Roughly 35% of American adults are vitamin D deficient — even in sunny Texas. A board-certified Texas physician explains the symptoms, who's at risk, what the 2024 Endocrine Society guidelines actually recommend, and how vitamin D deficiency is diagnosed and treated.
By Dr. Kathryn Kline, MD · Board-Certified Family Medicine Physician (ABFM) · Published 2026-05-04
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.
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 most common symptoms are persistent fatigue that isn't relieved by sleep, deep bone pain (especially in the lower back, ribs, and legs), proximal muscle weakness, frequent infections (especially respiratory), low mood or depressive symptoms, slow wound healing, and hair thinning. Because these symptoms overlap with many other conditions, vitamin D deficiency is one of the most underdiagnosed nutritional deficiencies in the United States.
Yes — and it's surprisingly common. Roughly 35% of American adults are deficient and another 40% are insufficient, even in sunny states. In Texas, extreme summer heat keeps many people indoors and in air conditioning for months, sunscreen use blocks UVB-driven vitamin D production, and indoor occupations limit sun exposure year-round. Studies have documented vitamin D deficiency in 38–40% of residents in cities as sunny as Miami.
The standard test is a serum 25-hydroxyvitamin D (25(OH)D) blood test — a simple, inexpensive blood draw that reflects both dietary intake and skin production. At Trinity Family Medicine, a Texas-licensed board-certified physician can order this lab during a telehealth visit starting at $49.99. You'll get a lab order to use at any major lab in Texas, and we review your results, start treatment if needed, and recheck your levels in 8–12 weeks.
For healthy adults under 75 with no risk factors, the 2024 Endocrine Society guideline endorses the IOM recommended daily allowance of 600 IU per day. Adults 75 and older, pregnant individuals, children and adolescents, and adults with high-risk prediabetes are now suggested to receive empiric supplementation at higher doses. If you are diagnosed with deficiency, the typical repletion protocol is 50,000 IU of ergocalciferol (D2) once weekly for 8 weeks, followed by a daily maintenance dose of 800–2,000 IU of cholecalciferol (D3).
No. Both the 2024 Endocrine Society guideline and the U.S. Preventive Services Task Force advise against routine screening of asymptomatic, low-risk adults. Testing is clinically appropriate when you have symptoms, risk factors (older age, dark skin, obesity, malabsorption, chronic kidney disease, osteoporosis, recurrent fractures), or conditions known to be associated with low vitamin D. Your doctor will decide whether testing is indicated for you based on your history and exam.
Both work, but cholecalciferol (vitamin D3) is generally preferred for maintenance because it more effectively raises and maintains serum 25(OH)D levels compared to ergocalciferol (vitamin D2). However, prescription D2 at 50,000 IU weekly is still widely used for repletion of confirmed deficiency, particularly because of its convenience and proven efficacy. Your physician will choose the form that's most appropriate for your situation.