Bile acid sequestrant resins can bind T4 in vitro and in the gut. Therefore, the effect of colestipol HCl on thyroid function was studied prospectively in 17 subjects with type II hyperlipoproteinemia. Serum T4, T3, T3 resin uptake, and TSH levels were measured before and during therapy. No change in T4 levels and only small transient, but significant, decreases in T3 were noted in a minority of patients; TSH levels did not change. Small increases in T3 resin uptake were noted. We conclude that no major alterations in thyroid function occurred with colestipol therapy.