Assessment of glycated hemoglobin levels in non-diabetic subjects with recently diagnosed hypo and hyperthyroidism

Sujesh K Narayanan; Renjith G Pillai

doi:10.4103/mgmj.mgmj_77_22

ORIGINAL ARTICLE

Year : 2022 | Volume : 9 | Issue : 3 | Page : 309-314

Assessment of glycated hemoglobin levels in non-diabetic subjects with recently diagnosed hypo and hyperthyroidism

Sujesh K Narayanan, Renjith G Pillai
Department of Biochemistry, Sree Uthradom Thirunal (SUT) Academy of Medical Sciences, Vencod, Vattappara, Kerala, India

Date of Submission	03-Jun-2022
Date of Acceptance	19-Jul-2022
Date of Web Publication	29-Sep-2022

Correspondence Address:
Dr. Sujesh K Narayanan
Department of Biochemistry, Sree Uthradom Thirunal (SUT) Academy of Medical Sciences, Vencod, Vattappara, Kerala 695028
India

Source of Support: None, Conflict of Interest: None

DOI: 10.4103/mgmj.mgmj_77_22

Abstract

Background: Thyroid disorders are highly prevalent in the general population and generally manifest as hypothyroidism or hyperthyroidism. Glycated hemoglobin (HbA1c) is commonly used for the assessment of the long-term glycemic status of diabetic patients. But the HbA1c concentration not only depends on prime glycemia but some other factors. Aims and Objectives: The objective of this study was to measure glycated hemoglobin and thyroid hormone levels and aimed to determine the effects of hypo and hyperthyroidism on glycated hemoglobin in non-diabetic patients. Materials and Methods: This case-control study was conducted on 70 euglycemic individuals each newly diagnosed with hypo and hyperthyroidism. Seventy age and gender-matched controls were recruited. Baseline HbA1c was estimated by high-performance liquid chromatography (HPLC) and thyroid status was measured by chemiluminescent immunoassay (CLIA) method in all cases and controls and then compared. Results: HbA1c values were found to be significantly higher in the hypothyroid group (P < 0.001), whereby its values in hyperthyroid subjects showed no significant difference from controls (P = 0.17). A positive correlation (r=0.67) exists between the levels of thyroid-stimulating hormone (TSH) and HbA1c in hypothyroid patients, but no significant correlation (r= -0.07) was observed for hyperthyroid subjects. Conclusion: HbA1c may not be a reliable indicator of glycemic status in patients with hypothyroidism, and so HbA1c results should be interpreted with caution in such patients.

Keywords: Glycated hemoglobin, HbA1c, hyperthyroidism, hypothyroidism

How to cite this article:
Narayanan SK, Pillai RG. Assessment of glycated hemoglobin levels in non-diabetic subjects with recently diagnosed hypo and hyperthyroidism. MGM J Med Sci 2022;9:309-14

How to cite this URL:
Narayanan SK, Pillai RG. Assessment of glycated hemoglobin levels in non-diabetic subjects with recently diagnosed hypo and hyperthyroidism. MGM J Med Sci [serial online] 2022 [cited 2022 Dec 7];9:309-14. Available from: http://www.mgmjms.com/text.asp?2022/9/3/309/357477

Introduction

Thyroid gland disorders are considered one of the common ailments affecting the endocrine system. Though second only to diabetes mellitus, it has a significant prevalence among the general population.^[1] The two frequent manifestations of thyroid gland disorders are hypothyroidism and hyperthyroidism. Hypothyroidism or underactive thyroid happens when the thyroid gland is unable to produce enough thyroid hormones (T3 and T4), which results in high production of thyroid-stimulating hormone (TSH) from the pituitary gland. Hyperthyroidism is due to increased levels of thyroid hormones with negative feedback to the pituitary gland resulting in low levels of TSH.

The prime parameter commonly used for the assessment of long-term glycemic status is glycated hemoglobin, which is hemoglobin A1c (HbA1c). American Diabetes Association (ADA).^[2] and World Health Organization (WHO).^[3] have suggested the use of HbA1c for the screening and diagnosis of pre-diabetes and diabetes in clinical practice. A value between 5.7% and 6.4% represents pre-diabetes, while a value of ≥6.5% is considered diabetes.^[4]

HbA1c signifies the fraction of hemoglobin that is formed by a non-enzymatic reaction between hemoglobin and glucose over the circulatory life span of the red blood cells. The concentration of HbA1c levels in blood not only depends on principal glycemia but also on the average period of exposure of the circulatory red blood cells (RBCs) or the life span of erythrocytes.^[5] So the conditions that prolong the life span of RBCs (e.g. iron deficiency anemia, vitamin B12/folate deficiency anemia, etc.) lead to increased exposure of erythrocytes to glucose and falsely high HbA1c results. Conversely, conditions that shorten the life span of RBCs (e.g. acute and chronic blood loss, hemolytic anemia, etc.) lead to reduced exposure of erythrocytes to glucose and falsely low HbA1c results.^[6],[7] That is HbA1c may not accurately reflect the level of glycemia in conditions of altered erythrocyte life span. In hypothyroidism, erythropoiesis is sluggish, or RBC turnover is low, which leads to increased exposure of erythrocytes to glucose, which may affect the glycation rate of hemoglobin and elevate HbA1c concentration independent of glucose level, while the thyrotoxic state may have the opposite effect.^[8]

We, therefore, hypothesize that HbA1c levels do not precisely reflect ambient glycemia in hypo and hyperthyroidism. It has been shown in a few studies that HbA1c was found to be significantly higher in non-diabetic hypothyroid patients.^[9],[10] A lot of studies have been listed indicating the positive association between diabetes mellitus and thyroid. But only limited studies are there demonstrating the influence of altered thyroid status on glycated hemoglobin in non-diabetic patients, and it is an area for extensive research.

Aims and objectives

➢ To determine the effects of hypo and hyperthyroidism on glycated hemoglobin levels in non-diabetic patients and to know its importance while interpreting data.

➢ To evaluate HbA1c levels in euglycemic individuals with altered thyroid status and to compare its levels with controls.

➢ To correlate TSH levels with HbA1c in hypo and hyperthyroid patients.

Materials and methods

This case-control study was conducted in the Department of Biochemistry at a tertiary level center for a span of twelve months from March 2021 to March 2022. The study proposal was approved by the Institutional Ethics Committee. Informed written consent was obtained from all study participants, and confidentiality was maintained throughout the study. A detailed history was taken from each participant and they were screened based on inclusion and exclusion criteria.

Inclusion criteria

Age >20 years and Age <60 years of both genders

Recently diagnosed primary hypo and hyperthyroidism patients (< 3 months)

Age and gender-matched euthyroid and euglycemic healthy individuals as controls

Exclusion criteria

Diabetes mellitus

Hemolytic anemia

Pregnancy

Renal dysfunction

Hepatic dysfunction

Hemoglobinopathies

Hypothyroid subjects on thyroid hormone replacement therapy

Sample selection

Based on the inclusion and exclusion criteria, we selected 210 study participants. Out of this, 70 (seventy) each newly diagnosed with hypo and hyperthyroidism, both without any history of pre-diabetes or diabetes, were considered as cases. Another 70 age and gender-matched normal healthy individuals were enrolled as controls.

Study procedure

Venous blood was drawn from all enrolled patients under strict aseptic conditions. Overnight fasting venous blood was obtained for fasting plasma glucose. Serum was separated by centrifugation at 3000 rpm for 10 minutes, within one hour after blood collection, and the parameters including T3, T4, and TSH were estimated on the same day of collection of the sample. Whole blood samples were used for testing HbA1c and plasma was used for testing glucose. HbA1c was estimated by the ion-exchange high-performance liquid chromatography (HPLC) method. Serum T3 (reference range: 0.97 – 1.69 ng/ml), T4 (reference range: 5.53 – 11.0 µg/dl) and TSH (TSH immunoassay kit) (reference range: 0.465 – 4.68 mIU/L), were estimated by chemiluminescent immunoassay (CLIA) method. Plasma glucose estimation was done by the glucose oxidase peroxidase (GOD-POD) method.

Statistical analysis

Statistical analysis was done using Statistical Package for Social Survey (SPSS) for Windows version 20.0. The mean and the standard deviation (SD) for all baseline variables were calculated. Comparisons between baseline variables were done using the student t-test (unpaired) and Mann-Whitney Rank sum test as applicable. Categorical variables were calculated using the Chi-square test. The correlation between HbA1c and TSH in cases and controls was obtained by Karl Pearson’s correlation coefficient. A P-value less than 0.05 was considered statistically significant and a P-value less than 0.001 was considered highly significant.

Results

This study enrolled a total of 210 subjects, out of whom 70 subjects each were in the hypothyroid and hyperthyroid group and another 70 subjects were in the control group. All the subjects were in the age group between 20–60 years. [Table 1] shows a Comparison of baseline characters between hypo and hyperthyroid groups with control groups. The mean age of the hypo and hyperthyroid groups was 41.81 and 39.64, respectively, whereas the mean age of the control group was 40.02. As age-matched controls were selected, there were no significant differences between hypothyroid and control groups (p value=0.37), and also between hyperthyroid and control groups (p value=0.59) in age.

Table 1: Baseline characteristics of study subjects

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Female predominance (71% in hypothyroid cases and 75% in hyperthyroid cases) was noted in our present study. Out of the total 140 cases, females were 73% and males were 27%. As gender-matched controls were selected, the control group had 74% females. [Figure 1] shows the gender distribution in percentage in study groups.

Figure 1: Gender distribution in percentage in study groups

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There was no significant difference in fasting plasma glucose between hypothyroid and control groups (p value=0.18), and also between hyperthyroid and control groups (p value=0.56). Also, there was no significant difference in postprandial plasma glucose between hypothyroid and control groups (p value=0.50), and also between hyperthyroid and control groups (p value=0.11).

Mean levels of serum T3, T4, and TSH between hypothyroid and control groups were compared, which showed a statistically significant difference. Mean levels of serum T3, T4, and TSH between hyperthyroid and control groups were also compared.^[11] which also showed a statistically significant difference. The comparison of mean levels of serum T3, T4, and TSH between hypothyroid v/s control and hyperthyroid v/s control are given in [Table 2] and [Table 3], respectively.

Table 2: Comparison of mean levels of Serum T3, T4, and TSH in Hypothyroid and Control groups

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Table 3: Comparison of mean levels of Serum T3, T4, and TSH in Hyperthyroid and Control groups

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The HbA1c levels in hypothyroid patients were found to be in a range of 5.4–6.3% with a mean value of 5.83 ± 0.26 in contrast to the range of 5.0–5.5% with the mean value of 5.28 ± 0.14 in controls. The difference was found to be statistically significant (p-value <0.001). The HbA1c levels in hyperthyroid patients were found to be in a range of 5.0–5.7% with a mean value of 5.33 ± 0.20, which showed no significant difference (p value=0.17) from controls. [Table 4] shows a comparison of HbA1c and TSH levels in cases and control subjects.

Table 4: Comparison of HbA1c and TSH levels in cases and control subjects

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A moderate positive correlation (r=0.67) was observed between TSH and HbA1c in patients with hypothyroidism by Pearson’s correlation coefficient, which was statistically significant (p-value <0.001). But only a weak correlation (r= -0.07) was observed between TSH and HbA1C in patients with hyperthyroidism, which was statistically insignificant (p value= 0.57). [Figure 2] and [Figure 3] show a correlation between the levels of TSH and HbA1c in hypothyroid and hyperthyroid patients.

Figure 2: Correlation between TSH and HbA1c in Hypothyroid patients

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Figure 3: Correlation between TSH and HbA1c in Hyperthyroid patients

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Discussion

The study was conducted to evaluate the effects of altered thyroid status on glycated hemoglobin in non-diabetic patients. It is imperative to study the impact of thyroid hormones on glycated hemoglobin, to help us a better interpretation of its levels in patients with thyroid dysfunction. HbA1c is a well-established diagnostic marker of diabetes mellitus, but there has been concern about the utility of HbA1c in certain conditions where it does not accurately reflect the level of glycemia.

In the present study, most of the patients belong to the age range between 30 and 50 years. The study also showed female preponderance, which resembles the study conducted by Meng Z et al.^[12] This indicates that thyroid dysfunction is more common in women than in men, which may be attributed to gender differences in immune function, similar to many autoimmune diseases.^[13] No significant difference in glycemic status was observed between case groups (hypothyroid and hyperthyroid subjects) and the control group, and these findings were similar to the study done by Bhattacharjee R et al.^[8]

In our study, mean baseline HbA1c levels were found to be significantly higher in hypothyroid patients in comparison to matched control subjects despite having similar glycemic status. Quite a few studies were conducted by researchers Kim MK et al.^[9] and Makadia et al.^[14] had shown a similar elevation of HbA1c in hypothyroid patients. In contrast, no significant difference exists in mean baseline HbA1c levels between hyperthyroid patients and control subjects. Similar results were reported by Bhattacharjee R et al.^[8] who didn’t observe any significant difference between hyperthyroid patients and control subjects regarding baseline HbA1c levels.

In the present study, a statistically significant correlation exists between the levels of serum TSH and HbA1c in hypothyroid patients. A study conducted by Kim MK et al.^[9] to determine the impact of thyroid hormones on glycated hemoglobin, showed a strong positive correlation between glycated hemoglobin and TSH in hypothyroid subjects. A similar correlation was also observed in studies conducted by Makadia et al.^[14] and Billic-Komarica E et al.^[15] The above observations infer that glycated hemoglobin levels were higher in hypothyroid subjects as compared to those of age and gender-matched euthyroid controls. In contrast, no significant correlation exists between the levels of serum TSH and HbA1c in hyperthyroid patients. The American Diabetes Association.^[2] endorsed an HbA1c of 5.7–6.4% as indicative of pre-diabetes. According to this criterion, 69% of our hypothyroid subjects (48 out of 70) were misclassified as having pre-diabetes.

Several factors like chronic kidney diseases, hemoglobinopathies, pregnancy, hemolytic anemia, etc. that can interfere with glycation of hemoglobin were excluded from our study. The concentration of HbA1c levels in blood not only depends on glucose levels but also on the average period of exposure of the circulatory red blood cells to glucose or the life span of erythrocytes.^[5] The lowered thyroid hormone (TSH) levels in hypothyroidism suppress the activity of bone marrow, resulting in reduced production (erythropoiesis) and turnover of red blood cells which may affect the life span of erythrocytes.^[16],[17] This leads to increased exposure of erythrocytes to glucose, which may affect the glycation rate of hemoglobin and thereby elevate HbA1c concentration independent of glucose level.^[8]

Reduced turnover of red blood cells and altered erythrocyte life span that coincide with hypothyroidism may be accountable for the false elevation of HbA1c levels.^[18],[19] So red blood cell survival time gives a better explanation for spurious elevation of glycated hemoglobin levels in hypothyroid subjects. A research by Anantarapu et al.^[10] demonstrated specious elevation of HbA1c values in patients with hypothyroidism, which was lowered by thyroid hormone replacement without a significant change in glucose levels. At the same time, Bhattacharjee R et al.^[8] in their study didn’t observe any noticeable post-treatment change in HbA1c levels among hyperthyroid subjects.

Strength and limitations

Several factors other than glycemic status like chronic kidney diseases, hemoglobinopathies, pregnancy, hemolytic anemia, etc. that can interfere with glycation of hemoglobin were excluded from our study, which reinforced our study by eliminating the possible confounding factors. The limitations of our study include the inability to estimate reticulocyte count and measure the lifespan of erythrocytes, and also we couldn’t evaluate the effect of thyroid hormone replacement therapy on erythropoiesis and glycated hemoglobin status in patients with hypothyroidism.

Conclusion

The study indicates that HbA1c levels were significantly higher among non-diabetic hypothyroid patients, which were in the pre-diabetic range in comparison to hyperthyroid patients and controls despite similar glycemic status. The reason might be due to the effect of hypothyroidism on erythropoiesis and thereby altering the life span of erythrocytes. So our study suggests that glycated hemoglobin may not be a reliable indicator of glycemic status in patients with hypothyroidism, and thereby HbA1c results should be interpreted with caution in such patients. Also, the effects of hypothyroidism on the glycated hemoglobin must be considered when interpreting the HbA1c value for the diagnosis of pre-diabetes.

Ethical consideration

The study protocol was reviewed and approved by the Institutional Ethics Committee, vide approval letter no.67/Acad/SUTAMS/2021, dated March 10, 2021.

Financial support and sponsorship

Nil

Conflicts of interest

There are no conflicts of interest.

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