The first chapter of this thesis comprises a review of thyroxine-binding proteins of man, with emphasis on TBG and, to a lesser extent, TBPA. The identification, isolation, physicochemical and physiological characteristics, and genetic variations of both proteins are included. The changes in protein concentration in disease are reviewed and the indications for the present study are presented. Subsequent chapters describe the purification of TBPA and partial purification of TBG. The production of monospecific antisera to TBG and TBPA, and development of immunoelectrophoretic assays for both proteins are described. TBG was measured in healthy persons, and the effects of age and sex on TBG were assessed, as was the effect of thyroid disease. TBG, TBPA and thyroid hormones were measured in patients after surgery, myocardial infarction and starvation. The inter-relationships of protein and free hormone concentrations were explored. Finally, nineteen families with inherited abnormalities of TBG concentration were studied. The typical case histories and biochemical findings of individuals with TBG abnormality were noted, especially when TBG abnormality coexisted with thyroid disease. Deficiencies of conventional thyroid function tests were found in TBG abnormality, and the use of T\(_4\) :TBG ratio as a means of assessing thyroid function was assessed retrospectively and prospectively.