Homozygous Mutation in Human Serum Albumin and Its Implication on Thyroid Tests.

An individual with familial dysalbuminemic hyperthyroxinemia (FDH) due to a homozygous mutation (c.653G>A, p.R218H) in the human serum albumin (HSA) gene is reported. The patient was identified during evaluation of abnormal thyroid tests in a large family with multiple levels of consanguinity. He showed a greater increase in total thyroxine (T4) relative to that observed in heterozygous family members. The higher affinity of mutant HSA for T4, together with the large molar excess of HSA relative to thyroid hormones in serum, results in preferential association of T4 with the mutant rather than wild-type HSA in heterozygous individuals. The twofold greater amount of T4 bound to the mutant HSA in the homozygote, relative to heterozygotes, is an adaptive requirement to maintain a normal free T4 concentration.