Autosomally transmitted low concentration of thyroxine-binding globulin.

The gene coding for thyroxine-binding globulin (TBG) is located on the long arm of the X chromosome; thus anomalies of TBG are transmitted in an X-linked fashion. We report a family with decreased concentration of circulating TBG and an unusual pattern of inheritance. In this family, a number of members showed decreased levels of serum TBG. The unusual finding was that the low TBG was transmitted from the male proband to his son. Thyroxine (T4) binding affinity and heat lability of serum TBG of the proband was not different from values obtained from a normal control. All exons and the critical region in the upstream of the TBG gene were sequenced and no alterations were found. The mechanism that causes decreased concentration of TBG of the proband and family members is unknown, but an abnormality in transcription factors that are important for TBG gene expression might be considered.