Reasoning GP · Algorithm · 1-page summary

Vitamin D deficiency — low 25-OH vitamin D

For: adults and children in primary care with suspected or confirmed low 25-OH-D, and deciding who to test or supplement. Not for: acute low calcium (use the hypocalcaemia algorithm) or a raised calcium (hypercalcaemia algorithm); CKD-related bone disease is nephrology-led.

Reviewed October 2026 · next review April 2027

SACN Vitamin D 2016ROS vitamin D 2020NICE NG247NICE NG203SPS vitamin D safety

Core rule: check adjusted calcium before any high-dose vitamin D. Symptomatic low calcium is an emergency; a raised calcium means investigate (primary hyperparathyroidism, malignancy, granulomatous disease), never load.

0Stop — call 999 now — whatever the vitamin D level

Tetany, seizure (adult or child), laryngospasm or stridor, or arrhythmia with low calcium; or adjusted calcium below 1.90 mmol/L → 999 emergency admission for IV calcium gluconate
Hypercalcaemia on high-dose vitamin D with confusion or severe symptoms, or adjusted calcium >3.5 mmol/L → 999 emergency hospital assessment
While waiting: stop vitamin D and calcium supplements if calcium is high; check magnesium if calcium is low. Do not delay transfer to arrange vitamin D replacement.

1Triage by 25-OH-D — calcium normal

Level	Action
<25 nmol/L deficient	Consider loading (~300,000 IU total) only when rapid correction is clinically needed; then maintenance.
25–50 nmol/L	May be inadequate for some: 1,000–2,000 IU daily (or 20,000 IU fortnightly) + diet advice.
>50 nmol/L	Adequate for most. 50–75 is not a universal target.
Calcium raised, any level	Hold vitamin D; investigate first (block 2).

2Screen — serious causes

Bone pain + proximal myopathy (can't rise from chair, waddling gait), raised ALP → suspected osteomalacia: Ca, phosphate, ALP, PTH, U&E + specialist metabolic bone advice ≤2 weeks, not loading alone
Child: bowed legs, wrist widening, craniotabes, late fontanelle closure, fractures → suspected rickets: Urgent paediatrics
Hypercalcaemia on supplements (nausea, vomiting, thirst, polyuria, weakness, renal impairment) → stop vitamin D and calcium; Same-day assessment
Low 25-OH-D + high calcium (± weight loss, night sweats) → hold vitamin D; PTH, chest imaging, specialist advice per the hypercalcaemia algorithm

3Assess

Test 25-OH-D only if: suspected osteomalacia or rickets; bone pain or proximal myopathy; malabsorption or bariatric surgery; deficiency despite treatment; complex osteoporosis work-up; before IV zoledronate or denosumab per local protocol.

☐ Adjusted Ca ☐ Phosphate ☐ ALP ☐ PTH ☐ eGFR ☐ LFT ☐ Magnesium if Ca low

Do not test well people just for darker skin, being housebound, older, pregnant or on an enzyme-inducing antiepileptic — supplement.

4Classify

Simple deficiency: raised PTH, low-normal Ca → replace.
Osteomalacia: + raised ALP, low phosphate, pain, myopathy, Looser zones → specialist.
PTH-independent hypercalcaemia: suppressed PTH, high Ca → malignancy, granulomatous disease, excess intake.
Low Ca + low or normal PTH: check magnesium; normal Mg → endocrinology (hypoparathyroidism).
Refractory: malabsorption, enzyme inducers, obesity, adherence. CKD: colecalciferol; alfacalcidol/calcitriol only via nephrology.
Uncertain — normal PTH and Ca: low Mg, mild deficiency or lab error → repeat; supplement if <50.

5Treat today — doses: NICE/SPC, BNF, local formulary

Loading (only if needed): 50,000 IU weekly × 6 or 4,000 IU daily × 10 weeks; document total dose, end date and maintenance (800–2,000 IU daily). Adjusted Ca ~1 month after.
10 micrograms (400 IU) daily without testing: everyone over 1 in autumn and winter; all year if housebound, care home or darker skin; pregnancy and breastfeeding. Breastfed infants: 8.5–10 micrograms from birth.
Calcium only if diet <700 mg/day, low calcium, or an osteoporosis/osteomalacia plan. Pregnancy: confirmed deficiency → obstetric advice.
Before antiresorptives: denosumab — correct Ca and vitamin D first; IV zoledronate — renal function, Ca, vitamin D.

6Refer routinely

Gastroenterology: low despite adequate supplementation, suspected malabsorption or coeliac disease. Bariatric service after surgery.
Endocrinology: PTH persistently raised despite adequate 25-OH-D. Nephrology: CKD-MBD or persistent secondary hyperparathyroidism.
No routine repeat 25-OH-D; recheck ~3 months only if symptoms persist, malabsorption, osteomalacia or doubtful adherence.

7Never reassure by

A loading course for bone pain and weakness — osteomalacia needs specialist work-up.
A low 25-OH-D when calcium is high — think granulomatous disease, PTH, malignancy.
Calling proximal weakness deconditioning, PMR or fibromyalgia without Ca, ALP and 25-OH-D.
A 25-OH-D level for toxicity — toxicity is hypercalcaemia on high-dose treatment.
High-dose vitamin D with no documented end date or calcium recheck.
Vitamin D for falls, infections or COVID-19 — it treats deficiency and bone health only.

8Safety-net — say it, then document it

999 “Call 999 for a fit, severe muscle spasms, noisy or difficult breathing, or confusion.” Same day “Contact us today for tingling around the mouth or fingers or muscle cramps; on high-dose vitamin D, for sickness, vomiting, thirst or passing a lot of urine; or if your child develops bowed legs.” ≤2 weeks “Book a review if bone pain or muscle weakness persists or worsens.”

✓Safe to manage in primary care only if — all five, documented

No block 0–2 trigger; calcium not high Adjusted calcium checked before any loading Tested only for a listed reason Dose, end date, maintenance and 1-month Ca booked Safety-net given

Summary of the full Reasoning GP vitamin D deficiency algorithm, which holds causes, special populations, CKD, bariatric and antiepileptic detail and evidence. Sources: SACN Vitamin D and Health 2016; Royal Osteoporosis Society vitamin D guideline (2018, updated February 2020); NICE NG247, NG203, NG249; SPS vitamin D safety; local denosumab shared-care protocol. Clinical reference — follow local pathways; doses: NICE/SPC, BNF, local formulary.