Effects of pharmacological fiber supplements on levothyroxine absorption

To determine the effect of pharmacological fiber supplements, we measured levothyroxine (LT₄) absorption without and with simultaneous ingestion of either calcium polycarbophil or psyllium hydrophilic mucilloid. Serum thyroxine (T₄) levels in 8 volunteers were measured following ingestion of 600 μg of LT4 on 3 separate occasions at 4-week intervals: (1) LT₄ alone; (2) LT₄ together with 1000 mg polycarbophil; and (3) LT₄ together with 3.4 g psyllium. The amount of absorbed LT₄ was calculated as the incremental rise in serum T₄ level during the first 6 hours multiplied by the volume of distribution for the hormone, and expressed as a percentage of the dose administered. Absorption of LT₄ alone averaged 89% (95% confidence interval [CI]: 75%-104%), occurring at a median of 180 minutes. After simultaneous ingestion of calcium polycarbophil, LT₄ absorption was 86% (95% CI: 74%-97%), occurring at 180 minutes. With simultaneous ingestion of psyllium and LT₄, the absorption was 80% (95% CI: 64%-95%), occurring at 240 minutes. In summary, neither calcium polycarbophil nor psyllium hydrophilic mucilloid are likely to cause malabsorption of LT₄ that could be detected by these methods.