SUMMARYThe abnormal intermediate‐affinity T4 binding to albumin which is characteristic of familial dysalbuminaemic hyperthyroxinaemia (FDH) is dependent on buffer, temperature, and ionic composition. Scatchard analysis of T4‐binding to isolated albumin preparations from FDH subjects showed that half the circulating albumin showed the higher‐affinity T4 binding site, assuming one site per molecule. Using dextran‐charcoal separation at 40C the T4 affinity (Ka) of purified albumin from FDH subjects was 7.5 nmol/l in phosphate and 17 nmol/l in Tris‐CI‐ buffer. T4 binding to FDH albumin was inhibited by a range of substances in the order: 8‐anilino‐l‐naphthalene sulphonic acid>merthiolate>propylthiouracil>methyl‐thiouracil>carbimazole>salicylate>barbitone. Binding of T4 was competitively inhibited by low concentrations of dithiothreitol (DTT). The effect of DTT 0.1–0.5 mmol/l was reversed by removal of DTT by dialysis. Competition with a range of iodothyronines indicated that the 3′, 5′‐iodine atoms are most important for binding to this site. Serum binding of salicylate. frusemide. fenclofenac and barbituric acid, and a range of steroid hormones was similar in FDH and normal sera. Serum levels of sex hormone binding globulin (SHBG), were not significantly different from sex‐matched controls. Nuclear [125I]‐T3 binding sites in circulating lymphocytes from two FDH subjects showed affinities (Kd) of 59 and 79 pmol/l (normal 67±7 pmol/l,n= 6. These findings suggest that the highly specific binding anomaly of FDH is due to a disulphide‐dependent structural change in albumin. The normal T3 affinity of lymphocytic receptors and normal SHBG levels are consistent with a normal relationship between free