The objective was to discuss the influence of the gut microbiota on the functions of the thyroid gland, elucidating the main aspects of rebalancing intestinal function and its importance in the regulation of thyroid functions. The present study followed a systematic review of the literature in periodicals published between 2010 and 2023. The microbiota of the healthy gastrointestinal system presents around 800 species of bacteria, and a hundred of these microorganisms together with bacteriophage viruses and fungi species characterize each human being, with maximum concentration in the colon. In the presence of dysbiosis, the malfunction of the epithelial barrier leads to intestinal and systemic disorders, mainly immunological and metabolic. The functions of the gut microbiota are fundamental and determinant in the metabolism of nutrients, drugs, and hormones, including exogenous and endogenous iodothyronines, as well as micronutrients involved in thyroid homeostasis. The state of the art of effects of the gut microbiota on the regulation of thyroid functions has not been fully elucidated. The intestinal tract is of great importance for the balance of exogenous and endogenous thyroid hormones, but the analysis of the composition of the microbiota is not an easy task. A recent study revealed that individuals with hyperthyroidism had significantly lower levels of Bifidobacteria and Lactobacilli and significantly higher levels of Enterococcus species compared to healthy controls.

Introduction: In addition to influencing physical performance, some studies verify the impact of vitamin D on body composition. It is known that their high levels favor the gain of fibers of fast contraction, favoring the increase of force due to the increase of muscular areas and it prevents loss of lean mass. Objective: It was present, through a systematic review, the main correlations and outcomes of clinical studies between vitamin D and physical exercise practitioners, analyzing the effects of vitamin D deficiency, sufficiency, and supplementation on sports performance. Methods: Following the criteria of literary search with the use of the Mesh Terms that were cited in the item below on "Search strategies", a total of 94 papers that were submitted to the eligibility analysis were collated and, after that, 65 studies were selected, following the rules of systematic review - PRISMA. The review protocol was based on literary search criteria using mesh terms in major databases such as PubMed, Scopus, Medline, Bireme, EBSCO, and Scielo. It followed the following steps: search for MeSH Terms: Vitamin D. Clinical trials. Physical exercises. Athletes. Quality of life. Conclusion: The present study aimed to present, through a systematic review, the main correlations and outcomes of clinical studies between vitamin D and physical exercise practitioners, analyzing the effects of vitamin D deficiency, sufficiency, and supplementation on sports performance.