone level and the coagulation system in subjects with acute ST-segment elevation myocardial infarction (STEMI) and normal thyroid function has not been definitely elucidated. In the present study, we prospectively explored this association in Chinese euthyroid subjects with STEMI.

Materials and methods

Study subjects

From 27 July 2010 to 21 March 2011, 231 consecutive euthyroid patients (177 males), aged 30 to 94 years (mean, 63 years) with acute STEMI at the First Affiliated Hospital of Nanjing Medical University, Nanjing, China, were enrolled in the study. The current guidelines for the ECG diagnosis of the ST segment elevation type of acute myocardial infarction require at least 1 mm (0.1 mV) of ST segment elevation in the limb leads, and at least 2 mm elevation in the precordial leads9. Because anticoagulation is an integral part of both fibrinolytic therapy and percutaneous intervention (PCI) in the reperfusion treatment of STEMI10, all patients were given antiplatelet therapy with aspirin and clopidogrel. Exclusion criteria were cardiac shock, severe liver and/or renal dysfunction, hyperthyroidism, hypothyroidism, severe hypovolemia, thyroid disease, and concurrent treatment with diuretics or amiodarone. Complete medical histories, including history of bleeding and smoking habits, were recorded.

Among the 231 patients with STEMI, the types of MI were anteroseptal MI in 35 cases, anterior wall MI in 72 cases, extensive anterior wall MI in 20 cases, inferior wall MI in 98 cases, and lateral wall MI in 6 cases. Patients were divided into 4 groups according to their levels of free triiodothyronine (FT3) and free thyroxine (FT4). The median (quartile range) for FT3 and FT4 were 3.7 pmol/L (3.1–4.3 pmol/L) and 16.9 pmol/L (14.9–18.8 pmol/L), respectively.

This study was approved by the Ethics Committee of the First Affiliated Hospital of Nanjing Medical University, and informed consent was obtained from each patient.

Clinical characteristics

At admission to the coronary care unit, the patients were immediately examined by the attending physician, who performed a complete physical examination, including blood pressure, heart rate, respiratory rate, and body temperature, and recorded demographic and historical data.

Systolic blood pressure (SBP) and diastolic blood pressure (DBP) were measured in the right arm with the participant seated and the arm bared. Three readings were recorded for each individual, and the average was recorded. After a rest of at least 5 min, the heart rate was measured by pulse palpation over a 30-s period and was multiplied by 2 to evaluate the heart rate per minute. The respiratory rate was measured by observing the frequency of thoracic ups and downs over a 60-s interval. The axillary body temperature was measured by placing a thermometer under the armpit, with the arm skin tightening the thermometer. The thermometer was removed and read after 5–10 min.

Thyroid level measurements

The 12-h fasting blood samples were collected from every patient upon admission to the coronary unit. All samples were collected in serum separator tubes and immediately centrifuged (at 3000 r/min for 20 min at room temperature) and analyzed. Thyroid-stimulating hormone (TSH), FT3, and FT4 levels were measured with full-automatic immune analyzer (cobas e601, Roche, Berlin, Germany), with normal reference ranges of 0.3–4.2 mIU/L, 3.10–6.8 pmol/L, and 12.0–22.0 pmol/L, respectively.

Blood coagulation measurements

Two common coagulation tests, PT and APTT, were performed with a computerized blood coagulation analyzer (CA 7000, Sysmex, Kobe, Japan). The reference ranges for PT and APTT were 11.0±3 s and 24.5±10 s, respectively. The international normalized ratio (INR) is the ratio of the PT of the patient to a normal (control) sample. The INR was measured by the coagulation analyzer (CA 7000, Sysmex, Kobe, Japan), with a reference range of 0.8–1.2. The platelet count was obtained with an automated blood analyzer (XE-2100; Sysmex, Kobe, Japan).

Statistical analysis

Significance was defined as a P value of <0.05. Data were analyzed with Statistics Package for Social Sciences (version 16.0; SPSS Inc, Chicago). All variables were checked by the Kolmogorov-Smirnov test.

Patients were classified into 4 groups according to their quartile FT3 and FT4 levels, respectively: 0.40–3.09 (n=52 patients), 3.10–3.69 (n=56), 3.70–4.29 (n=64), and 4.30–7.10 (n=59) for FT3; 4.9–14.8 (n=57 patients), 14.9–16.8 (n=58), 16.9–18.7 (n=57), and 18.8–29.0 (n=59) for FT4. In every group, the DBP, body temperature, heart rate, respiratory rate, PT, APTT, platelet count, and TSH were expressed as median (quartile range), and comparisons between the 4 groups were analyzed by Kruskal-Wallis test (for non-normal distribution). Age, SBP, FT4, FT3, and INR were expressed as the mean±SD, and comparisons were analyzed by analysis of variance (ANOVA)-Sheffe's F test. Categorical variables, including sex, were compared among the groups of patients by chi-squared analysis.

The independent relationship between FT3 or FT4 and other variables was assessed by stepwise or enter multiple regression analysis, respectively. Differences were considered to be significant if the null hypothesis could be rejected with >95% confidence. All reported P values are two-tailed.

Results

Demographic and clinical characteristics and coagulation parameters in patients according to the level of FT3 and FT4

Of the 231 patients with STEMI, 177 (76.6%) were male and 54 (23.4%) were female. Table 1 shows the baseline demographic and clinical characteristics and biochemical and coagulation parameters of the 4 groups. The frequency distribution of sex (P=0.04) differed significantly among the groups. Age (P=0.00), SBP (P=0.02), DBP (P=0.01), INR (P=0.01), PT (P=0.00), and APTT (P=0.00) differed significantly among the groups. However, platelet count (P=0.21), body temperature (P=0.55), heart rate (P=0.21), respiratory rate (P=0.53), FT4 (P=0.23), and TSH (P=0.88) were similar among the 4 groups.

Table 1 Clinical characteristics and biochemical and coagulation parameters in patients according to the level of free triiodothyronine.

Patients were classified into 4 groups according to their quartile FT4 levels. Sex (P=0.67), age (P=0.85), SBP (P=0.58), DBP (P=0.83), platelet count (P=0.24), INR (P=0.36), body temperature (P=0.86), heart rate (P=0.25), respiratory rate (P=0.12), PT (P=0.39), APTT (P=0.17), and TSH (P=0.26) were similar among the 4 groups (Table 2). FT3 (P=0.04) differed significantly among the groups.

Table 2 Clinical characteristics and biochemical and coagulation parameters in patients according to the level of free thyroxine.

Multiple linear regression analysis with FT3 and FT4 as the dependent variable

Multiple linear regression analysis was used to examine the independent association between FT3 or FT4 and INR in patients with STEMI. In this model, FT3 or FT4 was employed as the dependent variable, and other variables were considered as the independent variables. Table 3 shows that INR (β=-0.139, P=0.025), age (β=-0.344, P=0.000), and DBP (β=0.144, P=0.020) were significant independent factors associated with the level of FT3 after adjustment. Figure 1 presents the partial regression and shows the relationship between FT3 and INR.

Table 3 Multiple linear regression analysis to identify independent variables associated with serum free triiodothyronine levels.
Figure 1
figure 1

The relationship between free triiodothyronine (FT3) and the international normalized ratio (INR) in Chinese euthyroid subjects.

Sex (β=0.063, P=0.377), age (β=0.120, P=0.097), body temperature (β=-0.020, P=0.763), heart rate (β=0.065, P=0.336), respiratory rate (β=-0.056, P=0.413), SBP (β=-0.028, P=0.784), DBP (β=0.126, P=0.215), platelet count (β=0.086, P=0.218), and INR (β=0.024, P=0.725) were not significantly associated with the FT4 levels (Table 4).

Table 4 Multiple linear regression analysis to identify independent variables associated with serum free thyroxine levels.

Discussion

In the present study, we investigated the association between thyroid hormones and the coagulation system in patients with STEMI and normal thyroid function. Subjects with high FT3 levels had lower INR values than those with low FT3 (P=0.01). To our knowledge, this is the first study to report that an increased INR is associated with a decreased FT3 level in euthyroid patients.

The strong relationship between thyroid hormones and the coagulation system has been appreciated since the beginning of the past century11. For instance, hyperthyroid patients are known to have an increased prevalence of shortened APTT and higher fibrinogen levels than those with normal thyroid function. Because prolonged APTT and PT results indicate a reduced coagulation response and a bleeding tendency, these findings indicate that hyperthyroidism might be associated with hypercoagulability12. Previous studies largely have explored patients with clinically overt hypo- or hyperthyroidism who appeared to have an increased risk of bleeding or thrombosis13.

In contrast, conflicting results have been reported concerning the association between subclinical hyperthyroidism and coagulation. Bucerius et al reported that subclinical hyperthyroidism has no significant impact on coagulation metabolism14, whereas Smallridge reported that subclinical hyperthyroidism is associated with various cardiac effects, particularly atrial fibrillation that increases the thromboembolism risk15. Recently, a correlation between thyroid hormone levels and atherosclerosis was suggested in euthyroid patients, in whom the thyroid hormone levels were found to affect the presence and severity of coronary atherosclerosis11. Similarly, logistic regression analysis in the present study revealed decreased serum FT3 as an independent risk factor for elevated INR in patients with STEMI and normal thyroid function, after adjustment for confounders. Age was also observed to be an independent risk factor for elevated FT3, consistent with a previous study showing that the FT3 level in old subjects is negatively associated with age16.

Female gender by itself had a negative and independent influence on mortality in STEMI patients17. However, of the 231 patients with STEMI in our study, 177 (76.6%) were male and 54 (23.4%) were female. Among the subjects with a high serum FT3 level, there were significantly more males than females (P=0.04). A typical pattern of altered thyroid hormone metabolism called nonthyroid illness syndrome (NTI) occurs after acute MI. This syndrome is characterized by low serum and free T3 levels, increased serum reverse T3 levels, and, in the most severe condition, by decreased serum T4 and TSH levels18. In present study, among of 231 patients, 23% were lower than the normal reference of FT3 (3.1 pmol/L).

Thyroid hormones exert various effects on the coagulation system19. Modulation of the levels of T3 in hyper- and hypothyroidism is extremely important for the capability to increase or decrease the concentrations of fibrinogen and numerous blood clotting factors20. In particular, a decrease in active hormone T3 leads to further impairment in cardiac function21. Recently, Lymvaios et al reported that T3 levels are closely correlated with the cardiac function after AMI22. Everts et al also reported that T3, and not T4, is transported into the myocyte23. However, the exact mechanisms underlying the association between FT3 and cardiac function require further study. Regardless, these previous findings, combined with those of the present study, indicate the importance of assessing the FT3 level of patients. Acute myocardial infarction was the consequence of the acutely increased coronary thrombogenesis and the deficient blood. Whether or not substitution of thyroid hormone should be (routinely) considered as a treatment in patients with STEMI undergoing surgery should be considered.

The findings of the present study are consistent with previous observations that a rise in thyroxine level is associated with increased levels of factors VIII and IX, von Willebrand factor, and fibrinogen24. Several biological mechanisms have been proposed to explain this intriguing association, including the effects of thyroid hormones on the synthesis of coagulation factors and the thyroid-related autoimmune processes13, 19. However, the exact mechanism underlying the relationship between FT3 and INR remains unclear. Deficiency or excess of thyroid hormone may disturb the production and/or clearance of coagulation factors, such that a patient will bleed or develop thrombosis20.

Limitations of the present study include a small sample size and the patient selection. Future large clinical and intervention studies are needed to obtain more definitive information on the clinical relevance and the effects of pharmacologic treatment with acute MI. Prophylactic examination of FT3 might be proposed in cases of older people with cardiovascular disease.

In conclusion, free T3 is negatively associated with INR in patients with acute STEMI and normal thyroid function.

Author contribution

En-zhi JIA designed research; Li LI and Chang-yan GUO performed research; Tie-bing ZHU, Lian-sheng WANG, Ke-jiang CAO contributed new analytical tools and reagents; Wen-zhu MA, Zhi-jian YANG, and Jing YANG analyzed data; Li LI wrote the paper.