Effect of hypothyroidism on cognitive status: Evidence from stroop task

Shilpi Goyal; Abhinav Dixit; Neelam Vaney; SV Madhu

doi:10.4103/INJMS.INJMS_144_19

ORIGINAL ARTICLE

Year : 2020 | Volume : 11 | Issue : 1 | Page : 34-39

Effect of hypothyroidism on cognitive status: Evidence from stroop task

Shilpi Goyal¹, Abhinav Dixit², Neelam Vaney³, SV Madhu⁴
¹ Department of Physiology, Subharti Medical College, Meerut, Uttar Pradesh, India
² Department of Physiology, AIIMS, Jodhpur, Rajasthan, India
³ Department of Physiology, University College of Medical Sciences and GTB Hospital, Delhi, India
⁴ Medicine, University College of Medical Sciences and GTB Hospital, Delhi, India

Date of Submission	17-Nov-2019
Date of Decision	23-Dec-2019
Date of Acceptance	11-Jan-2020
Date of Web Publication	29-Jan-2020

Correspondence Address:
Dr. Shilpi Goyal
Department of Physiology, Subharti Medical College, Meerut, Uttar Pradesh
India

Source of Support: None, Conflict of Interest: None

DOI: 10.4103/INJMS.INJMS_144_19

Abstract

Background: Very less work has been done in the past to assess cognition in hypothyroid patients using classical color-word Stroop task. The Stroop color-word task is a well-recognized tool which estimates cognitive inhibition. Stroop task is based on the fact that reading words and identifying color of written words are two different information and they process with a common response channel but with different speeds. The aim of this study was to assess the cognitive inhibition process in drug-naïve hypothyroid patients and their comparison with the same patients after the attainment of euthyroid status and also with euthyroid controls using classical color-word Stroop task. Materials and Methods: Thirty drug-naïve newly diagnosed hypothyroid patients were study cases who underwent Stroop task once before initiating treatment and then after the attainment of euthyroid state. We performed the same test in euthyroid controls twice at an interval of 3 months for comparison purpose. Correlation analysis was also done for hypothyroidism and cognition. Results: The reaction time of Stroop task was decreased in all the three conditions of each block, i.e., neutral, incongruent, and congruent conditions after the attainment of euthyroid state, and the finding confirms posttreatment modulation of the attention processes in hypothyroid patients. Reaction time of Stroop task in hypothyroid patients was positively correlated with the serum thyroid-stimulating hormone levels. Processing of color identification was suppressed than reading words in hypothyroid patients as evidenced by increased reaction time of Stroop task. This increase in reaction time can be reversed by earlier identification of drug-naïve hypothyroid cases so that they can get timely treatment to restore their cognitive status. Conclusions: Stroop task scores are reduced in overt hypothyroid patients, but improve after thyroid replacement therapy.

Keywords: Attention, cognition, hypothyroidism, Stroop task

How to cite this article:
Goyal S, Dixit A, Vaney N, Madhu S V. Effect of hypothyroidism on cognitive status: Evidence from stroop task. Indian J Med Spec 2020;11:34-9

How to cite this URL:
Goyal S, Dixit A, Vaney N, Madhu S V. Effect of hypothyroidism on cognitive status: Evidence from stroop task. Indian J Med Spec [serial online] 2020 [cited 2023 May 29];11:34-9. Available from: http://www.ijms.in/text.asp?2020/11/1/34/277239

Introduction

An association of hypothyroidism has been seen with the deficits in somatic, neurological, and psychiatric domains for more than two decades.^[1],[2]

There is evidence of decrease in intelligence levels, working memory, attentional process, psychomotor performance, and visuoperceptual and constructional skills in hypothyroid patients.^[3] Cognitive difficulties such as memory impairment and difficulty in concentration are also common.^[4] Adequate thyroid functions are always required for the normal development and cognitive status of an individual.

Central nervous system development is significantly influenced by the thyroid hormone. Simultaneously, there is a major role of thyroid hormone in adult brain functions also, but the mechanism, which describes the role of this hormone in adult brain function, needs to be understood.^[5],[6]

One of the hypotheses which can explain the probable mechanism is that there are numerous nuclear thyroid hormone receptors in the hippocampus and frontal lobes, T₃ binds to these nuclear receptors, and these brain areas are well-known centers for memory and cognition.^[7],[8] Thus, we can conclude that thyroid hormones definitely have some neurogenic effects in the central nervous system.^[9]

Previous studies have shown conflicting reports about correlation of cognitive status with serum thyroid-stimulating hormone (TSH) values.^[10],[11] Studies have also shown the improvement in neuropsychological symptoms after hormonal restoration of euthyroid status.^{[12],[13],[14]} These studies have used various neuropsychological tests for cognitive assessment, which are mostly paper–pencil based.

The assessment of attention or vigilance can be done by computer-based neuropsychological test such as Stroop task. Sir John Ridley Stroop first described the Stroop task in 1935.^[15] This is the “gold standard of attentional measures” and used as a test of executive functions measuring cognitive inhibition.^[16]

Advantages of automated test are:

Speed of processing along with the accuracy of performance can be scored simultaneously
We can get printed copies of found scores
There are preset standard test conditions, which allow the uniform application of test for each and every individual
If required, repeat assessment of an individual's performance can be done because of saved data.

Stroop color-word test measures selective attention of relevant information such as naming color and response inhibition of irrelevant information such as reading words.^[17] The test is based on the ability of the subject to respond to a color when words (color name) written in the same ink color (congruent condition) or different ink colors (incongruent condition) are presented.

In the present case–control study, the aim was to explore the relation between cognitive inhibition and thyroid profile in newly diagnosed and yet untreated hypothyroid patients by computer-based neuropsychological test. This interventional study also assessed the changes in cognitive scores after the attainment of euthyroidism to find the role of thyroxine in restoration of cognitive status.

Scant work has been done in the past to assess cognitive inhibition in hypothyroid patients using classical color-word Stroop task, and most of them are in the Western population with a very few number of individuals. There are conflicting results of color-word Stroop task in hypothyroid patients.^{[18],[19],[20]}

In the present case–control study, we analyzed cognitive inhibition in hypothyroid patients by Stroop task. We compared the initial test scores of patients with posttreatment achievement of euthyroid state and with euthyroid controls to see the changes in attentional processes in different groups. Serum TSH levels were correlated with the reaction time of Stroop task to analyze the correlation between hypothyroidism and cognition.

Materials and Methods

Thirty drug-naïve newly diagnosed hypothyroid cases (age group of 18–50 years with mean age = 31.6 ± 8.4 years) were recruited as a case group on the basis of their thyroid hormone profile after taking detailed clinical history and performing thyroid examination. Thirty euthyroid age- and sex-matched volunteers (mean age = 31.0 ± 8.0 years) were recruited as controls after performing their thyroid hormone profile. Written informed consent was taken from both the cases and controls.

The institutional ethical committee approval was taken.

The cases and controls included in the study were all right handed with minimum of 5 years of schooling. Participants were tested for color vision, and none of them were found colorblind.

Individuals were excluded from the study on the basis of the following criteria:

Patients suffering from neuropsychiatric illness such as depression
Patients with a history of myocardial infarction, hypertension, or diabetes mellitus
Those who have drug addiction, history of smoking, or alcoholism
Those who are on any medications, especially anti-allergic, etc., during the past 2 weeks.

Serum levels of free T₃ (fT₃)_, free T₄ (fT₄), and TSH of patients and controls were measured by radioimmunoassay kit from Immunotech Beckman Coulter.

Color-Word Stroop task-Psych/Lab™ for Windows was used to perform the Stroop task. The task has three different blocks to be played at an interval of 5 min each. Each block consisted of three different conditions, i.e., neutral, incongruent, and congruent. Each condition separately has 72 trials. Among these, 72 trials of every condition correct responses and reaction time were measured:

Neutral condition – Letter “XXXX” was printed in red, green, blue, and yellow ink color
Incongruent condition – Words “red,” “green,” “blue,” and “yellow” were printed in different ink colors
Congruent condition – Words “red,” “green,” “blue,” and “yellow” were printed in the same ink color as that meant by the word itself.

The participants are supposed to identify the color of written text on computer screen by pressing the appropriate response button. The response buttons are as follows:

"z” button to be pressed on red color identification, similarly “x” for green, “.” for blue, and “/” to be pressed for yellow color.

The time taken to complete the tasks in the different blocks was taken as reaction time. The number of correct responses was also noted. Reaction time and correct responses are analyzed by the software at the end of the task.

Stroop interference (the difference between time taken for incongruent and neutral conditions) and Stroop facilitation (the difference between time taken for neutral and congruent conditions) were measured.

Participants were asked to take proper night sleep, and testing was done in the next morning between 9 and 11 am.

Hypothyroid cases underwent Stroop task once before initiating treatment and then after 3 months of thyroxine treatment to achieve the euthyroid state. The cases which did not achieve euthyroidism after 3 months were excluded from the study. We performed Stroop task in euthyroid controls twice at an interval of 3 months.

Serum TSH levels were correlated with reaction time of Stroop task in newly diagnosed hypothyroid patients.

Statistical analysis

SPSS version 20.0 (SPSS IBM Inc. Chicago, Illinois, USA) was used for the statistical analysis. Data are presented as mean values and standard deviation. Thyroid profile was compared at initial and final levels by paired t-test and between cases and controls at baseline by unpaired t-test. Within two factors, repeated measures ANOVA was employed to compare the differences for repeated measurement of Stroop task followed by Tukey's test at 5% for multiple paired comparisons. The relationship between serum TSH levels and performance in Stroop task was analyzed by Pearson correlation. P < 0.05 was taken as level of significance.

Results

Hypothyroid patients achieved euthyroidism after getting thyroxine treatment for 3 months as shown by restoration of thyroid hormone levels [Table 1].

Table 1: Thyroid profile of cases before and after treatment

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Stroop task has three successive blocks (1, 2, and 3). Block 2 has to be played at an interval of 5 min after completion of block 1, and then, after another 5 min, Block 3 has to be played. Each block separately consists of three different conditions, i.e., neutral, incongruent, and congruent. Each condition separately has 72 trials, which means that a participant has to perform 72 × 3 trials in Block 1 and also in Blocks 2 and 3.

At the start of test, we give a small trial block to be completed by the participant so that he/she becomes familiar to the test.

After the trial block participant completes Block 1, we start Block 2 after 5-min break.

As shown in [Table 2] and [Table 3], P value of interblock comparisons in hypothyroid patients for reaction time (P = 0.06) and correct responses (0.163) and, in euthyroid controls also, P value for reaction time (P = 0.156) and correct responses (P = 0.075) are nonsignificant.

Table 2: Stroop task reaction time (milliseconds) in case and control groups at 3-month interval

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Table 3: Correct responses in case and control groups at 3-month interval

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Thus, with the nonsignificant differences in results of Stroop task in terms of reaction time and correct responses on repeating the three successive test blocks, there is no role of practice on Stroop task.

Each block individually shows significant differences between neutral, incongruent, and congruent conditions, in terms of reaction time (P < 0.001) for both hypothyroid patients and euthyroid controls. The finding confirms the facilitation and interference effects.

Hypothyroid patients also show a significant difference in terms of correct responses (P = 0.003) although euthyroid controls do not show any significant difference in correct responses (P = 0.444) between the three conditions. This may be due to the fact that the control group is paying more vigilance for responding to the task according to varying levels of complexities in the three different conditions, so that their accuracy is maintained, and as a result, correct responses have nonsignificant differences among the three conditions.

Among hypothyroid patients, the reaction time of Stroop task was decreased significantly (P = 0.03) and number of correct responses increased significantly (P = 0.002) in all the three conditions of each block after the attainment of euthyroid state. This shows that there is posttreatment improvement of attention process in hypothyroid patients [Table 2] and [Table 3].

We also found a positive correlation (correlation coefficient = +0.2649) between reaction time of Stroop task and serum TSH levels in hypothyroid patients [Figure 1].

Figure 1: Correlation between Stroop task reaction time in milliseconds and serum thyroid-stimulating hormone levels in hypothyroid patients

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Hypothyroid patients did not reveal a significant change in interference and facilitation effects after treatment.

Euthyroid controls failed to show any significant difference in the reaction time and correct responses on repeating the test after 3 months [Table 2] and [Table 3].

Discussion

Cognition in hypothyroid patients and euthyroid controls was assessed by classical color-word Stroop task which is sensitive to attention.

Stroop task does not support the effect of learning on repeated performance, as evidenced by no significant difference in reaction time and correct responses between blocks; however, the task showed facilitation and interference effects in both hypothyroid patients and euthyroid controls as evidenced by significant difference between the three conditions in the reaction time (P < 0.001) within a block [Table 2].

When color names are written in different ink colors which it meant (word blue written in yellow ink), there occurs a behavioral conflict. It has been seen, naming the color requires more attention than reading the word. Naming an ink color when the written color name is not matching with ink color (incongruent condition, e.g. word blue with yellow ink) is still more difficult and time-consuming.

Incongruent condition (e.g., blue written in yellow) takes more time to identify the color relative to a neutral condition (i.e., multiple repeats of any random letter, e.g., xxxxx). Interference is denoted as the difference between time taken to perform incongruent and neutral condition. Congruent condition (e.g., blue written in blue) does not show any conflict, so it takes lesser time to identify the color than the neutral condition; this difference between neutral and congruent is called facilitation.

Stroop interference is a phenomenon of competitive response, whereas facilitation shows converging response. We can say with our experiences that reading word requires less time and attention than naming colors, and this is the fact on which phenomenon of facilitation and interference are based.

The Stroop task is a test of attention, which can measure the information-processing rate and parallel processing of attended and nonattended stimuli.^[21]

We found a significant decrease in reaction time and increase in correct responses in hypothyroid patients after treatment, but euthyroid controls did not show any significant changes after repeating the test at 3-month interval [Table 2] and [Table 3]. Similar to our study, Ergür et al.^[20] found lower scoring of Stroop task in subclinical hypothyroid children. But the study group was smaller in their study and follow-up after T4 replacement was not done contrary to ours.

Our study did not find a significant increase in facilitation and decrease in interference in hypothyroid patients after the attainment of euthyroid state in comparison to pretreatment state, showing the fact that Stroop task involves complex attention processing and modulation of this process does not occur immediately after the attainment of euthyroid state.

Reaction time of Stroop task was increased in pretreatment state, and the attainment of euthyroid state led to a significant improvement in terms of reaction time (P = 0.03) and correct responses (P = 0.002) in hypothyroid patients [Table 2] and [Table 3]; however, the difference in interference and facilitation effects between hypothyroid patients and euthyroid controls was not statistically significant, and this may be due to the fact that interference and facilitation effects require more complex attentional processes which may not differ in euthyroid controls and hypothyroid patients.

Correia et al.^[18] found no significant difference in scores of Stroop task, which is a measure of attention and cognitive inhibition in normal, hypothyroid, and subclinical hypothyroid patients; however, they found decreased scores in other domains of cognition such as visuospatial, verbal, and associative memories among overt hypothyroid patients in pretreatment state. After receiving treatment for 6 months, patients showed improvement in these cognitive domains.

Goyal et al. also found a deficit in information processing and decreased speed in various cognitive domains, for example, attention/concentration, executive functions, spatial working memory, visual attention, and task switching among drug-naïve newly diagnosed hypothyroid patients, and they restored cognitive functions after 3 months of thyroxine treatment on attending euthyroid state.^[22] Researchers used various paper–pencil cognitive tests for the assessment of these cognitive domains. Our finding of improved scores in reaction time and correct responses of Stroop task in hypothyroid patients after receiving treatment for 3 months further suggests a role of thyroxine in the restoration of impaired cognition.

Serum TSH levels were found to be positively correlated with reaction time of Stroop task in hypothyroid patients [Figure 1].

Jorde et al. did not find a different scoring of Stroop task in subclinical hypothyroid patients and euthyroid controls.^[19]

In a Brazilian article, patients with subclinical hypothyroidism did not show a different scoring of Stroop task from euthyroid controls.^[23]

This may be due to the fact that the case group in these studies^[19],[23] was subclinical hypothyroid patients, in which only TSH levels were raised and serum fT4 and fT3 were within the normal range and without any symptoms of hypothyroidism, but on the contrary, we took overt hypothyroid patients with overtly disturbed thyroid profile and clinical symptoms in our study.

These conflicting findings suggest the different age groups of study population and variations in severity of subclinical hypothyroid patients.^[19],[23] This reveals the fact that duration and severity of hypothyroidism are also deciding factors for cognitive impairment as suggested by positive correlation between serum TSH levels and reaction time of Stroop task in hypothyroid patients in our study [Figure 1].

In the present study, the reaction time of Stroop task was increased in pretreatment state and restored after the achievement of euthyroid status in hypothyroid patients. Our findings strongly support the previous studies suggesting the role of thyroxine in the restoration of cognitive functions in hypothyroid patients.^{[19],[22],[24]}

It was difficult to find drug-naïve hypothyroid patients for the study. Further studies with large number of patients can be done to see how the cognitive profile of subclinical hypothyroid patients differs from overt hypothyroidism using automated Stroop task.

Financial support and sponsorship

None.

Conflicts of interest

There are no conflicts of interest.

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