Introduction
Most of us know that Vitamin D is essential for bone health, that deficiency should be corrected, and that supplementation is often low-risk. But when you look more closely, the question becomes much less straightforward:
What actually defines vitamin D deficiency?
Over the past decade, two major groups—the Institute of Medicine (IOM) and the Endocrine Society—have offered different answers to that question. These differences have led to different recommendations for clinical practices, including:
- How to interpret lab values
- Who to test
- Who to treat
- And what targets to aim for
Many clinicians may not have even been aware of these differences (I wasn’t!). But it highlights that when it comes to Vitamin D there is uncertainty.
This series was inspired to explore and highlight the uncertainty that exists around Vitamin D testing and optimal thresholds.
What this series covers
This four-part series explores how the vitamin D controversy developed—and what it means for clinical practice today.
- Part 1: Where did the controversy start?
A breakdown of the 2011 IOM vs Endocrine Society guidelines - Part 2: What does the evidence actually show beyond bone health?
A closer look at vitamin D levels and outcomes (bone and beyond) - Part 3: What changed in 2024?
A major shift in Endocrine Society recommendations - Part 4: Science, bias, and conflicts of interest
How guideline development and industry relationships may shape practice
Why this matters
Vitamin D testing and supplementation is common practice.
But when:
- Thresholds are inconsistent
- Evidence is evolving
- And guidelines change over time
…it becomes easy for practice to drift away from the strongest available evidence.
This isn’t unique to vitamin D—but vitamin D is a particularly clear example of how:
- Well-intentioned recommendations
- Limited or evolving evidence
- And system-level influences
can shape clinical care in ways that are later reconsidered.
A note on approach
This series is not about “right vs wrong.”
Instead, it’s about:
- Understanding how different groups interpreted the same evidence
- Recognizing where uncertainty exists
- And applying that knowledge thoughtfully in clinical practice
Conclusion: So What Should We Do With All of This?
After working through this series, one thing becomes clear:
👉 The vitamin D story is less about a single “correct” number—and more about how we interpret and apply evidence.
What we know with reasonable confidence
- Vitamin D is essential for skeletal health
- Severe deficiency clearly leads to conditions like rickets and osteomalacia
- Levels around ~20 ng/mL (50 nmol/L) appear sufficient for most bone-related outcomes in the general population
Where uncertainty remains
- Whether higher levels provide meaningful additional benefit
- The role of vitamin D in non-skeletal outcomes
- Which patients truly benefit from testing and supplementation beyond general recommendations
What has changed
Perhaps the most important development is this:
👉 The Endocrine Society no longer endorses its previous definitions of vitamin D insufficiency and sufficiency.
That’s a significant shift—especially given how widely those thresholds were adopted in clinical practice and lab reporting.
Practical takeaways for clinicians
If there’s one message to carry forward, it’s this:
- Be thoughtful about testing
Ask whether the result will meaningfully change management - Know your lab’s reference range
Many still reflect older (2011) thresholds - Interpret results cautiously
Especially values labeled as “insufficient” based on outdated cut-offs - Stay curious about the evidence
Guidelines evolve—and so should practice
Final reflection
Vitamin D is a useful reminder that:
- Science is iterative, not static
- Guidelines are shaped by both evidence and interpretation
- And even widely accepted practices deserve periodic re-evaluation
As clinicians, our role isn’t just to follow guidelines—it’s to understand them.
One last question
Before you go:
👉 What vitamin D threshold does your lab use—and has it been updated recently?
Because sometimes, the most important insights come not from new evidence—but from looking more closely at what we’ve been using all along.
