Causality between thyroid disease and psoriasis: Bidirectional Mendelian randomization analysis

Both psoriasis and autoimmune thyroid diseases are characterized by chronic inflammation. The previous studies indicated a potential association between psoriasis and autoimmune thyroid diseases. However, the direction and nature of these relationships remain unclear. This study aimed to investigate the bidirectional causal relationships between multiple thyroid diseases (hypothyroidism, hyperthyroidism, Hashimoto thyroiditis, and Graves disease (GD)) and psoriasis (PsO) as well as its subtypes (psoriasis vulgaris (PV) and psoriatic arthritis (PsA)) through a two-sample Mendelian randomization (MR) approach. A bidirectional, two-sample MR analysis was conducted using genome-wide association study (GWAS) summary statistics from large European populations. Independent single nucleotide polymorphisms (SNPs) associated with thyroid diseases and psoriasis were selected as instrumental variables. The MR-PRESSO method was applied in 2 rounds to identify and remove outlier SNPs. MR-Egger regression, inverse-variance weighted (IVW), and weighted median methods were employed to assess causal relationships. This study provides genetic evidence of a suggestive association between hypothyroidism, the most common form of thyroid disease, and an increased risk of developing PsO (odds ratio (OR) = 1.059, P = .038). Additionally, genetic predisposition to hypothyroidism was significantly associated with its subtype, PsA (OR = 1.184, P = 2.40 × 10-5). In the inverse MR analyses, PV, the primary subtype of PsO, was suggestively associated with an increased risk of GD (OR = 1.113, P = .002) and hyperthyroidism (OR = 1.053, P = .040). However, it was associated with a decreased risk of hypothyroidism (OR = 0.977, P = .025). This study presents evidence supporting bidirectional causal relationships between thyroid diseases and psoriasis. These findings offer new insights into shared inflammatory pathways that may underlie these comorbidities.