The co-existence of thyroxine binding globulin (TBG) deficiency and euthyroid goitre in the same family raised the possibility that the disorders might be related. However, although both disorders co-existed in some members of the family, other members had either but not both conditions. These observations exclude the possibility that goitre development was solely due to alterations in thyroid activity brought about by TBG deficiency. It is possible, however, that the defect in protein binding might have enhanced goitre development in predisposed individuals as the two largest goitres occurred in TBG deficient subjects. Of the conventional parameters used to assess thyroid hormone levels in TBG deficient subjects, only the free triiodothyronine index consistently reflected the euthyroid status of these patients as established by clinical examination and TSH levels.