Association between the quality of plant-based diets and risk of thyroid dysfunction.

Background: Plant-based diets are generally considered to be associated with a decreased risk of several chronic diseases. However, there is limited prospective evidence on the association between plant-based diet quality and thyroid dysfunction (TD) risk. Therefore, we aimed to explore the longitudinal associations between plant-based diets and TD (encompassing hypothyroidism and hyperthyroidism) risk. Methods: A total of 199 382 TD-free participants were enrolled in the UK Biobank. The three plant-based diet indices (PDIs), including the overall plant-based diet index (PDI), healthy plant-based diet index (hPDI), and unhealthy plant-based diet index (uPDI), are calculated based on 17 major food groups from the Oxford WebQ questionnaire. Cox proportional hazard models were utilized to assess the associations of PDIs and their combinations with the risk of TD, hypothyroidism, and hyperthyroidism. Results: A total of 4283, 4086, and 935 cases of incident TD, hypothyroidism, and hyperthyroidism were documented over a median follow-up of 12.0 years. The multivariable-adjusted HRs (95% CIs) for the highest uPDI versus the lowest tertile of TD and hypothyroidism were 1.11 (1.03-1.20; P for trend = 0.014), and 1.14 (1.06-1.23; P for trend = 0.002), respectively, after adjusting for potential confounding factors. And lower risks of hyperthyroidism were linked to higher hPDI scores, with HRs (95% CIs) of 0.84 (0.72, 0.98; P for trend = 0.038). The associations between PDIs and TD were consistent in stratified analyses and several sensitivity analyses. Conclusions: The uPDI was associated with a higher TD and hypothyroidism risk, whereas the hPDI was associated with a lower risk of hyperthyroidism.