Why 5% of adults have thyroid dysfunction yet it remains one of medicine's most commonly mismanaged conditions. Peter Attia explains thyroid biology, blood test limitations, why symptoms alone cannot diagnose hypothyroidism, and how to navigate between underdiagnosis and overtreatment—including why some patients feel unwell despite normal test results. The thyroid's unique complexity stems from its inactive T4 hormone converting locally to active T3 at tissue level, creating visibility gaps that symptom-based diagnosis cannot bridge.