BACKGROUND: Plant-based diets are gaining popularity due to their well-documented cardiometabolic benefits and environmental sustainability. However, these diets are often lower in specific micronutrients such as iodine, raising concerns about their potential impact on thyroid health. Therefore, we examined the associations between plant-based diets and the risk of hypothyroidism. METHODS: We analysed data from the UK (United Kingdom) Biobank cohort. Multivariable Cox proportional hazards regression models were used to estimate hazard ratios (HRs) and 95% confidence intervals (95% CIs) for incident hypothyroidism across vegans, vegetarians, pescatarians, poultry-eaters, low meat-eaters, and high meat-eaters aged 40-69 years. Ancillary to this, we carried out logistic regression analyses to evaluate associations between the diet groups and prevalent hypothyroidism according to International Classification of Diseases (ICD) codes at baseline. RESULTS: We included 466,362 individuals from the UK Biobank, of which 220,514 followed a high meat, 221,554 a low meat, 5242 a poultry-based, 10,598 a pescatarian, 8057 a vegetarian, and 397 a vegan diet. During a median SD (Standard Deviation) follow-up of 12.7 (± 3.2) years, 10,831 participants developed hypothyroidism. In multivariable Cox regression models without adjustment for body mass index (BMI), none of the diets were significantly associated with the risk of hypothyroidism. However, there was a tendency for a higher risk of hypothyroidism among vegetarians compared to people following a high meat diet (HR = 1.13, 95% CI 0.98-1.30). After controlling for BMI, a potential collider, the association for vegetarians (HR = 1.23, 95% CI 1.07-1.42) became stronger and statistically significant. Furthermore, we observed a positive association between low meat-eaters (OR = 1.05, 95% CI 1.03-1.08), poultry-eaters (OR = 1.15, 95% CI 1.04-1.28), pescatarians (OR = 1.10, 95% CI 1.01-1.19) and vegetarian (OR = 1.26, 95% CI 1.15-1.38) with hypothyroidism prevalence. CONCLUSIONS: In the present study, we found a moderately higher risk of hypothyroidism among vegetarians, after controlling for BMI, a potential collider. This slightly higher risk of hypothyroidism among vegetarians requires further investigation, taking iodine status and thyroid hormone levels into account.

Center for Public Health Medical University of Vienna Vienna Austria

Centre for Research on Diabetes Metabolism and Nutrition of 3rd Faculty of Medicine Charles University Prague Czech Republic

Co Centre for Sustainable Food Systems and The Institute for Global Food Security Queen's University Belfast Northern Ireland UK

Department Food Safety Unit Human Study Centre Consumer Health Protection German Federal Institute for Risk Assessment Berlin Germany

Department of Internal Medicine University Hospital Královské Vinohrady and 3rd Faculty of Medicine Charles University Prague Czech Republic

Department of Nutritional Sciences University of Vienna Vienna Austria

Department of Paediatrics University Hospital Královské Vinohrady and 3rd Faculty of Medicine Charles University Prague Czech Republic

Vienna Doctoral School of Pharmaceutical Nutritional and Sport Sciences University of Vienna Josef Holaubek Platz 2 1090 Vienna Austria

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