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Familial euthyroid thyroxine excess: characterization of abnormal intermediate affinity thyroxine binding to albumin
The Journal of Clinical Endocrinology and Metabolism
|August 1, 1982
Summary
Familial euthyroid T4 excess is caused by an abnormal binding site on albumin, leading to increased total thyroxine (T4) levels while maintaining normal free T4. This sulfhydryl-sensitive site on albumin is key to understanding the condition.
Area of Science:
- Endocrinology and Metabolism
- Molecular Biology
- Protein Chemistry
Background:
- Familial euthyroid T4 excess is a rare genetic condition characterized by elevated total thyroxine (T4) levels without hyperthyroidism.
- Thyroid hormone transport in blood is primarily mediated by thyroxine-binding globulin (TBG), prealbumin (PA), and albumin.
- Previous studies suggested an abnormal T4 binding protein in affected individuals, but its identity remained unclear.
Purpose of the Study:
- To identify and characterize the abnormal T4 binding site responsible for familial euthyroid T4 excess.
- To compare the T4 binding properties of albumin from affected subjects with that of normal individuals.
- To elucidate the molecular mechanisms underlying the increased T4 binding in this condition.
Main Methods:
- Equilibrium dialysis was used to quantify T4 binding to albumin isolated from sera of normal and affected subjects.
- Competitive binding assays with radiolabeled T4 ([125I]T4) and various unlabeled ligands were performed.
- Studies using dithiothreitol investigated the role of disulfide bonds in T4-albumin association.
Main Results:
- Albumin from affected subjects exhibited an additional T4 binding site with intermediate affinity (Kd ~50 nM), distinct from normal albumin's high-affinity sites (Kd ~4 microM).
- This abnormal site, present in about one-third of albumin molecules, accounted for the approximate doubling of total T4 levels.
- The abnormal binding was sulfhydryl-sensitive, suggesting the involvement of disulfide bonds, and showed low affinity for T3.
Conclusions:
- Familial euthyroid T4 excess is caused by an abnormal, sulfhydryl-sensitive T4 binding site on albumin.
- This altered albumin binding capacity leads to increased total T4 levels while maintaining physiological free T4 concentrations.
- The findings highlight the critical role of albumin in thyroid hormone transport and the pathophysiology of T4 excess syndromes.