The Short Version
Strong evidence shows vitamin D *testing* is widely overused — with studies finding 57–77% of tests lack clinical justification — but the claim conflates overtesting with overdiagnosis. Overtesting means ordering tests without guideline indications; overdiagnosis means incorrectly labeling healthy people as deficient. While contested diagnostic thresholds may inflate deficiency labels in some populations, global data also show substantial true deficiency prevalence with meaningful health associations. The unqualified assertion that deficiency is "widely overdiagnosed" overstates what the clinical literature supports.