ABSTRACT Three children of a family of six, affected with familial sporadic hypothyroidism are presented. The characteristics of the cases are high 131I uptake, rapid turnover, normal perchlorate test (normal protein binding) and low PBI. Most of the protein trapped iodine appeared in the serum as diiodotyrosine which was also found in increased amounts in the urine. The authors think that these cases may belong to a mixed type according to the classification of Stanbury, partly due to a lack of coupling of the iodotyrosines to produce iodothyronines and partly to a defect in deiodination.