Vitamin D Test
Measure your 25-OH vitamin D level with a simple home fingerstick kit. Results reviewed by a GMC-registered physician and returned in 3 to 5 working days.
£29.00
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Overview
What Is Brittle Nails?
Brittle nails — medically termed onychoschizia (horizontal splitting) or onychorrhexis (longitudinal ridging and fracturing) — are nails that chip, peel, break, or split with minimal provocation. While occasional nail breakage is normal, brittle nails that affect all fingers uniformly and persist despite good nail care are almost always signalling an internal deficiency or hormonal imbalance rather than a purely cosmetic problem.
The nail plate is composed almost entirely of a tough structural protein called keratin, whose synthesis depends on a continuous supply of micronutrients. Iron is needed for the proliferation of nail matrix cells that produce keratin; deficiency results in thin, concave, or brittle nails — a classic finding clinicians call koilonychia. Zinc and vitamin B12 are required for DNA replication in the rapidly dividing nail matrix; low levels slow nail growth and impair structural integrity. Thyroid hormones govern the metabolic rate of every cell in the body, including those of the nail matrix — hypothyroidism characteristically produces dry, slow-growing, brittle nails across all digits.
Vitamin D and calcium together regulate keratinocyte differentiation, and deficiency in either is associated with soft, easily fractured nails. Omega-3 fatty acids maintain the hydrophilic layer of the nail plate that prevents dehydration and brittleness. Because multiple nutrients act in concert, a targeted nutritional blood panel is the most efficient way to identify which deficiency — or combination of deficiencies — is the primary driver.
Brittle Nails vs. External Damage
Nails that split or peel on the dominant hand only, or worsen after frequent handwashing and chemical exposure, are likely externally damaged rather than nutritionally deficient. Brittle nails caused by internal deficiency tend to affect all nails uniformly and are accompanied by other systemic signs such as hair thinning, fatigue, or dry skin — blood testing helps make this distinction confidently.
Related experience
Common Symptoms Associated With Brittle Nails
Brittle nails caused by nutritional or hormonal imbalance are frequently accompanied by other visible or systemic symptoms that reinforce the underlying diagnosis.
Hair Loss
Iron deficiency, hypothyroidism, and zinc deficiency simultaneously affect the hair follicle and nail matrix.
Dry Skin
Thyroid deficiency and low omega-3 impair the skin and nail barrier in tandem.
Fatigue
Iron and B12 deficiency cause both brittle nails and profound fatigue through impaired red blood cell production.
Cold Intolerance
Low thyroid function slows circulation and metabolic heat production, contributing to both nail changes and sensitivity to cold.
Muscle Weakness
Calcium and vitamin D deficiency affect both nail structure and neuromuscular function.
The big picture
What Causes Brittle Nails?
Brittle nails result from deficiencies in the building blocks required for keratin synthesis, or from hormonal changes that slow nail matrix cell turnover.
What to measure
Biomarkers Associated With Brittle Nails
These key blood markers assess the nutritional and hormonal drivers behind brittle nails most efficiently.
Ferritin
The single best marker of iron stores — low ferritin is one of the most common correctable causes of brittle, spoon-shaped nails.
Vitamin D (25-OH)
Vitamin D deficiency impairs keratinocyte differentiation; low levels are associated with soft, easily fractured nail plates.
Calcium
Calcium works alongside vitamin D in nail matrix cell development — low levels compromise structural nail quality.
Zinc
Zinc is essential for DNA replication in the rapidly dividing nail matrix cells; deficiency causes white spots, ridging, and brittleness.
Vitamin B12
B12 supports cell division in the nail matrix; deficiency slows growth and produces darkened or brittle nails.
TSH
TSH screening distinguishes thyroid-driven nail changes (affecting all nails equally) from isolated nutritional deficiency.
Omega-3
Omega-3 fatty acids maintain the hydrophilic lipid layer of the nail plate, preventing dehydration-related brittleness.
Underlying causes
Conditions Associated With Brittle Nails
Several systemic conditions present with brittle nails as a consistent outward sign of an internal imbalance.
Iron Deficiency
Iron deficiency reduces ferritin available to the nail matrix, producing thin, concave, brittle nails — among the earliest physical signs of low iron.
Hypothyroidism
Underactive thyroid slows the metabolic rate of all body cells, including nail matrix cells, resulting in slow-growing, dry, and brittle nails.
Vitamin D Deficiency
Vitamin D is required for normal keratinocyte proliferation and differentiation; chronic deficiency affects nail structural quality.
Hashimoto's Thyroiditis
This autoimmune thyroid condition is the most common cause of hypothyroidism in the UK and produces the same nail changes as overt hypothyroidism.
Anaemia
Any form of anaemia reducing red blood cell delivery of nutrients to the nail matrix will impair nail growth and structural integrity.
Getting answers
How Brittle Nails Is Investigated
Investigating brittle nails systematically begins with the most common correctable causes — iron, thyroid, and key vitamins.
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Step 1 — Iron & Ferritin Assessment
Ferritin is the most sensitive marker for depleted iron stores — it can be low even when haemoglobin is still normal. A low ferritin alongside brittle nails is sufficient justification to begin iron repletion under medical supervision.
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Step 2 — Thyroid Function Screen
TSH and free T4 identify hypothyroidism, which produces uniformly brittle, slow-growing nails across all digits. This is among the most commonly missed causes of persistent nail changes in women over 35.
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Step 3 — Key Micronutrient Panel
Vitamin D, vitamin B12, zinc, and calcium should be assessed together, as these micronutrients act synergistically in nail matrix function. Deficiency in multiple nutrients simultaneously is common in restrictive or low-variety diets.
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Step 4 — Omega-3 & Nutritional Review
An omega-3 fatty acid index assesses dietary fat quality, which governs the hydration of the nail plate. A comprehensive nutritional panel ensures no co-existing deficiency is being overlooked.
Recommended testing
Recommended Blood Tests
Private blood tests analysed by UK-accredited laboratories.
Essential Vitamin and Mineral Panel
Check your levels of vitamin D, B12, folate, ferritin, and zinc in one convenient test. Home fingerstick kit available. GMC physician review included.
£79.00
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Zinc and Copper Panel
Measure your zinc and copper levels together to assess balance between these two competing trace minerals. Home fingerstick kit available.
£49.00
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Magnesium Test
Check your serum magnesium level to investigate muscle cramps, fatigue, palpitations, or poor sleep. Simple home fingerstick kit. GMC physician review.
£29.00
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Comprehensive Nutritional Panel
An in-depth 12-marker nutritional screen covering fat-soluble vitamins, B vitamins, key minerals, homocysteine, and omega-3 index.
£149.00
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Everyday contributors
Lifestyle Factors That Can Contribute
Targeted nutrition and protective habits can improve nail strength once underlying deficiencies have been identified and addressed.
Prioritise Dietary Iron & Protein
Red meat, legumes, and leafy greens provide the iron and amino acids needed for keratin synthesis — vitamin C consumed alongside non-haem iron significantly improves absorption.
Increase Omega-3 Intake
Oily fish (salmon, mackerel, sardines) two to three times per week provides the fatty acids needed to maintain the nail plate's hydrophilic barrier.
Stay Well Hydrated
Chronic dehydration is an underappreciated driver of nail brittleness — adequate fluid intake maintains the water content of the nail plate.
Protect from Chemicals & Water
Wearing gloves for washing up and cleaning reduces external damage that can compound nutritional brittleness.
Address Vitamin D
In the UK, most adults require vitamin D supplementation from October to March at minimum — optimising levels supports nail matrix cell function.
Eat Zinc-Rich Foods
Pumpkin seeds, shellfish, and meat are excellent zinc sources — zinc deficiency is common in vegetarian and vegan diets and is easily correctable.
Safety first
When To Seek Medical Advice
Brittle nails are usually benign and nutritional, but certain accompanying changes to nail appearance warrant prompt medical review.
Red flags — speak to a doctor
These can point to a more serious underlying cause and should not be ignored.
- Nail changes accompanied by a skin rash, particularly scaly or silvery patches — may indicate psoriasis or an autoimmune skin condition.
- Nail pitting combined with joint pain and swelling — warrants assessment for psoriatic arthritis.
- Yellow, green, or significantly thickened nails — may indicate fungal infection or a respiratory condition and require medical assessment.
- Painful nail separation from the nail bed (onycholysis) — needs clinical examination to rule out infection, psoriasis, or thyroid disease.
Common questions
Frequently Asked Questions
The most common deficiencies linked to brittle nails are iron (ferritin), vitamin D, zinc, and vitamin B12. Each plays a distinct role in nail matrix cell division and keratin production. Because multiple deficiencies often co-exist, a comprehensive nutritional blood panel is more informative than testing a single marker in isolation.
Yes — hypothyroidism, including Hashimoto’s thyroiditis, is one of the most consistent causes of brittle, dry, slow-growing nails. Thyroid hormones govern the metabolic rate of nail matrix cells; when thyroid output is low, nail growth slows and the plate becomes structurally weaker. A simple TSH blood test is the standard screening test for thyroid dysfunction.
Yes. Iron deficiency is among the most common correctable causes of brittle nails, and in its more advanced form produces koilonychia — spoon-shaped nails with a concave surface. Importantly, ferritin (iron stores) can be depleted and causing nail changes even when haemoglobin is still within the normal range, which is why ferritin is a more sensitive marker than a standard blood count alone.
Emerging evidence suggests that vitamin D plays a role in keratinocyte differentiation — the process by which nail matrix cells produce the structural proteins of the nail plate. Vitamin D deficiency is extremely common in the UK, where sunlight exposure is insufficient for endogenous production for approximately six months of the year. Correcting deficiency with supplementation is safe, inexpensive, and may benefit both nail and overall musculoskeletal health.
Fingernails grow approximately 3–4 mm per month, meaning a full nail cycle takes around three to six months. Visible improvement in nail quality following correction of a nutritional deficiency — such as iron or zinc — typically becomes apparent after six to eight weeks of effective treatment, with the full benefit seen at three to four months. This timeline makes it important to retest biomarkers at the three-month mark to confirm levels have normalised.
Keep exploring
Related Symptoms
Related Biomarkers
Related Conditions
Sources
References
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British Journal of Dermatology — View source
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NICE Clinical Knowledge Summaries — View source
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British Nutrition Foundation — View source
This page is for general information only and does not replace personalised medical advice. If you are worried about your health, please speak to a qualified healthcare professional. Trupoint Health blood tests are analysed by UK-accredited laboratories.