A Comprehensive Review of Thyroid Hormone Metabolism in the Gut and Its Clinical Implications

The relationship between thyroid and human-associated microbiota: A systematic review of reviews

“…The analysis of tumoral specimens revealed different microbiota composition in peritumoral and tumoral tissues, with specific variations related to tumor subtype, metastases presence and patient’s age Jiang [ 41 ] To analyze the variations in gut microflora composition and metabolites in autoimmune, functional and tumoral thyroid disorders Several aspects of thyroid homeostasis might be related with gut microbiota composition but the evidence about the causative role of microflora on thyroid disorders are still lacking Belvoncikova [ 42 ] To summarize the results of the studies dealing with fecal microbiota transplantation experiments in thyroid autoimmune disorders field The different experiments strongly suggested the involvement of gut microflora on thyroid autoimmunity pathogenesis Calcaterra [ 43 ] To describe the reciprocal influence between gut microbiota, thyroid hormone metabolism and thyroid autoimmunity with a focus on probiotics use in a pediatric setting Deepening in understanding the connections between gut microbiota, thyroid hormones metabolism and thyroid-related micronutrients would implement microbiota-targeted therapies in thyroid disorders also in a pediatric perspective Sawicka-Gutaj [ 44 ] To clarify if microbiota composition is altered in patients with thyroid autoimmunity Significant alteration of the diversity indexes and overall composition have been described in autoimmune thyroid disorders. While higher diversity has been detected in patients with Hashimoto’s thyroiditis, lower diversity has been described in patients with Graves’ disease, with a higher relative abundance of Bacteroidetes and Actinobacteria Wu [ 45 ] To sum up the knowledge about gut microflora characteristics in patients with autoimmune and tumoral thyroid disorders More clinical research are needed to estimate the possible causative role on thyroid disorders of gut dysbiosis and the effects of its modulation Macvanin [ 46 ] To review novel findings on the connection between thyroid and gut microbiome with a specific focus on probiotics with antioxidant properties on thyroid disorders Despite the promising potential in improvement of thyroid function of probiotic supplementation, further human studies are needed Fenneman [ 47 ] To address the main features of thyroid-gut axis Dysbiotic state characterizes autoimmunity but the proof of a causative role is still lacking. Recent papers on microbiota tran...…”

Causal effect of gut microbiota of Defluviitaleaceae on the clinical pathway of “Influenza–Subacute Thyroiditis–Hypothyroidism”

“…Conversely, research also declared a reduction of Bacteroides in Hashimoto's thyroiditis patient fecal samples (Sawicka-Gutaj et al, 2022 ). Bacteroides were also discovered to be positively associated with anti-thyropreoxidase antibodies and negatively associated with thyroid-stimulating hormone levels (Sawicka-Gutaj et al, 2022 ; Fenneman et al, 2023 ). However, one species of Bacteroides, Bacteroides fragilis , is abundant in Hashimoto's thyroiditispatients.…”

Hypothalamic hormone deficiency enables physiological anorexia

“…In this study, we found that squirrels exhibit T3 deficiency in the hypothalamus during IBA, and that restoration of T3 reverses anorexia, demonstrating that long-term suppression of hunger during hibernation is, at least partially, due to central hypothyroidism. Thyroid hormone deficiency is specific to the CNS and does not affect peripheral levels of T3 and T4, obviating the adverse effects of hypothyroidism on internal organs 35,36 and preserving the involvement of thyroid hormones in increasing thermogenic capacity prior to squirrels entering hibernation.…”

“…Our findings demonstrate that thyroid hormone deficiency during hibernation is restricted to the CNS and does not extend to peripheral levels of T3 and T4. Hibernating squirrels thus present a remarkable animal model in which central thyroid hormone function is depressed while essential peripheral thyroid function on internal organs and general metabolic processes is preserved 49,50 . Central thyroid hormone has been implicated in the seasonal shifts in reproduction and food intake in non-hibernating animals, including Siberian hamsters, sheep, photoperiodic F344 rats [51][52][53][54] and hibernating arctic ground squirrels 55 .…”