Vitamin D Deficiency — Causes, Symptoms, Diagnosis & Treatment Guide — Symptoms, Causes & Treatment | MyMedicPlus

Vitamin D is a fat-soluble secosteroid hormone fundamental to calcium and phosphate homeostasis, bone mineralisation, immune regulation, and neuromuscular function. The principal form produced by humans is vitamin D3 (cholecalciferol — synthesised in the skin from 7-dehydrocholesterol under UVB radiation) and, to a lesser extent, vitamin D2 (ergocalciferol — from dietary plant sources). Both are converted to 25-hydroxyvitamin D (25-OH-D — calcidiol) in the liver, then to the active hormone 1,25-dihydroxyvitamin D (calcitriol) primarily in the kidneys. Serum 25-OH-D is the standard measure of vitamin D status. Deficiency is defined as 25-OH-D below 25 nmol/L (10 ng/mL); insufficiency as 25-75 nmol/L; sufficiency above 75 nmol/L (UK values — optimal thresholds are debated internationally). Deficiency causes rickets (children), osteomalacia (adults — painful bone disease with inadequate mineralisation), and secondary hyperparathyroidism driving bone resorption. Vitamin D deficiency is extremely common globally, affecting over 1 billion people.

Inadequate sunlight exposure (the main source — 80-90% of vitamin D comes from skin synthesis): indoor lifestyle (office workers, housebound elderly), sunscreen use (SPF 15 blocks approximately 99% of UVB), cultural practice of covering skin, living at high latitudes above 51 degrees North (UK — from October to March, UVB intensity is insufficient for skin synthesis), and winter months. Dark skin pigmentation: melanin reduces UVB skin penetration — individuals with darker skin require 3-5 times longer sun exposure to produce equivalent vitamin D; highest risk in South Asian and Afro-Caribbean communities in Northern Europe. Inadequate dietary intake: few foods are naturally rich in vitamin D (oily fish — salmon, herring, mackerel; egg yolks; UK margarine is mandatorily fortified; some cereals and plant milks are voluntarily fortified). Malabsorption: Crohn's disease, coeliac disease, short bowel syndrome, gastric bypass surgery — impaired absorption of fat-soluble vitamins including D. Obesity: vitamin D is sequestered in adipose tissue, reducing bioavailability. Liver disease: impaired 25-hydroxylation. Chronic kidney disease (CKD): impaired 1-alpha-hydroxylation — cannot convert 25-OH-D to active calcitriol. Medications: anticonvulsants (phenytoin, carbamazepine), rifampicin — increase vitamin D catabolism. Exclusively breastfed infants: breast milk is low in vitamin D — supplementation is essential from birth.

Mild-moderate deficiency: often asymptomatic or non-specific — fatigue and low energy, generalised musculoskeletal aches and pains, bone pain (particularly back, hips, and legs), muscle weakness (proximal myopathy — difficulty rising from a chair, climbing stairs, waddling gait), and low mood or depression. Severe deficiency and osteomalacia: bone pain (dull, worse at night and with pressure), bone tenderness on palpation (sternum, tibia, fibula), proximal muscle weakness, pseudofractures (Looser's zones — stress fractures on X-ray at sites of muscle insertion), and rarely hypocalcaemic tetany (perioral tingling, carpopedal spasm, positive Chvostek's and Trousseau's signs). Rickets (severe deficiency in children): bowing of weight-bearing legs, genu valgum (knock knees), rachitic rosary (costochondral junction beading), craniotabes (soft skull bones in neonates), Harrison's sulcus (horizontal groove in ribs from diaphragm traction on softened ribs), delayed dentition, and growth retardation. Secondary hyperparathyroidism: elevated PTH drives bone resorption to maintain serum calcium — long-term bone loss without treatment.

Serum 25-hydroxyvitamin D (25-OH-D): the standard test for vitamin D status — measured in nmol/L or ng/mL. UK reference ranges: severely deficient below 25 nmol/L (associated with rickets/osteomalacia), deficient 25-50 nmol/L (secondary hyperparathyroidism and impaired bone mineralisation), insufficient 50-75 nmol/L, sufficient above 75 nmol/L; toxicity risk generally above 250 nmol/L. Note: 1,25-dihydroxyvitamin D (calcitriol — the active form) is NOT the correct test for vitamin D status — calcitriol may be normal or elevated in deficiency states due to PTH stimulation. Serum calcium and phosphate (may be low in severe deficiency), alkaline phosphatase (elevated in osteomalacia — osteoblast activity), and parathyroid hormone (PTH — elevated in secondary hyperparathyroidism from deficiency). X-ray (for suspected rickets or osteomalacia): cupped and frayed metaphyses in rickets; Looser's zones (bilateral, symmetrical pseudofractures — pathognomonic of osteomalacia) in adults. DEXA scan (bone mineral density): assesses osteoporosis risk in longstanding deficiency. NICE does not recommend routine population vitamin D testing — targeted testing appropriate for high-risk individuals and those with symptoms.

Oral vitamin D3 supplementation (cholecalciferol — preferred over vitamin D2, which is less potent at raising and sustaining 25-OH-D levels): Loading dose for deficiency (25-OH-D below 25-30 nmol/L): total 300,000-600,000 IU given over 6-12 weeks (e.g., 50,000 IU weekly for 6-8 weeks or 20,000 IU twice weekly for 7 weeks); then maintenance 800-2,000 IU daily. Maintenance supplementation for insufficiency or prevention: 400-800 IU (10-20 mcg) daily — UK SACN recommends 400 IU (10 mcg) daily year-round for all adults and children over 1. At-risk groups requiring daily supplementation without testing: all infants from birth to age 1 (400 IU daily — NHS Healthy Start vitamins); children aged 1-4 (400 IU); all adults at risk (housebound, dark skin, pregnancy/breastfeeding, malabsorption). Calcium supplementation (1,000-1,200 mg daily) alongside vitamin D for osteomalacia and for elderly with low dietary calcium. Active vitamin D analogues (alfacalcidol, calcitriol): specifically for renal osteodystrophy (CKD — cannot 1-alpha-hydroxylate) and hypoparathyroidism — bypass renal regulation, require careful monitoring for hypercalcaemia. Safe sun exposure: 15-30 minutes of midday sun (April-September in UK) to arms and legs without sunscreen — effective for skin synthesis; avoid burning. Vitamin D toxicity requires sustained intake above 10,000 IU daily for months — symptoms: hypercalcaemia (thirst, polyuria, constipation, confusion, kidney stones).

Vitamin D deficiency causes a spectrum of complications ranging from skeletal disease to systemic immunological and metabolic consequences. Skeletal complications: in children, severe deficiency causes rickets — softening and deformation of bones including bowed legs, rachitic rosary of the ribs, craniotabes, soft skull bones, growth retardation, and delayed tooth eruption; in adults, osteomalacia causes diffuse bone pain, proximal muscle weakness, waddling gait, and stress fractures (particularly pelvis, femoral neck, and vertebrae) — Looser zones (pseudofractures) visible on X-ray are pathognomonic. Osteoporosis and fractures: chronic insufficiency (25-OH-D below 50 nmol/L) impairs intestinal calcium absorption, elevating PTH (secondary hyperparathyroidism), which drives bone resorption, accelerates bone loss, and increases hip fracture risk — particularly in elderly women. Falls and proximal myopathy: vitamin D deficiency impairs skeletal muscle function, causing proximal weakness — difficulty rising from chairs or climbing stairs — and increasing fall risk in the elderly independently of bone density effects. Immune consequences: vitamin D deficiency is associated with increased susceptibility to respiratory tract infections (influenza, tuberculosis, COVID-19 — vitamin D receptor activation upregulates cathelicidin antimicrobial peptides) and increased risk of autoimmune conditions (multiple sclerosis, type 1 diabetes, inflammatory bowel disease, SLE). Metabolic: deficiency is associated with insulin resistance and increased risk of type 2 diabetes. Cardiovascular: observational data link low vitamin D levels with hypertension and increased cardiovascular mortality, though causality remains under investigation.

UK SACN 2016 recommendation: all people aged 1 year and above in the UK should consider taking 10 mcg (400 IU) of vitamin D daily throughout the year, particularly during autumn and winter — dietary sources and limited UK sunlight are insufficient to maintain adequate status year-round for many people. Groups recommended to supplement year-round: people with limited sun exposure (housebound, institutionalised), those with dark skin pigmentation, pregnant and breastfeeding women, all infants and children under 5 (NHS Healthy Start scheme provides free vitamins). Safe sun exposure strategy: short periods of midday sun (11 am to 3 pm) to large skin areas from April to September — 15-20 minutes daily for lighter skin tones is sufficient for adequate synthesis; longer for darker skin tones; avoid sunburn at all times. Dietary sources: encourage oily fish (salmon — 500-600 IU per 100g), eggs, fortified foods (margarine, some cereals and plant milks). Be aware that UK sunlight (October to March) is too low in UVB for any skin synthesis regardless of time spent outdoors.

Consult your GP for persistent bone pain, muscle weakness, fatigue, or low mood — particularly if you have known risk factors for vitamin D deficiency (dark skin, minimal sun exposure, malabsorption, housebound status). Vitamin D testing is appropriate for: suspected osteomalacia or rickets (bone pain, fractures, characteristic X-ray changes), chronic kidney disease, liver disease, malabsorption conditions (coeliac disease, Crohn's, post-bariatric surgery), medications that accelerate vitamin D catabolism (anticonvulsants, rifampicin), and recurrent falls in older adults. Seek urgent assessment for symptoms of severe hypocalcaemia (muscle cramps, facial tingling, carpopedal spasm, seizures) — treated with IV calcium gluconate and vitamin D. Children with bowed legs, growth retardation, or rachitic changes require urgent paediatric assessment and immediate supplementation.