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Familial euthyroid thyroxine excess: an appropriate response to abnormal thyroxine binding associated with albumin
The Journal of Clinical Endocrinology and Metabolism
|August 1, 1981
Summary
Familial euthyroid T4 excess involves abnormal thyroid hormone binding to plasma proteins, specifically increased T4-albumin binding. This adaptation maintains normal free thyroxine levels despite elevated total T4 concentrations.
Area of Science:
- Endocrinology
- Molecular Endocrinology
- Thyroid Hormone Physiology
Background:
- Familial euthyroid thyroxine (T4) excess is an autosomal dominant condition characterized by high plasma T4 levels.
- Affected individuals exhibit a high free T4 index but normal free T4 by equilibrium dialysis, suggesting a regulatory abnormality.
Observation:
- Pituitary-thyroid axis responses and T4 binding to plasma proteins were investigated in three kindreds.
- Equilibrium dialysis revealed increased [125I]T4 binding in affected subjects, with abnormal high-capacity binding observed in the presence of excess unlabeled T4.
- While T3 binding was largely normal, serum concentrations of T4-binding globulin, prealbumin, and albumin were within normal ranges.
Findings:
- Gel electrophoresis and immunoprecipitation demonstrated a significant increase in T4-albumin bound fraction (25-30% vs. normal 10-12%).
- Abnormal T4 binding, studied via adsorption, was inhibited by barbitone and associated with normal albumin quantity.
- Exogenous T4 or T3 administration led to TSH suppression, indicating appropriate pituitary response to altered free hormone levels.
Implications:
- The study suggests that elevated total T4 is an appropriate compensatory response to abnormal T4 binding, ensuring euthyroidism.
- The underlying cause of increased T4-albumin binding requires further investigation.
- Understanding these binding abnormalities is crucial for diagnosing and managing thyroid hormone transport disorders.