(268) Human Breastmilk Microbiome in Individuals with Hypothyroidism Treated with Levothyroxine Compared to Healthy Controls

Background: Hypothyroidism, a common thyroid disorder, has been previously associated with alterations in gut microbiota, suggesting potential systemic effects on microbial communities within the body. Human breast milk plays an important role in the development of the infant gut microbiome. While studies have associated hypothyroidism with adverse maternal and infant outcomes, no studies have addressed the effect of treated hypothyroidism on the breastmilk microbiota.

Objectives: This study aimed to investigate potential differences in microbial composition of breastmilk samples between women with hypothyroidism treated with levothyroxine and samples collected from those without any reported medical history of hypothyroidism.

Methods: Samples were obtained through the US and Canada-wide Human Milk Research Biorepository at UC San Diego, 2014-2023. The sample consisted of those diagnosed with hypothyroidism treated with levothyroxine (n=24), and a comparison group of healthy individuals without any medical history of hypothyroidism (n=67). For microbial analysis, DNA extraction and 16S rRNA amplicon sequencing were performed using Earth Microbiome Project standard protocols. Microbiota data were normalized using Total Sum Normalization (TSS). Faith’s phylogenic diversity, Chao1 indexes, Shannon’s H diversity, and relative abundance of bacterial strains were determined. For comparison between the two cohorts, Mann-Whitney Wilcoxon Tests were used at an alpha=0.05 significance level.

Results: There were no significant differences between the hypothyroid and the comparison group on alpha diversity (p=0.91), phylogenetic diversity (p=0.71), or Shannon’s H diversity index (p=0.81). No significant differences were found in mean microbiota abundance per phyla. In each group, Proteobacteria was the most abundant phyla, followed by Firmicutes, Bacteroidota, Actinobacteriota, and Fusobacteriota, which is consistent with findings from previous literature.

Conclusions: These results suggest that hypothyroidism combined with levothyroxine treatment may not influence the diversity or composition of the breastmilk microbiome, contrasting with previous findings suggesting changes in gut microbiota associated with thyroid dysfunction in the general population. This study highlights a need for further research on how commonly occurring thyroid disorders and their treatments may affect maternal microbial environments, with implications for maternal and infant health.