Vitamin D on GLP-1 therapy: the 25-hydroxy test, the thresholds, and why routine testing is no longer recommended

Vitamin D is the most ordered and least necessary test on the typical GLP-1 baseline panel. It is worth understanding why it got there and why the professional guidance has moved away from it, so you can decide with your prescriber whether it earns its place on yours.

What the test measures

The blood test is 25-hydroxyvitamin D, the storage form the liver makes from the vitamin D you get from sun, food and supplements. It is reported in ng/mL in the United States and nmol/L elsewhere (ng/mL times 2.5 gives nmol/L). MedlinePlus says many experts recommend a level of 20 to 40 ng/mL (60 to 100 nmol/L) while others recommend 30 to 50 ng/mL (75 to 125 nmol/L), which tells you the "normal" range is a matter of opinion more than for most tests. The NIH Office of Dietary Supplements sets its thresholds from bone health: below 12 ng/mL (30 nmol/L) is associated with deficiency (rickets in children, osteomalacia in adults); 20 ng/mL (50 nmol/L) or more is considered adequate for bone and overall health in most people; above 50 ng/mL (125 nmol/L) has been linked to potential adverse effects.

Note the gap. A result of 25 ng/mL is adequate by the NIH threshold and low by the stricter opinion. Two clinicians can read the same number differently and both be within guidance.

What the 2024 Endocrine Society guideline says

The Endocrine Society's 2024 clinical practice guideline on vitamin D for disease prevention suggests against routine 25-hydroxyvitamin D testing in healthy adults, and against screening for the purpose of deciding whether to supplement. The reasoning is that thresholds that predict benefit from supplementation have not been established in trials, so a number does not reliably change what should be done. The guideline does identify groups where empiric supplementation may help (for example, adults 75 and older, and people with prediabetes) but frames those as supplementation decisions, not testing decisions.

This is the current position of the main professional body on the subject. It is why a prescriber who leaves vitamin D off your baseline panel is following guidance, not missing something.

Why it still gets ordered

Habit. Vitamin D testing grew enormously in the 2000s and 2010s. Panels built in that era still include it.

Bone concerns. Rapid weight loss is associated with loss of bone density, and people with a history of fractures, osteoporosis or a family history may reasonably have vitamin D and calcium looked at. That is a specific reason, which is what the guideline allows for.

Malabsorption. Prior bariatric surgery, coeliac disease and inflammatory bowel disease all reduce absorption of fat-soluble vitamins. These are established indications for testing.

A previous low level being treated. If you are already on a treatment dose of vitamin D, the level is rechecked to see whether the dose worked.

What the GLP-1 labels say

Nothing. The Wegovy, Ozempic, Zepbound and Mounjaro labels do not mention vitamin D, do not list deficiency as an adverse reaction and do not ask for monitoring. Any connection between GLP-1 therapy and vitamin D runs through diet and body composition, not through the drug.

How often

• No specific reason: the guideline suggests not testing.
• A specific reason (bone disease, malabsorption, treated deficiency): at baseline, then the interval your prescriber sets, usually a few months after a change in supplement dose.
• Levels well above 50 ng/mL: worth discussing, since the NIH flags potential harm above that point and it usually means a high supplement dose.

What to discuss with your prescriber

• Whether you have a reason to test at all, or whether empiric supplementation without a test fits the guideline for your situation.
• If you supplement: how much, and whether the dose is in line with the NIH upper intake level.
• If bone health is a concern during rapid weight loss: whether a bone density scan is the more useful test.

For nutrition and training while losing weight, including calcium and protein intake, see Weight Loss on GLP-1s. The glossary covers the unit conversions used on this page.