Adaptation and cross-cultural validation of the Spanish version of the Thyroid-Related Quality-of-Life Patient-Reported Outcome questionnaire.

Introduction: The ThyPRO questionnaire is the most widely used tool for measuring quality of life in patients with benign thyroid diseases. The purpose of this study was to adapt and validate a Spanish translation of the ThyPRO and its abbreviated version (ThyPRO-39).

Material And Methods: Adaptation to the Spanish language was performed using the forward-backward translation method, followed by a pretesting study on five representative patients. The final questionnaire (ThyPROes) was administered to 155 patients with thyroid disorders recruited in a tertiary Spanish hospital. Psychometric properties were evaluated by multitrait scaling and estimation of internal consistency reliability (Cronbach's alpha coefficient). Data from a previous sample of 902 Danish patients were used to analyze differential item functioning (DIF) between the Spanish and the original Danish versions of the questionnaire using ordinal logistic regression.

Results: Three of 85 items in ThyPROes and four of the 39 items in ThyPRO-39es lacked convergent validity, while lack of discriminant validity was found for in nine and 14 items of each version respectively. Cronbach's alpha was >0.7 for 12 of 13 scales in the ThyPRO and 10 of 12 scales in the ThyPRO-39es. Eight items in the ThyPROes were flagged with DIF (one with non-uniform DIF), as were two items in the ThyPRO-39es. DIF magnitude was small (explained variance in the item score <3%) in most cases, with a minor impact on scale scores.

Conclusions: The Spanish versions of the ThyPRO and ThyPRO-39 show acceptable psychometric properties and good cross-lingual validity, and are suitable for use in clinical studies.