Atherogenic index of plasma in subclinical hypothyroidism

 

Smita Kottagi¹, Triveni Jambale^2*, Srinivas Deshpande³, Amareshwar Malagi⁴

 

^1,2Assistant Professor, ³Professor, ⁴Associate Professor, Department of Biochemistry, Gadag Institute of Medical Sciences, Gadag, Karnataka, INDIA.

Email: trivenijambale@gmail.com

 

Abstract               Background: Subclinical Hypothyroidism (SCH) is increased thyrotropin-stimulating hormone(TSH) accompanied by normal T3 and T4 levels. SCH is associated with metabolic derangements, elevated cholesterol, increased incidence of atherosclerosis, leading to cardiovascular risk. Significance of dyslipidemia in SCH remains controversial. Many studies have involved various indices to detect cardiac events in SCH significant attention is focused on alternative biochemical markers to assess dyslipidemia. Aims and objectives: aim of present study was to assess lipid atherogenic risk based on Atherogenic index of plasma (AIP). Objective was to quantitatively detect levels of, HDL-Cholesterol, and triglycerides to evaluate the importance of AIP as better predictor of cardiovascular risk in SCH. Materials and Methods: 30 SCH cases compared with 30 euthyroid controls. Serum T3, T4, TSH estimated by ELISA method, Serum HDL-Cholesterol by enzymatic CHOD-PAP method, Triglycerides by GPO-POD method. Atherogenic index of plasma (AIP) log (TG/HDL-C) was calculated on all the samples. Results:: Serum levels of TSH(P<0.001), TC( p<0.001), HDL( p<0.001), LDL( p<0.001), AIP ( p<0.001),showed significant increase. No significant change in levels of serumT4 was observed. Conclusion: Results contribute to high atherogenic risk as indicated by AIP. So AIP can be used as better indicator of dyslipidemia as compared to isolated lipid profile parameters and highlights cardiovascular risk in SCH.

Key Words: Atherogenic index of plasma (AIP), Subclinical Hypothyroidism, cardiovascular risk.

 

 

 

INTRODUCTION

Subclinical hypothyroidism (SCH) is defined as increased serum TSH concentration above the upper limit of normal range with the serum T4 (T4) and T3 concentration within its normal range¹. Little decrease in levels of T4 within the normal range will cause an increase in serum TSH above the normal range. So, it is important to measure the levels of serum TSH for diagnosing the mild thyroid failure or subclinical hypothyroidism when peripheral thyroid hormone levels are in normal laboratory range². Subclinical hypothyroidism is a disorder based on laboratory diagnosis. Patients with subclinical thyroid disorder have few or no clinical signs and symptoms of thyroid dysfunction¹. Subclinical hypothyroidism is commonly detected now a days and has a prevalence rate of 3% to 8%³. The clinical significance and the treatment for subclinical hypothyroidism with TSH (<10mIU/L)¹ and the exact upper limit of the reference range for the serum TSH level remain debatable⁴. Levothyroxine therapy is preferred to be appropriate for TSH levels above 10mIU/L. But, therapy for patients having a serum TSH level lower than 10mIU/L is controversial⁵. Many studies have found that subjects with subclinical hypothyroidism have higher total cholesterol and low density lipoprotein cholesterol levels as compared to euthyroid subjects⁶. But the studies have shown conflicting results. This indicates the necessity of alternate biochemical parameter to show the importance of dyslipidemia in subclinical hypothyroidism.

MATERIALS AND METHODS

The study was performed in Dept. of Biochemistry, BLDEU’S Shri. B. M. Patil Medical College Hospital and Research Centre, Bijapur (Karnataka) India. 30 subclinical hypothyroid cases were compared with 30 euthyroid controls as per the inclusion and exclusion criteria mentioned below. This study was approved by the Institutional Ethics Committee. Consent was taken from the subjects before undergoing further investigations related to the study. The study was done during the period of November 2011 to May 2013.

Inclusion Criteria: Subclinical hypothyroidism cases with TSH within the range of 4.50 to 14.99 mlU/L, T3 and T4 within normal limits. The euthyroid controls having normal TSH [0.3-4.5 mlU/L.] were included in the study.

Exclusion Criteria: diagnosed cases of hypothyroidism, cases on which thyroidectomy is performed, patient with radiotherapy, radioactive iodine therapy, patients consuming drugs known to cause SCH, primary or secondary dyslipidemia, patients with diabetes mellitus, patients with systemic illness, renal and hepatic failure cases, patients on statins were excluded from the study.

Collection of blood samples: Venous blood samples were drawn at 8 a.m. after 12 hours of fasting, in a plain bulb with aseptic precautions. Blood samples were centrifuged within 30 minutes at 3000 rpm for 5 min. and serum was separated. Serum samples were stored at -20°C till assayed. Serum T3, T4, TSH were estimated by ELISA method.^7-9 Serum total cholesterol estimation and HDL-C was done by enzymatic CHOD-PAP method.(10) LDL-C was calculated by using Friedewald formula.¹¹ serum Triglycerides by enzymatic GPO-POD method¹². Atherogenic index of plasma (AIP) was calculated by using the formula log (TG/HDL-C)¹³.

RESULT

Table 1 shows that that serum mean levels of TSH, T3,TC,TG,, LDL,HDL,AIP, were highly significant in SCH patients as compared to controls where as T4 did not show statistically significant difference as compared to controls

 

Table 1: Parameters showing dyslipidemia in subclinical hypothyoidism subjects and control group

p<0.001- Highly Significant. T3=Tri-iodothyronine, T4=Tetra iodo- thyronine, TSH=Thyroid stimulating hormone, TC=Total cholesterol, LDL=Low density lipoproteins, HDL=High density lipoproteins, AIP =Atherogenic index of plasma.

 

DISCUSSION

Subclinical Hypothyroidism (SCH) is a common disorder as compared to overt hypothyroidism. Overt hypothyroidism is found to causes secondary hyperlipidemia and increase the risk of atherosclerosis is been proved¹⁴ but whether subclinical hypothyroidism cause causing dyslipidemia and atherosclerosis leading to cardiovascular disorders is still controversial aspect. The effects of alterations of thyroid function in SCH on lipid profile and atherogenesis is not very clear¹⁵. There are many studies showing SCH as an indicator of increased risk of atherosclerosis and myocardial infarction in elderly women¹⁶ so there is the need of alternative biochemical markers to prove dyslipidemia in subclinical hypothyroidism. Our study revealed that serum levels of TSH, T3 are significantly increased where as T4 levels are not significant. Our study also showed that the serum levels of total cholesterol, HDL-C, LDL-C TG and atherogenic index of plasma are highly significant. Our study is similar to the study done by Zoe Efstathiadou et al¹⁶ who studied lipid profile in subclinical hypothyroidism and came to the conclusion that serum total cholesterol and LDL-C were significantly increased in SCH as compared to controls. Our study show same results as study performed by Rafael Luboshitzky et al¹⁷, which showed that the percentage of subjects with increased total cholesterol, triglycerides, LDL-C were more in SCH as compared euthyroid controls. Nwagha UI et al¹⁸ studied Atherogenic index of plasma as useful predictor of cardiovascular risk and showed that AIP is highly significant as compared to other lipid profile parameters to assess dyslipidemia. Our study showed the similar results. Dobiasova, M¹⁹ studied atherogenic index of plasma (AIP) in patients AIP was significant in the cases as compared to controls. Our study showed the similar results. This indicates the importance of studying the ratio of atherogenic to the atheroprotective parameters rather than analysing the individual parameters to asses dyslipidemia in SCH.

 

CONCLUSION

AIP can easily be calculated from standard lipid profile and can act as an adjunct that significantly proves the dyslipidemic state as compared to that of the individual lipid profile parameter to assess atherogenic risk in subclinical hypothyroidism.

 

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