What to watch with FT3 and gut health?

The relationship between FT3 (free triiodothyronine) and gut health is bidirectional and complex. The gut influences thyroid metabolism via several pathways: gut microbiota participate in the deiodination of T4 to T3, and a healthy intestinal barrier is essential for absorbing iodine and selenium, both required for thyroid hormone synthesis. Inflammatory bowel diseases (e.g., Crohn’s, ulcerative colitis) or celiac disease can impair T3 conversion, leading to low FT3 levels even with normal TSH. Conversely, FT3 affects gut motility and mucosal permeability. Hypothyroidism (low FT3) can contribute to constipation and dysbiosis. Importantly, low FT3 with normal TSH and fT4 may indicate 'low T3 syndrome' (euthyroid sick syndrome), often seen in chronic inflammation or caloric restriction—not necessarily primary thyroid disease. Direct-to-consumer FT3 tests are often unreliable because values fluctuate diurnally and are affected by medications (e.g., glucocorticoids, beta-blockers). Evidence for a causal role of gut microbiota on FT3 is mostly mechanistic and observational. Clinically, an abnormal FT3 should always be interpreted with TSH, fT4, and possibly stool inflammatory markers (calprotectin). Caution: supplements claimed to boost T3 conversion (e.g., selenium, zinc) can be toxic in excess.