T4-binding globulin (TBG) is the major thyroid hormone transport protein in man. Inherited abnormalities in the level of serum TBG have been classified as partial deficiency, complete deficiency, and excess. Sequencing analysis of the TBG gene, located on Xq21-22, has uncovered the molecular defects causing partial and complete deficiency. However, the mechanism leading to inherited TBG excess remains unknown. In this study, two Japanese families, F-A and F-T, with inherited TBG excess were analyzed. Serum TBG levels in hemizygous males were 58 and 44 micrograms/mL, 3- and 2-fold the normal value, respectively. The molecule had normal properties in terms of heat stability and isoelectric focussing pattern. The sequence of the coding region and the promoter activity of the TBG gene were also indistinguishable between hemizygotes and normal subjects. The gene dosage of TBG relative to that of beta-globin, which is located on chromosome 11, and Duchenne muscular dystrophy, which is located on Xp, was evaluated by coamplification of these target genes using polymerase chain reaction and subsequent quantitation by HPLC. The TBG/beta-globin ratios of the affected male and female of F-A were 3.13 and 4.13 times, respectively, that in the normal males. The TBG/Duchenne muscular dystrophy ratios were 2.92 and 2.09 times the normal value, respectively. These results are compatible with three copies of TBG gene on the affected X-chromosome. Similarly, a 2-fold increase in gene dosage was demonstrated in the affected hemizygote of F-T. A 3-fold tandem amplification of the TBG gene was shown by in situ hybridization of prometaphase and interphase chromosomes from the affected male with a biotinylated genomic TBG probe, confirming the gene dosage results. Gene amplification of TBG is the cause of inherited TBG excess in these two families.