Endocrine Abstracts

Objective: Thyroid diseases have been shown to affect quality of life. A thyroid-specific patient-reported outcome (PRO) measuring quality of life, the ThyPRO, has recently been developed and validated using classical clinic- and psycho-metric methods. The purpose of the present study was to apply modern psychometrics to validate the measure as the extent of differential item functioning (DIF) according to sex, age, education and diagnosis, within the ThyPRO.

Methods: Eight hundred and thirty eight patients with benign thyroid diseases (non-toxic goitre, toxic nodular goitre, Graves’ hyperthyroidism, Graves orbitopathy and autoimmune hypothyroidism) completed the ThyPRO questionnaire (84 five-point items, 13 scales). Uniform and non-uniform DIF was investigated using ordinal logistic regression, testing for both statistical significance and magnitude (ΔRsq >0.02).

Results: Twenty instances of DIF were found. Seven according to diagnosis, where the goitre scale was the most affected scale, possibly due to differing perceptions in patients with autoimmune thyroid diseases compared to patients with simple goitre. Eight age-related DIFs were found, five of which were in positively worded items, which younger patients were more likely to endorse; one gender-related where women were more likely to report crying, and three according to educational level. The vast majority of DIFs only had minor influence on the scale scores (0.1–2.3 points on the 0–100 scales), but two DIFs corresponded to a difference of 4.6 and 9.8 respectively.

Conclusion: Ordinal logistic regression identified DIF in 17 of 84 items. The potential impact of this on the present scales was low, but the results can assist the planning and interpretation of future clinical trials. Shorter and more clinically implementable versions of the ThyPRO would be of virtue in the future, and items displaying DIF could be avoided when developing such abbreviated scales, where the impact of one items with DIF among fewer items will be larger.