Thyroid hormones (TH)

Thyroid hormones, primarily thyroxine (T4) and triiodothyronine (T3), are essential regulators of metabolism that undergo significant enterohepatic circulation. After being processed by the liver and excreted into the bile, these hormones enter the intestinal lumen, where they can be reabsorbed into the bloodstream (StatPearls, 2023). This physiological process becomes a therapeutic target in conditions of hormone excess, such as thyroid storm or refractory hyperthyroidism, where binding agents like cholestyramine are employed to trap the hormones in the gut and promote their elimination via feces (PubMed, PMID: 3904281). Conversely, the presence of certain minerals, fibers, or medications in the lumen can inadvertently bind thyroid hormones, leading to reduced bioavailability of oral thyroid replacement therapy in hypothyroid patients (NIH, 2022). Understanding the dynamics of thyroid hormones within the gut is crucial for optimizing treatment efficacy and avoiding drug-drug interactions. Clinicians must often adjust the timing of thyroid medication relative to other luminal binders to maintain euthyroidism. The sequestration of these hormones in the gut provides a rapid, non-systemic method to reduce toxic levels during acute thyrotoxic crises.

Other names

Triiodothyronine (T3)Thyroxine (T4)L-thyroxineLiothyronineEnterohepatic thyroid hormones

Bile acid sequestrants and other resins bind thyroid hormones in the intestinal lumen through ion-exchange or physical adsorption, preventing their reabsorption and increasing fecal excretion.