A kindred with deficiency of thyroxine binding globulin (TBG) was presented. The propositus, a 1-year old female also had congenital hypothyroidism with ectopic (sublingual) thyroid. Decrease in TBG-binding capacity (TBG) values ranging from 1.2-10.2 mug/100 ml) was detected in the seven relatives on the maternal side. Of these subject 2 were male and 5 female; the males had the lowest binding capacities. But six members of paternal relatives were entirely normal in this respect. The mode of inheritance of the abnormality in this family was compatible with the presumption of Nikolai et al. and Shishiba et al., indicating that the inheritance was an X-linked semi-dominant trait. The kinetics of thyroxine (T4) was investigated in a case of partial TBG deficiency with congenital hypothroidism (propositus). The half-life of T4 was shortened, turnover rate increased, extrathyroidal organic iodine and degradetion rate of T4 decreased compared to normal values.