Overview.
Persistent skin problems often have internal causes that topical treatments can't address. Hormonal acne is driven by elevated androgens (testosterone, DHEA-S). Dermatitis herpetiformis is cutaneous celiac disease detectable by tTG-IgA. Dry skin and hair loss accompany hypothyroidism. Butterfly rash signals lupus (ANA). Zinc and iron deficiency cause dermatitis and poor wound healing. A targeted lab workup identifies these systemic drivers.
Skin serves as a visible indicator of internal health. When skin problems are persistent, recurrent, or resist standard topical treatments, blood work can identify hormonal imbalances, autoimmune conditions, nutritional deficiencies, and metabolic disorders that manifest cutaneously.
Prevalence: Acne affects ~85% of people at some point (persistent adult acne in 12-22% of women). Eczema affects ~10% of adults. Psoriasis affects ~3%. Dermatitis herpetiformis (cutaneous celiac) is present in ~25% of celiac patients.
What to test.
First-line tests
- TSH — Hypothyroidism causes dry, coarse skin, brittle nails, and hair loss. Hyperthyroidism causes warm, moist skin and pretibial myxoedema. Both are common and treatable.
- CBC — Anaemia (pallor, poor wound healing), eosinophilia (allergic/parasitic conditions causing skin symptoms), and thrombocytopenia (petechiae/purpura).
- CMP — Liver dysfunction (jaundice, pruritus, spider angiomata), kidney dysfunction (uraemic pruritus), glucose (diabetic skin complications).
- Ferritin — Iron deficiency causes pale skin, angular cheilitis (cracked mouth corners), koilonychia (spoon-shaped nails), and poor wound healing.
Second-line tests
- Testosterone / DHEA-S (Women) — Hormonal acne in women is driven by androgen excess — elevated total testosterone, free testosterone, or DHEA-S. PCOS is the most common cause. Test if acne is jawline/chin pattern, persistent past teens, or with hirsutism.
- tTG-IgA (Celiac Screen) — Dermatitis herpetiformis — intensely itchy, grouped blisters on elbows, knees, buttocks, and scalp — is cutaneous celiac disease. tTG-IgA is positive in most cases. Can occur without GI symptoms.
- Vitamin D — Deficiency is associated with eczema severity, psoriasis flares, and impaired skin barrier function. Vitamin D receptors regulate skin cell differentiation and immune function.
- ANA (Antinuclear Antibody) — Positive in lupus (butterfly rash, photosensitivity, discoid lesions), systemic sclerosis (skin thickening), and dermatomyositis (heliotrope rash). Only order if clinical features suggest autoimmune disease.
Specialized tests
- Zinc — Zinc deficiency causes acrodermatitis (perioral and acral dermatitis), impaired wound healing, and hair loss. More common in vegetarians, alcoholics, and malabsorption conditions.
- Fasting Insulin / HOMA-IR — Insulin resistance drives acanthosis nigricans (dark, velvety skin in neck folds and armpits) and worsens hormonal acne through androgen stimulation. Screen if metabolic features present.
Common causes.
- PCOS / Androgen Excess — Adult female acne (jawline/chin), hirsutism, oily skin, androgenetic alopecia
- Hypothyroidism — Dry, rough, pale skin; brittle nails; hair loss (including outer eyebrows); non-pitting oedema
- Celiac Disease (Dermatitis Herpetiformis) — Intensely itchy, grouped papulovesicular rash on elbows, knees, buttocks, scalp
Diagnostic patterns.
- Adult female acne (jawline) + elevated testosterone + irregular periods — likely PCOS / androgen excess. Next step: Full PCOS workup (LH, FSH, AMH, fasting insulin); spironolactone or OCP for acne
- Butterfly rash + joint pain + positive ANA + cytopenia — likely Systemic lupus erythematosus. Next step: Anti-dsDNA, complement (C3/C4), urgent rheumatology referral
- Itchy blistering rash (elbows/knees) + positive tTG-IgA — likely Dermatitis herpetiformis (celiac disease). Next step: Skin biopsy (DIF), gastroenterology referral, gluten-free diet
- Dry skin + hair loss + elevated TSH + fatigue — likely Hypothyroidism. Next step: Full thyroid panel; levothyroxine — skin improves within 3-6 months of treatment
Lifestyle.
Non-medical causes
- Contact dermatitis (soaps, detergents, fragrances, metals)
- Stress-related skin conditions (eczema flares, urticaria, psoriasis triggers)
- Dietary triggers (dairy and high-glycaemic foods worsen acne in some individuals)
- Environmental factors (humidity, temperature, air pollution)
- Over-washing or harsh skincare products (disrupted skin barrier)
- Medication side effects (corticosteroids cause acne; anticonvulsants cause rashes)
Considerations
- Low-glycaemic diet: strongest dietary evidence for acne reduction
- Dairy reduction: some evidence for acne improvement (especially skim milk)
- Moisturisation with ceramide-containing products for eczema/dry skin
- Sun protection for autoimmune skin conditions (lupus, dermatomyositis)
- Stress management: stress directly worsens eczema, psoriasis, and acne through cortisol and neuroimmune pathways
- Adequate hydration and omega-3 fatty acids support skin barrier function
FAQs.
Should I get blood work for acne?
If acne is mild, localised to T-zone, and responds to topical treatment — probably not. But if acne is persistent adult-onset (especially jawline/chin in women), associated with hirsutism or irregular periods, or not responding to standard treatment, hormone testing (testosterone, DHEA-S, fasting insulin) is warranted to rule out PCOS or androgen excess.
Can celiac disease cause a skin rash without GI symptoms?
Yes. Dermatitis herpetiformis (DH) is 'celiac disease of the skin' and occurs in ~25% of celiac patients. Many have no diarrhoea, bloating, or abdominal symptoms at all. The rash (itchy blisters on elbows, knees, buttocks) may be the only manifestation. tTG-IgA screening catches it.
My skin is dry all over despite moisturising — what could it be?
Generalised dry skin that doesn't respond to moisturisers should prompt thyroid testing (TSH). Hypothyroidism is one of the most common systemic causes of refractory dry skin. Also consider: diabetes (glucose impairs skin hydration), zinc deficiency, essential fatty acid deficiency, and low humidity environment.