Gut microbiota, a reflection of our thyroid?

Thyroid diseases are steadily increasing, especially in women. Estrogens, body mass index, radioactivity, ethnicity or high consumption of iodine… there are many risk factors. Individuals with a thyroid disease have a thyroid hormone imbalance observed in the blood levels as well as a disrupted functioning. However, these hormones can impact the composition of the gut microbiota. On the contrary, the latter may be involved in the exchange of hormonal messages between gut and brain. Previous works have already established a link between gut microbiota on the one hand, and some autoimmune thyroid disorders (Hashimoto's thyroiditis and Graves’ disease*) on the other hand. What about thyroid nodules and cancer?

Patients with thyroid disorders have a richer microbiota

To test this hypothesis, a Chinese team analyzed and compared the stool composition of 74 participants: 20 with thyroid cancer, 18 with nodules and 36 healthy volunteers. The results showed a greater abundance of intestinal bacteria in patients with a thyroid disorder compared to healthy individuals. Patients also had lower contents of Lactobacillus, involved in the selenium retention, a trace element that is key to a properly-functioning thyroid, and Butyricimonas, which produces a substance with beneficial effects on the gastrointestinal tract. Other bacteria were, however, more abundant: Neisseria and Streptococcus in case of nodules, as well as Clostridium in case of cancer. According to the authors, these species seem to play a role in the proper functioning of the thyroid and onset of these disorders.

Useful bacterial biomarkers

The researchers also observed a link between the abundance of some bacterial species and TSH (thyroid-stimulating hormone) and T3 (triiodothyronine) levels, two hormones indicating the potential presence of thyroid disorders. However, it is still impossible to explain if observed thyroid disorders lead to a specific microbiota, or if some bacterial species trigger these disorders. All these discoveries could make thyroid nodules and cancer diagnosis easier and lead to the development of probiotics useful to their treatment.

^{*two autoimmune diseases caused by an attack of the thyroid gland by the immune system, which considers it as foreign to the body. Hashimoto's thyroiditis is characterized by hypothyroidism; and Graves’ disease by hyperthyroidism.}