Despite an accurate management, in about 20% of hypothyroid patients showing an increased need for oral levothyroxine (LT4), the leading cause of treatment refractoriness remains unknown. Some studies hypothesized the relationship between gut microbiota composition and thyroid homeostasis, but the possible involvement of microflora in the efficacy of LT4 treatment has not been evaluated yet, and this represents the aim of our study. We examined the faecal microflora composition of 48 euthyroid (TSH<0.8-2.5>mU/l) patients with Hashimotos thyroiditis, subdivided in three groups: A) 13 patients not treated with LT4 (median age=52 years); B) 22 patients reaching target TSH with a dose of LT4 of 1.24 mg/ kg/day (median age=49 years); C) 13 patients reaching target TSH with a high dose of LT4 of 1.75 mg/kg weight/day (+35%; P=0.0001) (median age= 56 years). All treated patients took LT4 in fasting state, avoiding food and drugs interfering with LT4 absorption and action. Moreover, none of the included patients followed unbalanced diets, had disorders or used drugs interfering with faecal microbial composition. From each patient fecal samples were provided. Microbiota composition was determined via 16s rDNA sequencing of the hypervariable region V3-4 on Illumina MiSeq. Alpha and beta-diversity indices and all statistical analysis were computed in Qiime2. The gut microbiota of groups A and B showed a similar composition, while patients with refractoriness to L-T4 treatment (group C) showed a lower relative abundance of Firmicutes (66.58%) as compared to those in group A (76.8%, P= 0.039) or in group B (80.8%,P=0.012). By contrast, patients of group C showed significantly increased Bacteroidetes (21.5%) as compared to both groups A and B (P=0.04), as well as significantly higher relative abundance of Proteobacteria (9.28 vs 2.89% P=0.04) as compared to patients with normal L-T4 absorption. By merging group A and B together, it appears that the ratio between Firmicutes and Bacteroidetes is more than doubled in patients with T4 refractoriness (3.05 to 6.28). Alpha-Diversity Analysis revealed that patients of group A who showed higher Shannons Diversity as well as Pielous evenness indexes as compared to patients of group C. Beta-Diversity Analysis measured as weighted UniFrac showed a statistically significant clustering (A vs C;P=0.006 and B vs C;P=0.018). This is the first analysis of fecal microbiota composition in patients with refractoriness to levothyroxine treatment, showing that a peculiar microbial signature characterizes patients with high levothyroxine requirement.
13 May 2023 - 16 May 2023