Background
Subclinical hypothyroidism (SCH) is defined as an isolated elevation of thyroid stimulating hormone (TSH) levels in conjugation with normal circulating levels of free triiodothyronine and free thyroxine. It is a highly prevalent disease especially in the elderly population. Thyroid hormones affect the heart and vasculature by both genomic and nongenomic pathways. However, the impact of SCH on the cardiovascular system is a matter of debate. Researches have been conducted to study the effect of SCH on cardiovascular system, yielding conflicting results. Although some studies support increased risk of cardiovascular events in patients with SCH, others show no significant increased risk.
Aim
This study was conducted to evaluate if SCH is associated with higher risk of coronary heart diseases in the elderly and if dyslipidemia, endothelial dysfunction as measured by flow-mediated dilatation (FMD) and carotid artery intima-media thickness were associated with SCH.
Patients and methods
Fifty elderly individuals aged 65 years and older were enrolled in this study and were divided into two groups, group I: 30 patients with SCH and group II comprised 20 age-matched and sex-matched euthyroid elderly serving as a control group. In all participants we performed serum TSH, free thyroxine, and antithyroperoxidase antibodies. SCH was defined as an elevated thyrotropin (TSH) (>4.5 mU/l) and normal free thyroxine level. Complete lipid profile, thyroid ultrasound, echocardiography to assess cardiac function and markers of endothelial dysfunction namely carotid artery intima-media thickness and FMD of the brachial artery (BA) after occlusion were done to all cases.
Results
The mean age of group I was 69.2±3.1 years and the mean age of group II was 68.6±3.2 years. Overall, 50% of the elderly patients with SCH (group I) were suffering from hypertension, whereas 35% in the elderly euthyroid group (group II) were hypertensive. The systolic and diastolic blood pressures are higher in group I as compared with group II (140±20 and 86±12, respectively vs. group II were 131±19 and 82±12, respectively), but the differences were statistically insignificant (=0.12 and 0.21, respectively). No significant statistical difference was observed when the elderly SCH patients were compared with a euthyroid control group as regards the mean cholesterol, triglycerides, low-density lipoprotein cholesterol, and high-density lipoprotein levels (=0.69, 0.79, 0.77, 0.42, respectively). For the elderly SCH group I the mean BA diameter before dilation was 3.12±0.44 versus 3.44±0.68 mm for euthyroid group II. However, the mean BA diameter after dilation was 3.64±0.60 mm compared with 4.05±0.73 mm for the euthyroid group. The mean percentage of FMD% of BA after occlusion was 16.2±5.7 in group I versus 17.9±5.5 in group II. There was no statistically significant difference between the two groups as regards flow-mediated vasodilatation% of the BA after occlusion (=0.29).
Conclusion
This study did not find a significant association between thyroid function, lipid profile, and vascular parameters in patients who were similar with respect to age, BMI, smoking and menopausal status, and endothelial function modifiers.