subclinical hypothyroidism ( sh ) , described as a condition characterized by elevated serum level of thyroid stimulating hormone ( tsh ) with normal levels of free thyroxine ( ft4 ) and free triiodothyronine ( ft3 ) , is a common disorder having a prevalence of about 7.58.5% in females and 2.84.4% in males , worldwide .
several studies have established overt hypothyroidism to be associated with abnormalities of lipid metabolism such as elevated levels of total cholesterol ( tc ) , low - density lipoprotein - cholesterol ( ldl - c ) , and triglycerides ( tg ) , thereby predisposing to cardiovascular diseases .
however , the association of sh with abnormalities in lipid parameters have been studied with conflicting results .
some studies have also established a link between sh and metabolic syndrome while others have failed to do so .
it has also been indicated that small changes in thyroid hormone levels within the reference range may influence the severity of atherosclerosis .
currently , serum lipid ratios such as tc / high - density lipoprotein - cholesterol ( tc / hdl - c ) , tg / hdl - c , and ldl - c / hdl - c have been shown to be better predictors of cardiovascular risk compared to conventional lipid profile .
small dense ldl ( sdldl ) , a subtype of ldl has been described as the main determinant of the atherogenicity of ldl - c .
atherogenic index of plasma ( aip ) has been defined as the logarithmically transformed ratio of tg to hdl - c .
aip , which is a good surrogate marker of sdldl , has been indicated as a superior predictor of cardiovascular risk than the previously used lipid parameters .
however , research on the complex interrelationship between sh and lipid ratios and aip has been extremely rare .
thus , though the effect of overt hypothyroidism on abnormalities in lipid parameters has already been established , the relationship between sh and lipid metabolism needs further study .
furthermore , there is insufficient information regarding the effect of sh on lipid ratios and aip .
this is particularly true in the indian scenario where conflicting results have been obtained , especially in the female population where sh is more prevalent .
therefore , this study was conducted to understand the association of sh with abnormalities in the serum lipid profile and aip in apparently healthy adult females .
the study also aims to find out whether any correlation exists between aip and serum tsh levels among sh cases .
the study was a retrospective , time - bound , hospital - based , case  control study carried out by detailed analysis of patient records in the department of biochemistry at a medical college in southern india .
the study was approved by the institutional research committee and ethics committee , which follow the guidelines set by the helsinki declaration ( reference number - rc/14/123 ) .
data collection was done for 2 months from may 1 , 2015 , to june 30 , 2015 .
study subjects were taken from the records of apparently healthy adult females coming for a master health check to our hospital from june 1 , 2014 , to may 30 , 2015 .
adult females with high tsh ( > 4.2 iu / ml ) but normal ft3 ( 2.04.4 pg / ml ) and ft4 ( 0.931.7 ng / dl ) levels were included in the sh group .
adult females having normal tsh ( 0.274.2 iu / ml ) , ft3 ( 2.04.4 pg / ml ) , and ft4 ( 0.93 - 1.7 ng / dl ) levels were included in the euthyroid ( eu ) group .
individuals with a history of overt hypothyroidism or levothyroxine replacement therapy , history of radioiodine treatment or thyroid surgery , history of alcohol abuse and smoking , history of drugs such as estrogens , diabetic individuals , subjects with renal , hepatic or other systemic diseases , and known history of dyslipidemia , were excluded from the study .
totally , 504 female subjects came for master health check during the period that had been selected . of these
, 300 patient records could be studied during the 2 months when the study was carried out .
based on the ft3 , ft4 , and tsh values , the subjects were classified into sh and eu groups according to the inclusion criteria .
based on age - matching between the sh and eu groups , seventy consecutive patients in each group , according to their date of presentation were taken for statistical analysis .
detailed epidemiological information along with a detailed history was taken from the hospital records of the patients who came for a master health check to our hospital .
specific emphasis was given to age , history of smoking and alcohol intake , history of overt hypothyroidism or levothyroxine replacement therapy , history of thyroid surgery , history of hypertension , diabetes or dyslipidemia , history of liver , hepatic or any other systemic disease and history of medications such as oral contraceptive pills .
next , the reports of the subjects were collected with special emphasis on serum fasting glucose , liver function tests , renal function tests , complete blood count with peripheral smear , and routine examination of urine .
for the study , the following results were noted : serum tsh , ft3 , ft4 , tc , tg , hdl - c , and ldl - c . for each subject
included in sh group and eu group , the lipid ratios were calculated as tc / hdl - c , tg / hdl - c , and ldl - c / hdl - c .
the routine serum parameters in all the study subjects had been measured using automated routine chemistry analyzer ( roche cobas integra 400 plus , roche diagnostics limited , switzerland ) with commercial kits ( roche cobas c packs , roche diagnostics limited , switzerland ) according to manufacturer 's protocol .
thyroid profile of all the study subjects had been measured using automated electrochemiluminescence immunoassay analyzer ( roche cobas e411 , roche diagnostics limited , switzerland ) with commercial kits ( cobas cassettes containing working solutions ) according to manufacturer 's protocol .
the data were analyzed using spss statistical software ( v 16 ; ibm corporation , armonk , ny , usa ) . to describe continuous variables , mean and standard deviation was used .
since the data were not normally distributed , the difference in lipid parameters , lipid ratios , and aip between sh group and eu group , were compared using mann 
whitney u - test and a p < 0.05 was considered statistically significant . in the sh group , aip and tsh
logistic regression was performed to ascertain the effects of age , tc , tg , hdl , ldl , tg / hdl , tc / hdl , ldl / hdl , and aip on sh . variables which are significant at < 0.1 in simple regression were considered for multiple regression analysis and because of multicollinearity , only one of the following variables were included in the model at a time : tg , tg / hdl , and aip .
the data were analyzed using spss statistical software ( v 16 ; ibm corporation , armonk , ny , usa ) . to describe continuous variables , mean and standard deviation was used .
since the data were not normally distributed , the difference in lipid parameters , lipid ratios , and aip between sh group and eu group , were compared using mann 
whitney u - test and a p < 0.05 was considered statistically significant . in the sh group , aip and tsh were correlated using spearman 's correlation .
logistic regression was performed to ascertain the effects of age , tc , tg , hdl , ldl , tg / hdl , tc / hdl , ldl / hdl , and aip on sh . variables which are significant at < 0.1 in simple regression were considered for multiple regression analysis and because of multicollinearity , only one of the following variables were included in the model at a time : tg , tg / hdl , and aip .
there was no statistically significant difference ( p > 0.05 ) among the mean ages of sh and eu groups .
however , there was a statistically significant ( p < 0.05 ) difference between the means of ft3 , ft4 , and tsh of the sh and eu groups .
comparison of baseline characteristics of the study subjects the data in table 2 represent a comparison of lipid profile parameters of eu group and sh group .
it includes mean and standard deviations of tc , tg , hdl - c , and ldl - c of eu and sh groups . the differences between the means of tc , tg , and ldl - c were found to be statistically significant while that for hdl - c was found to be insignificant ( p = 0.712 ) .
the data presented in table 3 shows the comparison of lipid ratios such as tc / hdl - c , tg / hdl - c , and ldl - c / hdl - c between sh and eu groups .
the difference in the mean tg / hdl - c between the two groups was found to be significant ( p < 0.005 ) .
comparison of lipid profile parameters between the study groups comparison of lipid ratios between the study groups figure 1 represents the comparison between the mean aip of sh group and eu group .
the mean aip of eu group was 0.078 , and that of sh group was 0.211 . when the means were compared using mann 
whitney u - test , the difference was found to be statistically significant ( p = 0.002 ) .
the spearman 's correlation coefficient r is 0.174 ( p = 0.149 ) indicating that there is no significant correlation between aip and tsh in the sh group .
eu : euthyroid group , sh : subclinical hypothyroidism group , aip : atherogenic index of plasma .
* difference of the means of aip between eu and sh groups is statistically significant ( p = 0.002 , < 0.05 ) correlation between atherogenic index of plasma and thyroid stimulating hormone in subclinical hypothyroidism group .
no significant correlation , spearman 's correlation coefficient r is 0.174 ( p = 0.149 ) table 4 represents the results of logistic regression analysis .
aip has a significantly higher risk ( or = 22.38 ) than tg ( or = 1.01 ) or tg / hdl ( or = 1.42 ) .
the link between dyslipidemia and sh has been explored in a large number of studies of varying methodologies in different parts of the world .
however , studies on the abnormalities in lipid ratios and aip which are better markers of cardiovascular risk than conventional lipid profile are rare . with this background
control study , where we retrospectively studied patient records to investigate the abnormalities in conventional lipid profile , lipid ratios , and aip in sh .
the study also aimed to find out if there was any correlation between tsh and aip levels in sh . in our study
the mean age of sh patients in our study was 42.5 similar to other studies done in tamil nadu ( india ) , greece , and turkey but higher than studies done in kashmir ( india ) , italy , and turkey .
the differences can be explained by the different study populations used in the different studies which differed on ethnicity , gender preponderance , race , etc .
, however , tsh levels were significantly higher in sh as compared to eu similar to other studies . in our study
, we also reported that the mean ft3 and ft4 was significantly lower in sh as compared to eu though in both groups the values were within normal reference ranges .
this is in contrast to the other study done in tamil nadu where no significant difference was noted .
the difference could be explained by our larger sample size whereby the study in tamil nadu had a sample size of 30 whereas our study had a sample size of 70 . regarding lipid profile parameters ,
our study showed a significantly lower tc and ldl - c level and a significantly higher tg level in sh as compared to eu .
several other studies conducted in india and abroad have also reported similarly higher tg levels , but tc and ldl - c were also reported to be higher in sh as opposed to our study . since our study was based on retrospective analysis of patient records , it can be suggested that a prospective study could lead to a better agreement . on the other hand ,
a contrasting result was obtained in national health and nutrition examination survey iii , a large study of 215 subjects with sh and 8013 euthyroid individuals , where no significant difference in lipid parameters between sh and euthyroid individuals was reported , when adjusted for age , race , sex , and the use of lipid - lowering agents . in overt hypothyroidism
, the molecular basis of this dyslipidemic pattern has been elucidated . in overt hypothyroidism , due to the reduced activity of ldl - c receptors ,
the catabolism of ldl - c , and intermediate density lipoprotein - cholesterol are affected , consequently increasing serum tc and ldl - c .
moreover , in overt hypothyroidism , the activity of the enzyme lipoprotein lipase is lowered , thereby inhibiting the removal of tg - rich lipoproteins leading to a high serum tg level . in our study , the mean hdl - c was lower in sh than in eu , but the difference was not statistically significant .
this result is in contrast to the study done in tamil nadu where the hdl - c was significantly lower in sh compared to eu . in our study ,
the lower than desirable hdl - c levels in both sh and eu can be explained by the lack of exercise and sedentary lifestyle among the study participants . in the indian context , this poses a serious health risk and needs to be investigated further by large population - based studies as well as experimental studies to elucidate the molecular etiopathogenetic links .
several studies have also shown that levothyroxine therapy has led to significant improvement in dyslipidemia in sh patients , but we have not studied this aspect . in contrast , in overt hypothyroidism , elevation of serum hdl - c levels may be seen due to increased concentration of hdl2 particles .
considering our results about the lipid ratios , our study showed a significantly higher tg / hdl - c in sh as compared to eu , but no significant difference was seen in tc / hdl - c and ldl - c / hdl - c levels . in a study , it was proved that 6-month treatment with levothyroxine led to normalization of tsh , and this improved the adverse lipid profile of elderly women with sh .
this effect was reflected in the lipid ratios and thereby concentrations of tc / hdl - c , tc / ldl - c , lp(a ) and apob were significantly reduced but apoai and hdl - c were also reduced but not significantly .
however , the difference in the means of lipid ratios between sh and eu have not been studied .
thus , though controversies exist regarding the association of sh with cardiovascular risk , to the best of our knowledge , no other study has investigated the status of lipid ratios in sh .
thus , to fill this knowledge lacuna , we decided to include lipid ratios in our study . in our study , aip was significantly higher among sh subjects than eu subjects .
the mean value of aip in sh subjects was found to be 0.211 which falls in the at risk category ( aip > 0.11 ) for cardiovascular diseases .
aip has already been established as strongly correlating with serum sdldl level , which is the main atherogenic fraction of ldl . on the other hand ,
sh has been shown to be associated with highly atherogenic pattern b of ldl - c subfractions .
it has been reported that as a marker of lipoprotein particle size , aip adds predictive value beyond that of the individual lipids , and/or tc / hdl - c ratio .
this is corroborated in our multiple regression analysis , where aip emerged as the single most significant factor associated with sh among the study parameters .
this could also indicate that sh is associated with high aip and thereby a highly atherogenic lipid profile . nevertheless , despite extensive literature search
in other metabolic disease states such as metabolic syndrome , hyperuricemia , and diabetes mellitus , aip was found to be a good predictor of cardiovascular risk . in overt hypothyroidism
also aip levels have not been studied , but conflicting results were seen in studies investigating the impact of overt hypothyroidism on ldl - c subfractions , particularly sdldl . though a recent study conducted on newly diagnosed hypothyroid patients showed an association between hypothyroidism and elevated sdldl , other studies could not conclusively prove such an association .
all these results reiterate the need for studies on the association of aip and abnormal lipid ratios with sh . in our study , we have attempted to bridge this gap and from our results , it could be speculated that though serum ldl - c was lower in sh subjects , their pattern is significantly atherogenic with a high level of sdldl .
we failed to find any correlation between aip and tsh levels in sh subjects thereby indicating that though sh patients were at a greater risk of cardiovascular diseases , the risk did not proportionately increase with an increase in tsh levels .
several studies have reported that dyslipidemic changes were not apparent when serum tsh levels were below 10 miu / l but above this level , dyslipidemia was more overt .
an indian study reported that there was no association between lipid abnormalities and sh when the value of serum tsh was < 10 miu / l . on the other hand , patients with tsh concentrations
mu / l had significantly greater levels of tc and ldl - c as well as lower hdl - c compared with controls . based on these findings
, it would not be wrong to speculate that since a majority of our patients had a tsh level < 10 miu / l , aip could not be correlated with tsh levels in sh patients .
ours is a retrospective , hospital - based , time - bound study where hospital records of study subjects were analyzed .
large , well - planned , community - based , prospective studies need to be conducted to gain a better understanding of dyslipidemia in sh , particularly the study of aip .
thus , the positive association of dyslipidemia and sh indicates a need for regular screening of sh patients for dyslipidemia to enable early diagnosis and treatment of the condition . even in patients who have a normal conventional lipid profile , lipid ratios , and aip
the high prevalence of low hdl levels reiterates the need for counseling of patients regarding healthy dietary habits and regular exercise .
high aip among the sh population too underlines the need for large - scale , community - based programs for health awareness and lifestyle modification .
however , this is a cross - sectional study and so , whether sh leads to high aip could not be determined . moreover ,
the molecular basis for this association can only be elucidated by experimental studies with animal models .
this study helped to bridge the knowledge gap as well as paucity of indian data available regarding the complex interplay between thyroid hormones and atherogenic lipid parameters , particularly in sh .
the study also emphasizes the importance of aip in the detection of atherogenic risk in sh patients , even in the presence of minor dyslipidemia .
abnormalities found in conventional lipid profile as well as in lipid ratios and aip imply sh to be a risk factor for dyslipidemia and atherosclerosis .
an atherogenic lipid profile is a prequel to cardiovascular and cerebrovascular diseases , and hence , its association with sh in general and calculation of aip , in particular , could provide avenues for early diagnosis and rigorous management of dyslipidemia in hypothyroidism .
 background : a controversy exists regarding the association between subclinical hypothyroidism ( sh ) and dyslipidemia . 
 moreover , studies on lipid ratios and atherogenic index of plasma ( aip ) in sh are rare , particularly in the indian scenario.aim:this study aimed to investigate abnormalities in conventional lipid profile , lipid ratios , and aip in sh and attempted to correlate thyroid stimulating hormone ( tsh ) and aip in sh.materials and methods : in this retrospective analysis of patient records of sh subjects and euthyroid subjects , age , free triiodothyronine , free thyroxine , tsh , total cholesterol , triglycerides , high - density lipoprotein - cholesterol ( hdl - c ) , low - density lipoprotein - cholesterol , lipid ratios , and aip were compared between the two groups . 
 the correlation of tsh and aip in sh was studied . 
 spearman 's correlation , mann  
 whitney u - test and logistic regression analysis were performed.results:triglyceride , triglyceride / hdl - c , and aip were significantly higher in sh as compared to euthyroid group , but there was no correlation between tsh and aip in sh . 
 aip emerged as the significant single factor associated with sh in multiple logistic regressions.conclusion:the positive association of dyslipidemia and sh indicates a need for regular screening of these patients to enable early diagnosis and treatment of dyslipidemia . even in patients who have a normal conventional lipid profile , lipid ratios , and aip 
 have to be calculated for better assessment of atherogenic risk . 