Patients with hypogammaglobulinaemia have been regularly found to have abnormal conventional thyroid function test results. The abnormality is due to an increased plasma concentration of T4-binding globulin (TBG). As the prevalence of autoimmune thyroid disease is probably increased in hypogammaglobulinaemia this further abnormality in the plasma proteins may lead to diagnostic confusion. Administration of gammaglobulin by infusion causes a rapid but transient fall in plasma concentrations of TBG and T4 which is probably due to a temporary redistribution of the plasma proteins.