Selenium is an essential trace element incorporated as selenocysteine (the 21st amino acid) into selenoproteins. Which clinically important selenoprotein protects thyroid hormone from excess oxidation during synthesis?

• A Glutathione peroxidase 1 (GPx1) in red blood cells
• B Thioredoxin reductase 1 (TrxR1)
• C Iodothyronine deiodinases (types 1, 2, 3) and GPx in the thyroid ✓
• D Selenoprotein P (SelenoP) as the plasma selenium transport protein only

Explanation

The thyroid gland has the highest selenium concentration of any organ. Both iodothyronine deiodinases (which convert T4 → active T3 or inactive rT3) and thyroidal glutathione peroxidase (which neutralises H2O2 generated by thyroid peroxidase during hormone synthesis) are selenoproteins. Selenium deficiency in iodine-deficient regions worsens hypothyroidism because without GPx, excess H2O2 destroys thyroid tissue, and without deiodinase, T4→T3 conversion is impaired. GPx1 in red blood cells protects against haemolysis; it is a selenoprotein but is not thyroid-specific. TrxR is also a selenoprotein involved in redox homeostasis. SelenoP transports selenium in plasma but is not the primary functional protein in the thyroid.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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