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 disproportionate ly 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 consta nt level. Serum thyrotropin (TSH) was assessed before and after remova l of the dietary fiber supplements. T-4 requirements, reflected by eit her decreased serum TSH or by decreased T-4 dose, was observed in conj unction with decreased dietary fiber intake compared with T-4 requirem ent during increased dietary fiber intake. In vitro experiments carrie d out to determine the mechanism of interaction between dietary fiber and T-4 revealed dose dependent, nonspecific adsorption of levothyroxi ne by wheat bran. These results indicate a decrease in T-4 bioavailabi lity by dietary fiber through a mechanism involving nonspecific adsorp tion of T-4 to dietary fibers. Increased intake of dietary fiber may a ccount for the need for larger than expected doses of T-4 in some hypo thyroid patients.