Vitamin D Deficiency: The "Sunshine Vitamin" Gap Hiding Behind Your Fatigue, Bone Pain, and Low Mood

You live in Texas — one of the sunniest states in the country. So there's no way you could be low on vitamin D, right? Not quite. Roughly 35% of American adults are vitamin D deficient, and an additional 40% fall into the "insufficient" range, according to data from the National Health and Nutrition Examination Survey (NHANES). That means the majority of adults in the United States may not have optimal vitamin D levels — even in states with year-round sunshine. Vitamin D deficiency is one of the most common nutritional deficiencies in the world, affecting an estimated one billion people globally, and it is a condition your primary care doctor can identify and treat, often through a single telehealth visit.

What Vitamin D Actually Does in Your Body

Vitamin D is not just a vitamin — it functions more like a hormone. Your body produces it when ultraviolet B (UVB) rays from sunlight hit your skin, triggering a chemical conversion in the liver and kidneys that creates its active form: 1,25-dihydroxyvitamin D. This active form plays a central role in calcium absorption and bone mineralization, which is why severe deficiency leads to softened bones (a condition called osteomalacia in adults and rickets in children).

But vitamin D's influence extends far beyond bones. Vitamin D receptors are found in nearly every tissue in the body, including the brain, immune cells, muscles, and the gut. It helps regulate immune function, supports muscle strength, influences mood through its effects on neurotransmitter pathways, and plays a role in cell growth and differentiation. When your levels are low, the consequences can show up in ways you might never connect to a single vitamin.

The Symptoms Most People Miss

One of the reasons vitamin D deficiency is so underdiagnosed is that its symptoms are nonspecific — they overlap with dozens of other conditions and are easy to dismiss as "just getting older" or "being stressed." Here are the most common signs your doctor would look for:

Fatigue and low energy

Persistent tiredness that doesn't improve with sleep is one of the most frequently reported symptoms. It's also one of the most commonly overlooked, since fatigue accompanies so many conditions.

Bone pain and muscle aches

Vitamin D deficiency can cause a deep, throbbing bone pain — particularly in the lower back, ribs, and legs — that is often mistaken for arthritis or fibromyalgia. Proximal muscle weakness (difficulty getting up from a chair or climbing stairs) is another hallmark finding.

Frequent infections

If you catch every cold that comes through your household, low vitamin D may be contributing. Vitamin D is critical for activating T-cells and other immune defenses. A meta-analysis published in the British Medical Journal found that vitamin D supplementation reduced the risk of acute respiratory infections, with the strongest benefit seen in those who were deficient at baseline.

Low mood or depression

Research increasingly supports a link between vitamin D deficiency and depressive symptoms. A 2024 systematic review and dose-response meta-analysis published in Psychological Medicine found that vitamin D supplementation significantly reduced depressive symptoms, particularly in individuals with baseline levels below 20 ng/mL and those with chronic inflammatory conditions. Vitamin D receptors and activating enzymes are widely distributed in brain regions involved in mood regulation, and deficiency appears to increase neuroinflammatory markers.

Slow wound healing

Vitamin D plays a role in the inflammatory response and in producing compounds essential for new skin formation. Slow recovery from cuts, surgical wounds, or infections may be a subtle sign of inadequacy.

Hair thinning

While not the most common presentation, vitamin D deficiency has been associated with telogen effluvium and alopecia areata, particularly in women.

Who Is Most at Risk?

Certain populations are significantly more likely to develop vitamin D deficiency, even in sunny climates like Texas:

How Vitamin D Deficiency Is Diagnosed

The definitive test is a serum 25-hydroxyvitamin D (25(OH)D) level — a simple blood draw. This is the major circulating form of vitamin D and reflects both dietary intake and skin production.

Historically, the medical community has used these reference ranges: levels below 12 ng/mL indicate deficiency, levels between 12 and 20 ng/mL indicate insufficiency, and levels above 20 ng/mL (some guidelines say 30 ng/mL) are considered sufficient.

However, it is worth noting that the landscape has shifted. The 2024 Endocrine Society Clinical Practice Guideline — a major update that replaced its 2011 guidance — no longer endorses specific numerical cutoffs defining "sufficiency," "insufficiency," or "deficiency" for healthy adults. The panel found that the evidence did not clearly support a single optimal target level for disease prevention. Instead, the guideline focuses on identifying populations that benefit most from empiric supplementation, regardless of a specific blood level.

The USPSTF has similarly concluded that there is insufficient evidence to recommend universal vitamin D screening in asymptomatic adults. This does not mean testing is never appropriate — it means your doctor should use clinical judgment. If you have symptoms, risk factors, or conditions that are commonly associated with low vitamin D (osteoporosis, recurrent fractures, malabsorption, chronic kidney disease), testing is clinically indicated and can guide treatment decisions.

What the 2024 Guidelines Actually Recommend

The 2024 Endocrine Society guideline represents the most current, evidence-based framework for vitamin D supplementation. Here is what it recommends for specific groups:

For all groups, the guideline recommends daily dosing rather than intermittent high-dose supplementation (such as monthly megadoses), particularly in adults over 50.

Treatment: What Your Doctor Will Typically Prescribe

If a blood test confirms deficiency (particularly levels below 20 ng/mL with symptoms), the standard treatment protocol involves two phases:

Repletion phase: Ergocalciferol (vitamin D2) at 50,000 IU once weekly for eight weeks, or an equivalent regimen using cholecalciferol (vitamin D3). Some physicians prefer D3 because it more effectively raises and maintains serum 25(OH)D levels.

Maintenance phase: Once levels normalize, a daily maintenance dose of cholecalciferol (vitamin D3) at 800 to 2,000 IU per day, depending on the individual's risk factors and response.

Your doctor will typically recheck your levels after 8 to 12 weeks of repletion therapy to confirm that you have responded and to adjust your long-term plan. For patients with malabsorption or obesity, higher doses may be necessary.

The Texas Paradox: Sunny State, Low Levels

It may seem counterintuitive, but living in a sun-drenched state does not protect you from vitamin D deficiency. A study of vitamin D levels in the Southern United States found that even in Miami, Florida — one of the most sun-exposed cities in the country — 38 to 40% of the population was deficient at the end of winter. In Texas, the combination of extreme summer heat (which keeps people indoors and in air conditioning from May through September), widespread sunscreen use, and the prevalence of indoor occupations creates a perfect storm for low vitamin D, even at our latitude.

This is why symptoms matter more than assumptions. If you are fatigued, achy, getting sick frequently, or struggling with your mood — and especially if you have any of the risk factors listed above — it is worth having a conversation with your doctor about checking your vitamin D level.

When to See Your Doctor

Vitamin D deficiency is one of the most straightforward conditions to diagnose and treat in primary care — and it is ideally suited for a telehealth visit. Your doctor can review your symptoms and risk factors, order the appropriate blood work at a lab near you, and develop a treatment plan based on your results, all through a secure video visit from your home.

At Trinity Family Medicine, we offer telehealth visits across the entire state of Texas* starting at $49.99 — no insurance required. If you have been dealing with persistent fatigue, unexplained aches, frequent colds, or low mood that just will not lift, a vitamin D level may be one of the first things worth checking.

Sometimes the answer to months of feeling "off" is simpler than you think — and it starts with a blood test and a conversation with your doctor.

Frequently Asked Questions About Vitamin D Deficiency

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