Background: Abnormalities of the serum thyroid hormone binding proteins are not uncommon but, when properly assessed, they do not present diagnostic difficulties. In contrast, the presence of two inherited defects of thyroid hormone transport, of the type presented in the family described here, may cause a major problem in diagnosis and has not been described previously.
Methods: All conventional thyroid function tests were carried out. In addition, thyroid hormone binding to serum proteins was assessed by agarose gel electrophoresis, and thyroxine binding globulin by immunoassays and by immunodiffusion. The affinity of TBG for thyroxine and its maximal binding capacity were assessed by Scatchard analysis.
Results: Tests carried out on 22 members of the family revealed familial dysalbuminaemic hyperthyroxinaemia in 10 family subjects. All five living siblings of the propositus had familial dysalbuminaemic hyperthyroxinaemia and two tested transmitted this trait to their children and grandchildren. This was not the case with the propositus. Partial thyroxine binding globulin deficiency only, inherited presumably from the propositus' mother, was found in two family members. Both thyroxine binding globulin deficiency and familial dysalbuminaemic hyperthyroxinaemia were detected in the propositus and in his male nephew, masking the typical laboratory abnormalities associated with each of these defects.
Conclusions: Coexistence of two inherited defects of thyroid hormone transport proteins produce atypical thyroid function test abnormalities, which can be misinterpreted as thyroid hormone dysfunction.