In a family expressing euthyroid hyperthyroxinemia, an increased association of plasma thyroxine (T4) with transthyretin (TTR) is transmitted by autosomal dominant inheritance and is secondary to a mutant TTR molecule with increased affinity for T4. Eight individuals spanning three generations exhibited the abnormality. Although five of eight individuals had elevated total T4 concentrations, all affected individuals were clinically euthyroid and all had normal free T4 levels. Purified TTR from the propositus had an affinity for 125I-T4 three times that of control TTR. Exons 2, 3, and 4 (representing greater than 97% of the coding sequence) of the TTR gene of DNA prepared from the propositus' peripheral blood leukocytes were amplified using the polymerase chain reaction (PCR) and were sequenced after subcloning. Exons 2 and 3 were indistinguishable from normal. In 50% of clones amplified from exon 4, a substitution of adenine (ACC) for guanine (GCC) in codon 109 resulted in the replacement of threonine-for-alanine, a mutation confirmed by amino acid sequencing of tryptic peptides derived from purified plasma TTR. The adenine-for-guanine substitution abolishes one of two Fnu 4H I restriction sites in exon 4. PCR amplification of exon 4 of TTR and restriction digestion with Fnu 4H I confirmed that five affected family members with increased binding of 125I-T4 to TTR are heterozygous for the threonine 109 substitution that increases the affinity of this abnormal TTR for T4.