• There are several familial syndromes — Euthyroid dysalbuminemic hyperthyroxinemia: abnormal binding of T4 (but not T3) to albumin — Increased serum transthyretin level (transthyretin is a plasma protein that transports 15–20% of circulating T4) — Mutations in transthyretin that markedly increase its affi ffinity for T4
• In all three of these syndromes, total T4 is elevated, but free T4 is normal and the patients are clinically euthyroid • A fourth syndrome has also been described in which there is both pituitary and peripheral resistance to thyroid hormone; this condition may be due to point mutations in the human thyroid receptor (hTR-β1) gene, resulting in abnormal nuclear T3 receptors
2. Describe unusual syndromes of familial euthyroid hyperthyroxinemia.
103 Familial Euthyroid Hyperthyroxinemia, B