Dyslipidemia In Thyroid Disorders

Background : Thyroid hormones play an important role in regulating lipid metabolism; and thyroid dysfunctions can result in lipid abnormalities which increase the risk of endothelial dysfunction, hypertension and cardiovascular disease. Methods : A cross-sectional study was conducted on 50 patients with suspicion of thyroid disorders were taken as cases. 50 patients with normal thyroid profile and no history of other chronic diseases were taken as control group. Results: The serum TC, TG and LDL levels in hypothyroid individuals (both overt and subclinical) were significantly higher than euthyroid subjects but the levels were comparable between hyperthyroid and euthyroid group. Conclusion : We conclude that, dyslipidemias are associated with thyroid disorders, so biochemical screening for thyroid dysfunction in all dyslipidemic patients. Therefore, patients presenting with dyslipidemia are recommended for investigation to explore thyroid dysfunction.


Introduction
Hypothyroidism is characterised by reduced thyroid activity resulting from less secretion of both T3 and T4.It is a clinical manifestation irrespective of the cause. It is the most common pathologic hormone deficiency among other endocrine disorders. Hypothyroidism may be due to primary disease of the thyroid gland itself or lack of pituitary TSH. Biochemical decrease in T4 and T3 concentrations causes hypersecretion of pituitary TSH, thus an amplified increase in serum TSH levels. This is a key laboratory finding, particularly in the early detection of thyroid failure. [1][2] Thyroid hormones significantly effect on synthesis, mobilization and metabolism of lipids.
In general, overt and subclinical hypothyroidism is associated with hypercholesterolemia which is mainly due to elevation of low density lipoprotein (LDL) cholesterol levels, whereas high density lipoprotein (HDL) cholesterol concentration is usually normal or even elevated. 3 In hypothyroid patients, despite the reduced activity of HMG CoA reductase, there is often an increase in the serum total cholesterol concentration, mainly due to raised levels of serum LDL cholesterol and intermediate density lipoprotein (IDL) cholesterol. 4 Decreased thyroid secretion greatly increases the plasma concentration of cholesterol because of decreased rate of cholesterol secretion in the bile and consequent diminished loss in the faeces due to decreased number of low density lipoprotein receptors on liver cells. Decreased activity of LDL receptors resulting in decreased receptormediated catabolism of LDL and IDL is the main cause of the hypercholesterolemia observed in hypothyroidism.
Hypertriglyceridemia in hypothyroidism is due to decreased activity of lipoprotein lipase (LPL), which results in decreased clearance of triglyceride-rich lipoproteins. 5

Materials and Methods
A cross-sectional study was conducted on 50 patients with suspicion of thyroid disorders were taken as cases. 50 patients with normal thyroid profile and no history of other chronic diseases were taken as control group. Detailed informations of the patients were collected after taking informed consent with the help of pre-test proforma that included age, sex and family or personal history of chronic diseases.
After 12 hours overnight fasting, 5 ml blood was collected by standard venipuncture method, and the serum was separated. T3, T4 and TSH were quantitatively estimated by Enzyme linked immunosorbent assay (ELISA) method. The serum TC, TG and LDL levels in hypothyroid individuals (both overt and subclinical) were significantly higher than euthyroid subjects but the levels were comparable between hyperthyroid and euthyroid group.

Discussion
Thyroid dysfunction, along with a higher prevalence of goiter, is a major public health problem in India population. In this study, the prevalence of hypothyroidism was higher than hyperthyroidism similar finding observed by findings by Baral et al. 6 and Holowell et al. 7 The serum TC and LDL levels in hypothyroid individuals (both overt and subclinical) were significantly higher than euthyroid subjects but the levels were comparable between hyperthyroid and euthyroid group in our study. Jung 8 found mean plasma total cholesterol and LDL cholesterol levels elevated in hypothyroid cases than in normal controls.
In another study, average serum total cholesterol level was found elevated in primary and secondary hypothyroidism 9 .
Keyes & Heimberg 10 , Laker & Mayes 11 found triglyceride level elevated in hypothyroid patients. Thompson 12 and Abrams & Grundy 13 have stated decreased activity of LDL receptors as the main cause of hypercholesterolemia in hypothyroidism.

Conclusion
We conclude that, dyslipidemias are associated with thyroid disorders, so biochemical screening for thyroid dysfunction in all dyslipidemic patients. Therefore, patients presenting with dyslipidemia are recommended for investigation to explore thyroid dysfunction. As our sample size was small and duration of study was limited, another study with large sample size and longer duration is also recommended.