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-lin
ked fashion. We report a family with decreased concentration of circulating
TBG and an unusual pattern of inheritance. In this family, a number of mem
bers showed decreased levels of serum TBG. The unusual finding was that the
low TBG was transmitted from the male proband to his son. Thyroxine (T-4)
binding affinity and heat lability of serum TBG of the proband was not diff
erent from values obtained from a normal control. All exons and the critica
l 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.