Levothyroxine and conjugated thyroid hormone in the intestinal lumen

Levothyroxine and conjugated thyroid hormones in the intestinal lumen represent the pool of thyroid hormones available for absorption or reabsorption via the enterohepatic circulation. Levothyroxine is a synthetic form of thyroxine (T4) used for hormone replacement therapy, with absorption occurring primarily in the jejunum and ileum (StatPearls, 2023). Conjugated thyroid hormones, such as T4-glucuronide and T4-sulfate, are metabolites formed in the liver and excreted into the bile (Virili et al., 2019). Once in the intestinal lumen, these conjugates are hydrolyzed by commensal bacterial enzymes, such as beta-glucuronidase, to release free hormones for reabsorption into the systemic circulation (Hays, 1988). This luminal pool is highly susceptible to interactions with various medications, including bile acid sequestrants, phosphate binders, and metal cations like calcium or iron, which can bind the hormones and prevent their absorption (Colucci et al., 2013). Such interactions can significantly impair the therapeutic efficacy of levothyroxine, leading to subtherapeutic hormone levels and necessitating clinical monitoring of TSH levels. Consequently, this entity is primarily discussed in the context of pharmacokinetic interference rather than as a classical therapeutic target.

Other names

Intestinal T4 poolLuminal thyroid hormonesThyroxine conjugatesT4-glucuronideT4-sulfateL-thyroxine

Physical adsorption, chelation, or pH-dependent inhibition of gastrointestinal absorption of thyroid hormones.