Vitamin D and the Functional Panel: Deficiency, Sufficiency, and Dosing

The standard vitamin D blood test measures 25-hydroxyvitamin D (25-OH D) — the main circulating form and the best indicator of overall vitamin D status. It reflects both sun-derived synthesis and dietary/supplemental intake. The test does not measure the active form (1,25-dihydroxyvitamin D), which is tightly regulated and not a useful marker of status.

25-OH D is measured in ng/mL (or nmol/L internationally). It is stable, unaffected by recent meals, and does not require fasting — making it an easy marker to include in any panel.

Most labs define deficiency as below 20 ng/mL and insufficiency as 20–30 ng/mL, with sufficiency generally considered 30 ng/mL and above. However, functional and preventive practitioners often target higher levels — typically 40–60 ng/mL — based on evidence that parathyroid hormone suppression plateaus and immune function optimizes in this range.

Severe deficiency (below 12 ng/mL) can produce overt symptoms — bone pain, muscle weakness, fatigue — but milder deficiency is often asymptomatic, contributing to subtle immune and metabolic dysfunction over time. This is why testing, rather than guessing, is the right approach.

Vitamin D synthesis depends on UV-B exposure, which is minimal at northern latitudes during winter months. Metro Detroit, at roughly 42°N, sees essentially no cutaneous vitamin D synthesis from roughly November through March. During these months, intake and stored status are the only sources — and stored status depletes over time without replenishment.

This geographic reality makes vitamin D testing particularly relevant for Metro Detroit patients. A level checked in late winter reveals the nadir of seasonal status, while a summer check reveals the peak. Serial tracking across seasons provides a complete picture.

Repletion depends on the starting level. For frank deficiency, clinicians often prescribe higher-dose short-term repletion (e.g., 50,000 IU weekly for 8–12 weeks) followed by a maintenance dose. For insufficiency, a daily maintenance dose of 1,000–2,000 IU is common, with some individuals requiring more based on absorption, body weight, and genetics.

Vitamin D3 (cholecalciferol) is generally preferred over D2 (ergocalciferol) for supplementation, as it raises 25-OH D more effectively. Because vitamin D is fat-soluble, taking it with a meal containing fat improves absorption.

Vitamin D responds measurably to supplementation, but the response varies widely between individuals. Two people taking the same dose can reach very different 25-OH D levels based on absorption, body weight (vitamin D is sequestered in fat), and genetic factors. This is why serial testing — not formula-based dosing — is the right way to confirm sufficiency.

A common approach is to test, supplement for 2–3 months, and re-test to confirm the level has reached the target range. Once stable, a lower maintenance dose and annual or seasonal monitoring are sufficient. This is a textbook use case for at-home serial testing.

Given the regional winter deficiency risk, vitamin D testing is one of the highest-value preventive interventions for Metro Detroit patients. Monarch Lifestyle Medicine (www.monarchlifestylemed.com) offers lifestyle-medicine-informed evaluation that can incorporate vitamin D status into a broader preventive plan.

The broader Detroit wellness network — including HOK Detroit (www.hokdetroit.com) and Detroit Face MD (www.detroitfacemd.com) — reflects a regional infrastructure for coordinated preventive care where seasonal vitamin D trend data can inform more targeted clinical guidance.