European Journal of Cardiovascular Medicine

In India thyroid disorders are the   most common among all endocrine disorders and hypothyroidism is more common than hyperthyroidism and carcinoma thyroid.¹ The prevalence of hypothyroidism has been reported to be 1-2% in women and 0.1%in men. Hypothyroid patients have dyslipidemia, impaired glucose tolerance and increased insulin resistance leading to various complications.² Insulin resistance is assessed by the calculation of homeostatic model assessment-insulin resistance [HOMA-IR]. (HOMA- IR) is a method for assessing basal glucose and insulin or C-peptide concentrations.³ The relationship between glucose and insulin in the basal state reflects the balance between hepatic glucose output and insulin secretion, which is maintained by a feedback loop between the liver and beta-cells of pancreas ⁴    Hypothyroidism and obesity frequently coexist. Hypothyroidism leads to increased body weight by increasing muscle deposits of fat and by salt and water retention. Extreme obesity also leads to increased thyroid stimulating hormone. Hyperlipidemia, particularly hypercholesterolemia, is a well documented, significant health consequence associated with hypothyroidism and obesity.

Several studies have shown that a significant  increase  in  insulin, HOMA-IR and glucose levels  in  hypothyroidism  cases  when  compared  to  controls. Total  cholesterol[TC], low density lipoprotein[LDL], very  low  density  lipoprotein [VLDL]  and  triglycerides[TG]  are also  significantly increased, where as high  density  lipoprotein [HDL]  was  significantly  decreased  in  hypothyroidism cases  when  compared  with  controls⁵ Another study on hypothyroidism   concluded  that  hypothyroidism  is  associated  with  metabolic  syndrome and  females  are  more  at  risk⁶ . Thyroid hormone influences all aspects of lipid metabolism including synthesis, mobilization, and degradation. Thyroid hormone increases the activity of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase, cholesteryl ester  transfer protein (CETP), hepatic lipase, and lecithin: cholesterol acyltransferase^7,8 Insufficient thyroid hormone levels would  be  predicted  to  be  associated with increased TC,LDL and TG, accompanied by a decrease in high density lipoprotein [ HDL].  In  hypothyroidism,  the  dyslipidemia  is  mainly  caused  by  a shift  to  increased   synthesis  over  degradation  rate, with  the elevated  levels  of TC, chiefly  LDL, providing  the   substrate  for  lipid    peroxidation   by   reactive   oxygen   species (ROS),  thus

  resulting   in  oxidative  stress ⁹. Moreover, the  synthesis  and  rate  of  catabolism  of  fatty acids  in  hypothyroidism  is  decreased  and  the  lipolytic  sensitivity  of white  fat cells  is  blunted¹⁰ .

 Clinical hypothyroidism is characterised as a risk factor for resistance to insulin^11,12.There  is  a  decline  in  the  rate  of  absorption  of  intestinal  glucose  in  hypothyroidism, as adrenergic activity decreases, resulting in a decrease in muscle and liver glucose breakdown, along  with a reduction in new glucose synthesis and secretion of  insulin. However due  to massive  peripheral  insulin  resistance  associated  with  higher  free  fatty  acid  concentrations, impaired  glucose  absorption  and  elevated  glucose  oxidation,  there  was a  post-prandial rise in insulin secretion¹³ 

Many mechanisms have been proposed to explain the observed association between obesity and thyroid dysfunction. Several studies have found a positive correlation between increased serum leptin and Thyroid stimulating hormone [TSH] levels in obese individuals¹⁴. Leptin promotes Thyrotropin releasing hormone [TRH] expression and synthesis in the paraventricular hypothalamic and arcuate nucleus, which, in turn, can cause an increase in serum TSH levels ¹⁵. Leptin could also increase the conversion of T4 to T3 in peripheral tissues resulting in decreased levels of T4 that is observed in many studies. Moreover, this can lead to compensatory activation of the hypothalamus-pituitary-thyroid axis in an attempt to maintain serum thyroid hormone levels within the euthyroid range.^16,17 Adipocytes of obese individuals generate a state of peripheral resistance to thyroid hormones due to their lower-than-normal expression of TSH receptors, leading to increased plasma TSH levels as a compensatory mechanism ¹⁸ Although it is not possible to precisely assert whether thyroid function changes are primary or secondary to obesity, several studies suggest obesity to be the primary factor that triggers thyroid function changes. This conclusion is based on the reversion or alleviation of thyroid dysfunction after weight loss in obese individuals ¹⁹. Indeed, weight loss is proposed to be a key factor in the restoration of hormonal imbalances observed in obesity ²⁰ It is likely that both states, thyroid dysfunction primary or secondary to obesity, to some degree, are involved in all studies. Adipokines and inflammatory cytokines released by the adipose tissue in obese individuals might play a role in altering the thyroid function by inducing chronic low-grade inflammation in the thyroid tissue of obese individuals. ^{21, 22} obese individuals are more prone to autoimmune and inflammatory processes in the thyroid gland. ^{23, 24}

 In light of the above facts, this work intends to see the possible association of impaired glucose tolerance, insulin resistance, abnormal lipid profile and obesity in hypothyroid patients

Aim and objectives:

1. To assess impaired glucose tolerance in hypothyroid patients.
2. To determine lipid profile in hypothyroid patients.
3. To assess the degree of lipid abnormality in hypothyroid patients with impaired glucose tolerance
4. To establish the association of metabolic syndrome in hypothyroid subjects.
5. To establish the association of insulin resistance in hypothyroid subjects.

After taking institutional ethical clearance and informed consent from the patients the study was undertaken.

 The study comprised of hypothyroid individuals who visited General Medicine outpatient Department, Government   General Hospital, Guntur, during the period of 14 months from 1-08-2015 and 31-10-2016. Age and sex matched healthy volunteers served as controls. The test group consisted of 50 subjects both male and female in the age group of 20-50yrs who were diagnosed to be having hypothyroidism. Exclusion Criteria: 1. Patients with diabetes mellitus, hypertension and other metabolic disorders. 2. Pregnant women 3. Post thyroidectomy patients. 4. Patients on any other treatment like oral contraceptive pills and statins. Control group consisted of 50 normal individuals both male and female of age between 20 to 50 years, with normal waist circumference (male < 94cm, female <80cm), without hypertension and diabetes or any other clinical abnormality.  For both control and test groups   following data were recorded.  Waist Circumference (WC), waist circumference was obtained at the level of the umbilicus with a plastic anthropometric tape.

Blood samples were collected after 10-12 hours in the fasting state from both control and test groups for the analysis of following parameters.  Fasting serum glucose and oral glucose tolerance test [GTT] , Thyroid profile, triiodothyronine (T3], thyroxine [T4],thyroid stimulating hormone [TSH) , Lipid Profile (Triglycerides, Total Cholesterol and HDL Cholesterol) , Fasting Serum Insulin level , Calculation of Homeostatic model assessment for insulin resistance (HOMA-IR)=(Fasting Serum glucose (mg/dl) × fasting serum insulin in µI.U/ml) / 405.

 Using standard kits the tests were done. Clinical chemistry analysis was done on semi automated analyzer-ERBA .Thyroid profile and insulin was done on ELISA, MINDRAY instrument.  Appropriate Quality Control done before doing the tests.

Statistical methods: Numerical variables were presented as mean ± standard deviation (Mean±S.D.). Correlation analysis was done by using Pearson correlation analysis. The data was entered in excel and analyzed in SPSS Version 22. Descriptive statistics was done with Mann-Whitney U test, Wilcoxon test

 p value<0.05 was considered significant.

Table 1: Thyroid profile and insulin resistance in cases and controls.

Table 2: Oral glucose tolerance test [OGTT] in cases and controls.

 Table 3:  Lipid Profile in cases and controls.

Graph 1: Correlation of TSH and Insulin

 The relation between TSH & Insulin was positive correlation (0.64) p value<0.001 and it was statistically significant.

Graph 2: Correlation of TSH and HOMA- IR.

The relation between TSH and HOMA-IR was positive correlation (0.69)  p value <0.001 and it was statistically significant.

In 50 cases of hypothyroid patients studied, the proportion of females (80%) when compared to proportion of males (20%) was found to be high. Hypothyroidism is a common condition affecting females more than males and more commonly the middle aged population²⁵_. In this study, it was found that hypothyroidism is associated with obesity. The mean ± S.D of Body mass index [BMI] in hypothyroid group was 31.08 ± 1.40, compared to the mean ± S.D in control group was 29.18 ± 3.45. The increase in Body mass index[ B.M.I] in hypothyroid group is statistically significant (p = 0.002) The mean ± S.D of waist circumference in hypothyroid group was 101.18 ± 9.28cm, compared to the mean ± S.D of waist circumference in control group was 94.6 ± 12.55cm. The increase in waist circumferences in hypothyroid group is significant (p = 0.004).  The probable mechanism of weight gain and central obesity in hypothyroidism is thyrotropin’s influence on adipogenesis and on adipokinin production directly by acting on thyrotropin receptors in adipocytes and preadipocytes including differentiation and expansion. It also includes synthesis and release of adipokines and leptin. The weight gain that occurs in hypothyroidism is partly due to fluid retention by hydrophilic glycoprotein deposits in tissues²⁶.

 The increase in fasting glucose in hypothyroid group is highly significant (p<0.001). Serum insulin assay values in hypothyroid patients are significantly increased, HOMA - IR values in hypothyroid patients are significantly increased (P<0.001 value) when compared to controls. Similar study was done by Eirini Maratou et al and it correlates with this study. They assessed the sensitivity of glucose metabolism to insulin and concluded that hypothyroid patients have higher plasma insulin than euthyroids (p<0.05) and an increased risk for insulin resistance associated disorders such as cardiovascular diseases.²⁷ A similar study conducted by Rochon et al demonstrated that hypothyroidism induced a decrease in the insulin mediated glucose disposal.²⁸ According to Czetch et al²⁹ and other studies possible explanation for the decreased insulin responsiveness in hypothyroidism may be due to dysregulation of leptin in hypothalamus and adipocyte – myocyte cross talk by adipokines also play a significant role in skeletal muscle insulin resistance in hypothyroidism. Altered blood flow, impaired Glut 4 translocation, decreased  glycogen synthesis and decreased muscle oxidative capacity also may be other reasons for insulin resistance and impaired glucose tolerance in hypothyroid patients.³⁰ TSH correlates positively with insulin (0.64) and HOMA-IR(0.69) and it was statistically significant. HOMA-IR values in Hypothyroid patients are significantly increased (P<0.001 value) when compared to controls.  The dyslipidemia as shown in table 3 was significant and it correlates with the following study. In this study, out of 50 hypothyroid patients studied 32 patients are found to have impaired glucose tolerance. That means 64% of hypothyroid patients in our study are having impaired glucose tolerance .The mean ± S.D of Fasting Glucose (FG) in hypothyroid group was114.04+8.96mg/dl compared to the mean ± S.D of control group fasting glucose was 105.7 ± 7.35mg/dl. The increase in Fasting Glucose in hypothyroid group is highly significant (p<0.001). The mean ± S.D of Serum glucose at 2nd hour in hypothyroid group was156.94 ± 18.06mg/dl compared to the mean ± S.D of control group serum   glucose at 2nd hour was 137.10+9.26mg/dl. The increase in serum glucose at 2nd hour in hypothyroid group is highly significant (p<0.001 value) Ashrafuzzaman SM et al study concluded that the prevalence of pre diabetic state with impaired glucose tolerance is higher among hypothyroid subjects, and their results   correlated   with this study.³¹ Serum insulin assay values in hypothyroid patients are significantly increased (p<0.001 value) when compared to controls. HOMA- IR values in Hypothyroid patients are significantly increased (p<0.001 value) when compared to controls.  TSH correlate positively with insulin (0.64) and HOMA-IR (0.69) and it was statistically significant.

Study conducted by Ujwal Upadya B et al correlates with this study. There was a significant increase in insulin, HOMA-IR and glucose levels in hypothyroidism cases when compared to controls, cholesterol, LDL, VLDL and triglycerides were significantly increased whereas HDL was significantly decreased in hypothyroid cases when compared with controls.⁵

Manoj Kumar et al conducted a study which correlates with this study. He concluded that total cholesterol and low density lipoprotein, Triglycerides and very low density lipoprotein cholesterol are higher whereas high density lipoprotein cholesterol is reduced in hypothyroid patients as compared to Euthyroid individuals.³²Khan et al reported a significant correlation of Triglyceride and HDL levels with HOMA - IR values in human subjects with insulin resistance syndrome³³ which correlates with this study. Decreased activity of LDL receptors resulting in decreased receptor mediated catabolism of LDL is probably the main cause of hypercholesterolemia observed in hypothyroidism. It is believed that the dyslipidemia associated with insulin resistance is a direct consequence of increased VLDL secretion by the liver³⁴ Hypertriglyceridemia is commonly associated with reductions in HDL-C. This in part relates to the transfer of triglycerides instead of cholesteryl ester from the core of triglyceride rich lipoproteins like VLDL and/or LDL to HDL, a process catalyzed by cholesteryl ester transfer protein (CETP^).,35,36,37 This generates a smaller, triglyceride-rich HDL that is a better substrate for hepatic lipase which is more rapidly cleared by the Liver^{. 38} In the setting of hypertriglyceridemia, LDL particles are small and dense as they are triglyceride enriched. Evidence supports an association of small dense LDL with Insulin Resistance^.39 Of interest, the metabolic syndrome  has been associated with increased CETP mass that results in reduced LDL particle diameter in addition to reduced HDL.⁴⁰ Accumulation of triglyceride rich lipoproteins explains the associated small dense LDL in hypertriglyceridemic patients . VLDL   and chylomicron remnants provide a pool of triglycerides for exchange with other lipoproteins. Cholesterol ester transfer protein actively exchange triglycerides and cholesterol esters between lipoproteins. Both LDL and HDL have cholesterol rich cores that participate in .cholesterol ester transfer protein mediated exchange for triglycerides with remnants. Result is net transfer of cholesterol ester to remnants and triglyceride enrichment of LDL and HDL. That is the basis for formation of small dense LDL and small dense HDL although the mechanism by which small dense HDL is cleared rapidly from plasma is poorly understood. It is believed that It occurs mainly via kidney. The accumulation of triglyceride rich lipoprotein and their remnants may be the primary events explaining the association of small dense LDL and decreased mass of HDL found in hypertriglyceridemic patients.⁴¹

In the present study, hypothyroid patients were observed to have increased prevalence of elevated waist circumference, increase in B.M.I, dyslipidemia, impaired glucose tolerance, elevated insulin resistance.  This is suggestive that hypothyroid patients are more prone for developing metabolic syndrome in due course leading to the other consequences of it including cardiovascular, degenerative and other metabolic consequences. 

The present study found that women are at more risk of developing hypothyroidism in comparison to men Central obesity in hypothyroidism is well established in this study with elevated waist circumference, and BMI levels. Awareness of weight reduction and regular physical exercise are to be advised to patients with hypothyroidism. Hypercholesterolemia is a constant feature of hypothyroidism along with elevated LDL-cholesterol and decreased level of HDL-cholesterol. So all hypothyroid cases should be investigated for the lipid profile for early detection of cardiovascular diseases.  Impaired glucose tolerance is found to be more prevalent in hypothyroids, they are also found to have elevated insulin resistance. Together impaired glucose tolerance and elevated insulin resistance imply that hypothyroids are more prone to develop type 2 diabetes mellitus. Hence hypothyroid patients should be monitored regularly for glucose levels.

 The study implies hypothyroid patients are more prone for developing metabolic syndrome in due course leading to the other known sequence of it including cardiovascular, degenerative and other metabolic consequences. Therefore early detection and treatment of hypothyroidism with hormone replacement and not allowing the disease to progress to severity are to be followed.

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