The condition in three affected euthyroid males and two affected euthyroid females from two families with X-chromosome-linked hereditary deficiency of thyroxine binding globulin was misdiagnosed as thyroid disease because of the low protein bound iodine (PBI) content. In addition, one affected woman was found with undiagnosed hypothyroidism. The low PBI level in these patients led to the treatment for hypothyroidism although they appeared to be clinically euthyroid. The combination of the PBI value and triiodothyronine resin uptake is found most useful in patients with an abnormal PBI value who have suspected thyroid disease or thyroxine binding protein abnormality or both. Both these tests are readily available.