SELENOS and hyperinsulinemic hypoglycemia, familial, 4: This pattern of abnormal thyroid function, together with low plasma selenium levels, reflecting decreased levels of the major circulating selenoproteins (SELENOP, GPX3), provides a biochemical signature whereby selenoprotein deficiency due to pathogenic variants in SECISBP2 or TRU‐TCA1‐1 can be identified.35, 39, 40