Confirmatory factor analysis of the thyroid-related quality of life questionnaire ThyPRO
© Watt et al.; licensee BioMed Central Ltd. 2014
Received: 11 March 2014
Accepted: 6 August 2014
Published: 10 September 2014
Background and aim
Thyroid diseases are prevalent and chronic. With treatment, quality of life is restored in most, but not all patients. Construct validity of the thyroid-related quality of life questionnaire, ThyPRO, has been established by multi-trait scaling, but not evaluated with more elaborate methods. The purpose of the present study was to evaluate dimensionality of the ThyPRO scales and to attempt to understand possible item misfit through structural equation modeling for categorical data.
The current 85-item version of ThyPRO consists of 13 scales, covering domains of physical (4 scales) and mental (2 scales) symptoms, function and well-being (3 scales) and participation/social function (4 scales). The data were collected from a cross-sectional sample of 907 thyroid patients. One-factor confirmatory models were fitted to each scale, and evaluated by model fit statistics (comparative fit index >0.95, root mean square error of approximation <0.08), magnitude of factor loadings, model residual correlations and modification indices (MI). Indications of multi-dimensionality were tested in bi-factor models. Possible item misfit was evaluated in a combined, investigational model.
Each ThyPRO scale was adequately represented by a unidimensional model after minor revisions. Eleven items were identified in the unidimensional models as potentially misfitting and were investigated further by multidimensional modeling.
Elaborate psychometric modeling supported the construct validity of the ThyPRO. However, 11 potentially misfitting items and 18 items with local dependence to other items are candidates for removal in future item reduction processes.
Thyroid diseases are diseases related to the thyroid gland, which is an endocrine, i.e. hormone producing, gland located in the front of the neck. Thyroid diseases are prevalent, affecting approximately 15% of individuals of all ages, with a 4 to 1 women/men ratio ,. The main disease groups comprise non-toxic goiter (enlargement of the gland), hyperthyroidism (either as toxic nodular goiter or Graves' disease -with or without Graves' orbitopathy (GO, inflammation and protrusion of the eyes)) - and autoimmune hypothyroidism. The symptomatology is often diffuse, sharing features with many other diseases (fatigue, palpitations, dry skin, depression, uneasiness, etc.) as well as with the non-pathological fluctuations of well-being and function in life. Therefore, thyroid diseases may go un-diagnosed for many years in some patients and at the time of diagnosis, most patients have reduced quality of life ,. The diseases are chronic, but relevant treatment is available. In general though, there is a lag in treatment effect for thyroid diseases of up to several months and population-based studies document excess morbidity and mortality, also when adequately treated ,. Eventually, the quality of life of the majority of patients is restored ,. However, studies indicate that a substantial minority do not regain their premorbid level of well-being and function ,. Valid and reliable measures of health-related quality of life are necessary in order to describe the patients' experiences of the diseases adequately and for intervention studies attempting to improve treatment efficacy. Therefore, there has been a growing interest within thyroidology in measuring patient-reported outcomes (PRO), leading to the development of a comprehensive PRO measuring thyroid-related quality of life, the ThyPRO. Due to the fact that individual thyroid diseases often co-exist (e.g., goiter and hyperthyroidism) and that treatment of one disease entity may lead to another (e.g., removal of a goiter leading to hypothyroidism), the ThyPRO was developed as a comprehensive thyroid-related measure, aimed at any benign thyroid disease.
The content of the ThyPRO addresses the impact of all benign thyroid diseases ,. The validation of the current version has included evaluation of clinical validity in terms of known-groups comparisons and reliability in terms of internal consistency and test-retest reliability ,. Further, the ThyPRO's dimensionality or construct validity has been established by multi-trait scaling . However, within such a framework, it is not possible to test the overall fit of a model , nor can misfit of items be modeled specifically.
The growing interest in applying the ThyPRO in clinical studies ,, and even in daily clinical practice has motivated efforts to develop shorter versions of the instrument as well as versions applicable to ecological momentary assessments. Development of such versions can be informed by the application of item response theory (IRT) models, which also provide a more detailed description of measurement precision and can provide data for interpretability of the ThyPRO. However, IRT models require additional, more detailed examinations of the dimensionality of the ThyPRO scales.
Structural equation models provide a latent variable modeling framework that is useful in detailed examinations of dimensionality. The measurement part of structural equation models can be used to assess the dimensionality of measured variables such as questionnaire items, using confirmatory factor analysis (CFA) for categorical data. Structural equation modeling can also test relationships among modeled latent variables (i.e., structural part of the models) -. We will exploit the former in the detailed analyses of the dimensionality of the ThyPRO scales, including overall test of model fit. We will use the structural part of the modeling approach when attempting to understand, through investigative modeling, any possible item misfit identified during the CFA step.
Thus, the purpose of the present study was to evaluate dimensionality of the ThyPRO scales in a sample of patients with a broad spectrum of thyroid diseases and to attempt to understand possible item misfit through investigative structural equation modeling.
The ThyPRO questionnaire
The current 85-item version of ThyPRO measures quality of life in 13 scales, covering physical (4 scales) and mental (2 scales) symptoms, function and well-being (3 scales) and participation/social function (4 scales) and one single item about overall quality of life. Content and scale structure were derived from a literature search  and from expert and patient interviews  and the development was conducted within a classical health-related quality of life theoretical framework -. Items are rated on a five-point scale from 0 = not at all to 4 = very much, with a reference period of 4 weeks. Thirteen scales are scored by reverting positively worded items and rescaling item scores from 0 (best QoL - absence of symptoms) to 100 (worst QoL – maximum level of symptoms) and taking the average across the items in the scale – i.e., standard summation and linear transformation.
Characteristics of the N = 907 patients
Age (mean (SD))
Diagnosis (n (%)):
Diffuse non-toxic goitre
Multinodular non-toxic goitre
Uninodular non-toxic goitre
Multinodular toxic goitre
Uninodular toxic goitre
Other thyroid disease
Months since diagnosis (median (range))*
Thyroid treatment (n (%)):
No thyroid treatment (ever)
Prior to any of the statistical analyses mentioned below, a content analysis of each scale was performed to identify items which might be less associated with the remaining items in the same scale, and item pairs which might be closely related to one another after being accounted for by the scale (local item dependence). This was done to provide a content-based guidance to model fitting.
Then a one-factor confirmatory model for ordinal data was fitted to each individual scale ,, using Mplus (version 7.11) . The ordinal items were regressed on the scale-factor by probit regressions estimated by a robust weighted least squares estimator with mean and variance adjustment (WLSMV) ,. Appropriateness of the initial one-factor model for each scale was assessed by: 1) overall goodness-of-fit statistics including the comparative fit index (CFI) and the root mean square error of approximation (RMSEA), where CFI >0.95 and RMSEA < 0.08 were regarded as appropriate fit -; 2) magnitude of factor loadings; 3) model residual correlations (RC) and 4) modification indices (MI) ,. For the latter three criteria, their magnitude was evaluated in comparison to other items in the scale and in an integrative manner, taking all three under consideration at once, so no strict thresholds were applied for each criterion. In general though, modification indices >100 and residual correlations >|.10| were taken as indices of lack of fit (local dependence or lack of convergent validity), but smaller values could also give rise to model revision considerations, if several indices pointed in the same direction; e.g., if an item had a modification index of 40 for a specific residual correlation (a "WITH"-statement in Mplus) and also had residual model correlations with several items. Revisions to improve model fit were based on both confirmatory factor modeling and content analysis, including specification of residual correlations among items, omission of poorly associated items from the models, and specification of sub-factors (for example among positively worded items in a scale). For scales where secondary factors seem plausible, a bifactor model was fitted to evaluate the dominance of the primary factor when secondary factors were modeled. A bifactor model specifies that each item is regressed on both a general and a group (secondary) factor, and the general and group factors are uncorrelated with each other ,-. The magnitude of loadings on the general and group factors were compared. The two-item scale on impaired sex life was not examined in this step, since a separate factor analysis of a two-item scale is not useful.
In an attempt to understand any possible item misfit identified through individual scale analyses, hypotheses which could explain the misfit were sought. These hypotheses were evaluated in a combined, investigational multidimensional model, where the individual scale factors were allowed to correlate freely. Also items were cross-loaded on multiple scale factors when necessary to explore a better understanding of item misfit. For example, if an item in a physical symptoms scale, e.g., "Palpitations", had low own-factor loadings, it could be hypothesized that this was due to palpitations being influenced by mental health, e.g., as part of anxiety. Then cross-loading of this item on the mental symptoms scales would be specified and evaluated in the combined model.
In order to examine the stability of the model across various estimation techniques, the overall final model was compared with graded response multidimensional IRT models , fitted with the Mplus program . For computational reasons, a 13-dimensional IRT model could not be estimated, so the model was broken down to four separate models, each containing scales with cross-loadings across scales. Stability was examined by comparing the estimated factor scores for each patient from the SEM vs. the IRT-model using intra-class correlations.
Fitting unidimensional models to each individual ThyPRO scale
Content analysis and confirmatory factor analyses of the individual ThyPRO scales
Scale and item
Possible misfit from content analysis
Initial unidimensional modela
Abbreviated item content
Indication of local dependenceb
Indication of item misfitc
Sense of fullness in neck
MI: LD with 2b
Visible swelling on neck
MI and RC: LD with 2a
Pressure in throat
RC: LD with 2g
Pain in front of neck
Throat pain felt in ears
Low loading and low IC
Lump in throat
Clear throat often
MI: LD with 2l, RC: LD w. 2c
MI: LD with 2i
MI: LD with 2h
Sense of suffocating
MI: LD with 2g
With 2o, 2p, 2q
MI: LD with 2q
With 2n, 2p
Shortness of breath
With 2n, 2o
Sensitive to heat
MI: LD with 2n
Low IC and large neg. RCs
Sensitive to cold
Swollen hands or feet
RC: LD with 2hh
RC: LD with 2gg
With 2y, cc, dd
MI and RC: LD with 2x
Bags under the eyes
MI and RC: LD with 2w
Grittiness in eyes
With 2w, 2cc, 2dd
Pressure in eyes
MI: LD with 2cc
Pain in eyes
With 2w, y, dd, aa
MI: LD with 2aa
Sensitive to light
With 2w, y, cc
MI: LD with 3b
MI: LD with 3a
Difficult get motivated
Felt worn out
Full of life
With 4b, 4c
MI and RC: LD with 4b, 4c
With 4a, 4c
MI and RC: LD with 4a, 4c
Able to cope with life
With 4a, 4b
MI and RC: LD with 4a, 4b
RC: LD with 5d
Slow or unclear thinking
Difficulty finding words
RC: LD with 5a
MI: LD with 5f
MI: LD with 5e
MI: LD with 6b
Afraid or anxious
MI: LD with 6a
Afraid being seriously ill
Low loading, neg. RC's
MI: LD with 6f
MI: LD with 6e
MI: LD with 7f
MI: LD with 7e
With 7i, 7f
MI: LD with 7i
MI: LD with 7g
Not like yourself
MI: LD with 8i
Large neg. RC
MI: LD with many other items
MI: LD with many other items
Felt in control
MI: LD with many, large neg. RC's
Felt in balance
MI: LD with 8i, 8c
Impaired Social Life
Difficult with people
A burden to people
Conflicts with people
Others lack understanding
Low loading, neg. RC's
Impaired Daily Life
Difficult manage life
Limit leisure activities
MI: LD with 11f
Difficult participate in life
Difficult getting around
MI: LD with 11e
Everything takes longer
MI: LD with 11d
Difficulty managing job
MI: LD with 11b
Disease affect appearance
MI: LD with 13b
MI: LD with 13a
Camouflage visible signs
Other people looking
Influence on clothes worn
Felt too fat
Three items were problematic (2b Visible swelling in front of neck, 2e Throat pain felt in ears and 2l Hoarseness), with relatively low loadings and indication of local dependence with other items. Two of these items were identified prior to the modeling as potentially less related to the concept. Two instances of local dependence among other items were identified (2c Pressure in throat vs. 2 g Need to clear throat often and 2 h Discomfort swallowing vs. 2i Difficulty swallowing, Table 2). When omitting the three items and modeling the local dependencies, an appropriately fitting unidimensional model was reached (Figure 1, CFI = 0.99, RMSEA(90%CI) = 0.08(0.07-0.09)).
For one pair of items (2n Increased sweating vs. 2q Sensitive to heat), the modification index suggested local dependence and one item (2t Loose stools) had large negative residual correlations with other items, when the initial model was estimated. When omitting the latter and fitting the local dependence, a unidimensional model obtained an appropriate fit to the data (Figure 1, CFI = 0.97 RMSEA(90%CI) = 0.06(0.05-0.08)).
When modeling the expected local dependence between the items concerning skin (2gg Dry skin vs. 2hh Itching skin), an appropriate fit between an overall unidimensional model and data was demonstrated for this scale (Figure 1, CFI = 1.0 RMSEA(90%CI) = 0.00(0.00-0.09).
With the specification of two local dependence-pairs (2w Watery eyes vs. 2x Bags under eyes and 2aa Pressure in eyes vs. 2cc Pain in eyes), an appropriate fit of a unidimensional model was found (Figure 1, CFI = 0.99 RMSEA(90%CI) = 0.06(0.04-0.07).
All items had high loadings in the initial model (Table 2). When specifying two pairs of local dependence, suggested by modification indices (5a Problems remembering vs. 5d Been confused and 5e Difficulty learning vs. 5f Difficulty concentrating), overall model fit was appropriate (Figure 1, CFI = 1.0 RMSEA(90%CI) = 0.07(0.05-0.09)).
According to overall goodness-of-fit indices, the initial model did not obtain an appropriate fit to the data (Table 2). When fitting a model by excluding the item identified as less related with the other items (6d Afraid being seriously ill) and by specifying two item pairs with local dependence (6a Nervous vs. 6b Afraid or anxious and 6e Uneasy and 6f Restless), appropriate fit was obtained (Figure 1, CFI = 1.0, RMSEA(90%CI) = 0.07(0.04-0.10)).
All items had high loadings (Table 2). However, only after specification of two local dependence pairs (7e Crying easily vs. 7f Unhappy and 7 g Happy vs. 7i Self-confident), was an appropriate overall fit to data reached (Figure 1, CFI = 1.0 RMSEA(90%CI) = 0.07 (0.05-0.09)).
In contrast to most other concepts measured by ThyPRO, this scale measures a unique aspect of mental health identified through qualitative analysis of patient interviews. Thus, it is not classically described as a separate concept. It is, however, an important aspect according to the patients and a prominent feature particularly among patients with thyroid autoimmunity . According to the overall fit indices, these items do not appropriately conform to a unidimensional model, despite high factor loadings (Table 2). Several items had high inter-item residual correlations and were attempted to be modeled as a separate "Anger" sub-factor (Figure 2, Panel B). However, as shown in Figure 2, the sub-factor loadings were rather low. Four items had to be omitted in order to obtain appropriate fit between a unidimensional model and the data (Figure 1, CFI = 1.0 RMSEA(90%CI) = 0.08(0.05-0.11)). A local dependence (8c Easily stressed vs. 8i Felt in balance) was also modeled.
Impaired Social Life
Appropriate, albeit not good overall goodness-of-fit indices were found for the initial unidimensional model. Excluding the lowest-loading item (10d People lack understanding), which was also pre-specified as possibly less associated, resulted in a just-identified model, hence with perfect fit (Figure 1, CFI = 1.0 RMSEA(90%CI) = 0.00(0.00-0.00)).
Impaired Daily Life
With the specification of one local dependence (11d Difficulty getting around vs. 11e Everything takes longer), a unidimensional model fit the data appropriately (Figure 1, CFI = 1.0, RMSEA (90%CI) = 0.08(0.07-0.10)).
The initial unidimensional model had almost appropriate goodness-of-fit indices (Table 2). When modeling one local dependence (13a Disease affect appearance vs. 13b Unsatisfied with appearance) and leaving out the very nonspecific item concerning feeling too fat (13g), a good fit between model and data was found (Figure 1, CFI = 1.0 RMSEA(90%CI) = 0.05(0.02-0.08)).
Investigative modeling of possible item misfit within one combined multidimensional model
For each item which was omitted during the single-scale analyses, hypotheses regarding possible reasons for misfit were formulated, modeled and tested as specified
Hypothesized reason for misfit
Investigative modeling of the hypothesized reason for misfit
Results of the investigative modeling
2b Visible swelling on neck from the Goiter Symptoms scale
May relate to cosmetic concerns, rather than being a symptom
Item was allowed to cross-load on the Cosmetic Complaints factor
Loaded –0.23 on the Cosmetic Complaints factor.
Loading on own factor: 0.68
2e Throat pain felt in ears from the Goiter Symptoms scale
May be relevant only for patients with subacute thyroiditis, during the acute inflammatory phase.
No marker of acute inflammation is available in the clinical database describing the patients. Only 9 patients in this sample had subacute thyroiditis
Extraneous modeling not possible.
Loading on own factor in the full model: 0.75
2l Hoarseness from the Goiter Symptoms scale
Hoarseness is also a classical symptom of hypothyroidism. Might relate more to hypothyroidism than to goiter.
Item was allowed to cross-load on the Hypothyroid Symptoms factor
Loaded 0.22 on Hypothyroid Symptoms factor.
Loading on own factor: 0.46
2t Loose stools from the Hyperthyroid Symptoms scale
Might be a non-specific physical symptom
Item was allowed to load on the other physical symptoms factors, except for Eye Symptoms
Loaded –0.15 on Goiter Symptoms factor and –0.55 on Hypothyroid Symptoms.
Loading on own factor: 1.20
6d Afraid of being seriously ill from the Anxiety scale
May be related to not being fully examined yet, and thus an initial fear of e.g. cancer has not yet been ruled out completely
Item was regressed on time since diagnosis.
A significant negative association with time since diagnosis was found
10e Other people lack understanding from the Impaired Social Life scale
May relate more to depressive mood and emotional distress than the other items in the Social Life scale
Item was allowed to cross-load on the Depressivity and the Emotional Susceptibility factor
No significant loading on Depressivity or Emotional Susceptibility was found.
Loading on own factor: 1.08
13g Felt too fat from the Cosmetic Complaints scale
Weight gain is often experienced during hypothyroidism. Feeling too fat may also relate more to a negative self-esteem aspect of depressive mood
Item was allowed to cross-load on the Hypothyroid Symptoms and Depressivity and Anxiety factors
Loaded –0.16 on Hypothyroid Symptoms factor, –0.22 on Anxiety and 0.15 on Depressivity factor.
Loading on own factor: 0.53
Comparison of individual factor-scores derived from the ordinal confirmatory factor analysis approach with the factor scores derived from the item response theory (IRT) approach
Ordinal vs. IRT factor scores intra-class correlation coefficients
Impaired Social Life
Impaired Daily Life
Impaired Sex Life
The purpose of the present study was to evaluate the dimensionality of the ThyPRO scales and to detect and understand potential item misfit. Since an established scale structure already exists for the ThyPRO, we used a combination of confirmatory factor analyses of the individual scales and a combined multidimensional model comprising all 13 ThyPRO scales. In case of misfit for each individual scale, we revised the model to achieve the best description of data.
In general, items had high loadings on their own factors and the comparative fit indices were high, but for the majority of the scales, the root means square error of approximation indicated that a simple unidimensional model was not fitting the data sufficiently well. Based on prior expectations informed by content analyses, modeling results (model inter-item correlations and model residual correlations) and on model modification indices, the models were adjusted in order to reduce the overall misfit. For all scales, an appropriate fit according to the overall goodness-of-fit indices could be reached. During this process, a total of 11 items were left out of the models and 18 residual correlations indicating local dependence were specified.
In most instances, the magnitude of the residual correlations representing local dependencies was small, and the loading on the relevant general factor was still high. Most of the residual correlations were among very similarly worded items. Such local dependencies are not problematic for the current scoring of the ThyPRO, but may lead researchers to overestimate the precision gained by the instrument, because locally dependent items provide less measurement precision than assumed by standard psychometric analyses . Moreover, one of the items involved in such pairs would be potential candidates for omission in future IRT-modeling of the instrument and in the development of abbreviated versions of the ThyPRO.
However, such item reduction should be done with caution and should take clinical analyses and considerations into account.
Although positively worded items did tend to exhibit residual correlations, we found no consistent evidence of a method factor among the positively worded items. Similar studies with other outcome measures have previously found substantial influence of the value of the wording ,-, whereas other studies either did not identify such an effect  or the identified effect had only minor influence on the results regarding the substantive factor .
We attempted to model potential item misfit identified during the dimensionality analyses of the existing ThyPRO scales. This was done within a model including all scales, which were allowed to correlate, in order to allow for cross-loadings of items to be examined and in order to evaluate if possible misfit identified during individual scale analyses was due to interrelation with other factors. In doing so, the hypothesized reason for misfit was confirmed in five of seven items: Item 2b, about visibility of the goiter, cross-loaded on Cosmetic Complaints. Item 2t, Loose stools, had a large negative loading on Hypothyroid Symptoms, as had 2l, Hoarseness. Both constipation and hoarseness are indeed salient and classical features of hypothyroidism . The rather non-specific item 13g, Feeling too fat, which is a common complaint among hypothyroid patients and among hyperthyroid patients after treatment, had cross-loadings on several other scales and low loading on its own factor, also when modeled multidimensionally. Thus, these four items are very strong candidates for item reduction when developing abbreviated and focused versions of the scales or when fitting models where unidimensionality is a strong assumption, for example as in unidimensional IRT models.
A unique "duration of disease"-effect was observed for one item. Item 6d, Afraid of being seriously ill was negatively associated with time since diagnosis, indicating that the responses to this item reflects a relevant concern early in the disease course, for instance of a goiter being malignant, a concern that wanes as the diagnosis becomes more firmly established and malignancy thus ruled out. It thus measures something different from the other items in the scale, which are more classical indicators of an anxious state.
As an analysis of the robustness and appropriateness of the ordinal confirmatory WLSMV factor analysis, an alternative multidimensional IRT-based analysis was performed. Individual factor scores derived from each of these approaches were very similar, as illustrated by very high intra-class correlation coefficients. This corroborates the current simple scoring approach and the results of the present analyses.
The use of theoretically driven analyses within a clinically well-described and relatively (for thyroid diseases) large sample was a strength of this study. However, the analyses were carried out in one sample and should ideally be confirmed in a new independent sample. Furthermore, although the present sample comprised patients in all stages of disease and treatment, stability of the factor structure across time could not be evaluated, since the data did not contain longitudinal measurements.
In conclusion, each of the ThyPRO scales could be appropriately represented by a unidimensional model after minor revisions. Eleven items were identified in the unidimensional models as potentially misfitting and understood further by multidimensional modeling. Thus, overall the previous initial examinations of the construct validity of the scales  were corroborated using a more elaborate technique. Further, advanced psychometric modeling such as IRT, with strong assumptions about dimensionality, can be applied to the reduced scales. Finally, the locally dependent items identified here are strong candidates for removal, in future item reduction processes.
This study has been supported by grants from the Danish Medical Research Council, Agnes and Knut Mørk's Foundation, Aase and Ejnar Danielsen's Foundation, Else and Mogens Wedell-Wedellsborg's Foundation, the Genzyme Corporation, the Novo Nordisk Foundation, Arvid Nilsson's Fund and the Danish Thyroid Foundation.
*Researchers who want to use the ThyPRO may contact the first author (firstname.lastname@example.org).
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