Iodine

An old finding, still not fully explained

Iodine is essential for making thyroid hormones, and the thyroid gland depends entirely on absorbing it from the digestive tract first. The idea that gut bacteria influence this isn't new. A rat study from 1972 found that gut microbiota affected how much iodine was absorbed from the intestine. More recent research has identified a plausible mechanism: gut bacteria appear to influence the sodium/iodine symporter, a protein on cell surfaces that transports iodine into the thyroid, partly through the short-chain fatty acids bacteria produce when fermenting fibre.

A two-way relationship, seen in thyroid disease

The gut-thyroid axis, as researchers now call this relationship, appears to run in both directions. People with hyperthyroidism have shown reduced levels of Bifidobacterium and Lactobacillus alongside increased Enterococcus, a shift that reduces short-chain fatty acid production and may impair iodine absorption further, potentially reinforcing thyroid dysfunction rather than resolving it. Conversely, inflammatory bowel disease, which reduces overall gut bacterial diversity, has been associated with poorer iodine absorption, suggesting the same disrupted gut environment that causes digestive symptoms may also affect thyroid nutrient status.

Source: Microbiota dysbiosis impact on the metabolism of T3 and T4 hormones and its association with thyroid cancer

Genuinely early-stage evidence

This is a mechanistically interesting area, but it remains early. One review specifically notes that while gut bacteria plausibly affect iodine absorption, the evidence for an effect on how much iodine is actually excreted in urine, a key marker of overall iodine status, is too weak to draw firm conclusions from yet. This is a case where the biological story is compelling, but hasn't yet been confirmed with the same rigour as some other nutrient-microbiome relationships on this site.

What this means for you

Iodine deficiency and thyroid problems need proper testing through blood tests measuring thyroid hormones, not assumptions based on gut symptoms, since fatigue, weight changes, and mood shifts common to thyroid conditions have many other possible causes too. People with an existing gut condition, particularly inflammatory bowel disease, may be worth discussing thyroid function with a doctor given this research, especially if they also have unexplained thyroid symptoms. Iodine-rich foods, such as dairy, seafood, and iodised salt, remain the established way to maintain healthy levels. A varied, plant-rich diet supports the broader gut ecosystem this research increasingly ties to thyroid health. That's the same idea behind GutLab, everyday gut health starts with regularly feeding the microbiome a broad range of plant-rich ingredients.