Thyroid diseases are often associated with cardiovascular morbidity, but the mechanisms that mediate this risk are unclear. Certain mechanisms have been proposed to be potentially responsible for this scenario and the MetS is one of these. The potential contributory role of the MetS to cardiovascular risk and its scope in subjects with TD was the focus of this study.
The main findings of this report are the documentation of the presence of the MetS in about a fourth of our patients with thyroid diseases. One limitation of this study lies in its nature which comprised a small study population of patients who were already on treatment for thyroid disorders.
In our report, we found the prevalence of the MetS to be 28%, with subjects with thyrotoxicosis having the least frequency of occurrence of the MetS compared with subjects with hypothyroidism and those with nontoxic goiters. The comparison of the MetS in this report showed that the occurrence of this cardiovascular risk factor was comparable in the three categories of study subjects. It is pertinent to note that the study subjects as classified in these three groups had comparable mean ages and anthropometric indices. Of the components of the MetS, dysglycemia was the prevalent MetS defining criterion documented in the subjects with TD. Reported prevalence rates of glucose intolerance in subjects with thyrotoxicosis and hypothyroidism are 40% and 50%, respectively.[17
] In subjects with hypothyroidism, insulin resistance is suggested as the possible underlying pathophysiological basis for glucose intolerance when present.[18
] The effects of excess thyroid hormone on carbohydrate metabolism are complex, with alterations in insulin secretion, insulin clearance, gluconeogenesis, glycogen synthesis, glucose oxidation, nonoxidative glucose metabolism, adipokine signaling, and lipid oxidation contributing to the state of hyperglycemia and insulin resistance observed in thyrotoxicosis.[19
Abnormal lipid profile is an often documented abnormality in thyroid disorders, and some reports[20
] have demonstrated that thyroid hormones influence LDL-C by various mechanisms which include catabolism of LDL-C–independent alterations in metabolism, stimulation of the synthesis of cholesterol as well as the influence on biliary lipid metabolism. Well-documented lipid abnormalities in hypothyroidism include hypercholesterolemia and elevated LDL levels, but HDL-C levels may be normal or elevated in severe hypothyroidism.[21
] In this study, the mean levels of TG, TC and LDL-C were higher in persons with hypothyroidism than in those belonging to the other categories of TD, but statistically significant differences in the lipid parameters, TG and LDL-C, were noted between subjects with hypothyroidism and other subjects with TD. Thyroid hormones are known to affect the cardiovascular system both directly and indirectly and result in increased cardiac contractility and reduced systemic vascular resistance. Ogbera et al
] in a previous report had noted that thyrotoxicosis is a significant cause of cardiac mortality and morbidity in Nigerians and that systemic hypertension is a comorbidity that is found in half of the subjects with thyrotoxicosis. In this study, systemic hypertension occurred least in subjects with thyrotoxicosis, with a prevalence rate of 37%. Our findings on hypothyroidism and hypertension are similar to those by Saito et al
] who compared the occurrence of hypertension in subjects with hypothyroidism and those with euthyroidism. They found the rate of occurrence of hypertension to be 15.8% and this was significantly higher than that in euthyroid subjects in whom the rate was 5.5%.[22
] In our study, subjects with hypothyroidism had the greatest frequency of occurrence of hypertension. Hypothyroidism is a potentially important but overlooked cause of hypertension, and possible pathophysiological mechanisms responsible for the occurrence of hypertension in hypothyroidism include changes in circulating catecholamines, their receptors and renin–angiotensin–aldosterone.[23
] Our results indicate that persons with hypothyroidism had higher mean levels and higher proportions of some of the MetS defining criteria compared with persons with nontoxic goiters and those with thyrotoxicosis. A Korean study[24
] not only reported a relationship between thyroid function and cardiovascular risk factors, such as BP, TC, TG, HDL-C and fasting glucose, but also showed that higher levels of TSH indicative of hypofunctioning of the thyroid gland may predict the MetS in Koreans. A significant relationship between thyroid hormone and the components of the MetS has also been shown in the study by Kim et al
] in a report that sought to clarify the association of serum FT4 level with the presence of the MetS and its components in healthy euthyroid subjects. Although we did not set out to evaluate the insulin resistance which forms the basis of the MetS, one form of thyroid dysfunction, specifically hypothyroidism, has also been found to be associated with a syndrome – polycystic ovary disease – a condition with underlying insulin resistance.[26
] Some other unusual association that has been found in relation to thyroid dysfunction is increased risk of dysglycemia – a MetS defining criterion – in TD subjects with impaired sleep disorder.[27
Central obesity has a role in the development of the MetS and is reported to sometimes precede the appearance of other MetS components.[28
] The occurrence of central obesity was comparable in the three categories of subjects with thyroid diseases and was noted in 38% of the study subjects.
The overall prevalence rate of the MetS in this study was much lower than the 86% rate reported in the diabetes population[29
] and the 59% rate reported in the general population among Nigerians.[30
The results of this report should be interpreted with caution, given the small number of the study population and also the fact that the patients were not thyroid treatment naive.