Evidence for a clinically important adverse effect of fiber-enriched diet on the bioavailability of levothyroxine in adult hypothyroid patients

To evaluate the effect of dietary fiber supplements on levothyroxine (T-4) bioavailability in hypothyroid patients, dietary fiber-containing supplementation was withheld from patients requiring disproportionately high doses of T-4, in whom a dietary history revealed ingestion of a dietary fiber supplement. The dose of T-4 was maintained at a constant level. Serum thyrotropin (TSH) was assessed before and after removal of the dietary fiber supplements. T-4 requirements, reflected by either decreased serum TSH or by decreased T-4 dose, was observed in conjunction with decreased dietary fiber intake compared with T-4 requirement during increased dietary fiber intake. In vitro experiments carried out to determine the mechanism of interaction between dietary fiber and T-4 revealed dose dependent, nonspecific adsorption of levothyroxine by wheat bran. These results indicate a decrease in T-4 bioavailability by dietary fiber through a mechanism involving nonspecific adsorption of T-4 to dietary fibers. Increased intake of dietary fiber may account for the need for larger than expected doses of T-4 in some hypothyroid patients.