Hidden Causes of Hair Loss: Thyroid, Iron, Medications and More
The medical and lifestyle factors that can cause or worsen thinning, and the questions worth asking your doctor.
FOLIGAIN Editorial Team · 13 min read · Updated 2026-09-28
Why look beyond genetics
Pattern hair loss is the most common cause of thinning, but it is not the only one. Other factors can cause hair loss on their own or make existing pattern hair loss worse. Some are simple to correct. Before investing months in a routine, it is worth ruling out the most common medical and lifestyle contributors, particularly if the thinning is diffuse, started suddenly or comes with other symptoms.
Thyroid disorders
The thyroid gland regulates metabolism, and both hypothyroidism (under-active) and hyperthyroidism (over-active) can cause diffuse hair thinning. Hair may also become dry, brittle or change texture. Other signs of hypothyroidism include fatigue, weight gain, feeling cold and constipation. Hyperthyroidism can cause weight loss, palpitations, heat intolerance and anxiety.
Thyroid function is assessed with a simple blood test. When thyroid levels are brought back into range with treatment, hair often improves over several months.
Iron deficiency and ferritin
Iron deficiency, with or without anemia, is one of the most frequently discussed contributors to hair shedding, especially in women who menstruate, people with heavy periods, vegetarians and vegans, frequent blood donors and people with some digestive conditions. Ferritin, a measure of stored iron, is commonly checked.
Iron supplements should only be taken if a deficiency is confirmed, and under medical guidance, because excess iron can be harmful.
Other nutritional factors
The message from dermatology is consistent: correct genuine deficiencies, eat a balanced diet, and be careful with high-dose supplements marketed for hair unless a doctor recommends them.
| Nutrient | Why it matters | Notes |
|---|---|---|
| Protein | Hair is made mainly of keratin, a protein | Low intake and crash diets can trigger shedding |
| Vitamin D | Involved in follicle cycling | Low levels are common; test before high-dose supplementing |
| Zinc | Supports tissue growth and repair | Deficiency is uncommon; excess can cause problems |
| Biotin | B vitamin involved in keratin production | True deficiency is rare; high doses can interfere with lab tests |
| Vitamin A | Needed in small amounts | Too much vitamin A can itself cause hair loss |
| Overall calories | Hair growth is metabolically demanding | Rapid weight loss is a classic trigger |
Medications
Many medications list hair loss as a possible side effect, usually through telogen effluvium. Examples include some anticoagulants, retinoids, certain beta-blockers, some antidepressants and mood stabilisers, some anticonvulsants, some cholesterol-lowering drugs and hormonal treatments. Chemotherapy causes a different, rapid type of hair loss (anagen effluvium).
Never stop a prescribed medication on your own because of hair concerns. Talk to the prescriber: sometimes an alternative is available, and sometimes the shedding settles over time.
Hormonal changes
Pregnancy, the postpartum period, perimenopause, menopause and stopping or switching hormonal contraception can all affect hair. Conditions such as PCOS are associated with increased androgen activity and can contribute to pattern thinning. In men, anabolic steroids and testosterone therapy can accelerate pattern hair loss in genetically susceptible individuals.
Scalp conditions
Significant seborrheic dermatitis, psoriasis, fungal infections (tinea capitis) and contact dermatitis can cause flaking, redness, itching and sometimes hair loss. Treating the scalp condition often improves hair comfort and appearance, and a calm scalp makes any topical routine easier to tolerate.
Styling and hair care
Tight styles cause traction alopecia. Bleaching, relaxing, perming and frequent high heat damage the hair shaft and cause breakage. None of these usually affect the follicle unless the damage is severe or chronic, but they can make hair look significantly thinner.
Questions to ask your doctor
- Could this be pattern hair loss, telogen effluvium, or both?
- Should my thyroid, ferritin, vitamin D or hormones be checked?
- Could any of my medications be contributing?
- Are there signs of a scalp condition that needs treatment?
- Is topical Minoxidil appropriate for me, and which strength and format?
- When should we review progress?
Frequently asked questions
Can low iron cause hair loss?
Iron deficiency is associated with hair shedding, particularly in women. A doctor can check ferritin and advise on supplementation if needed.
Do biotin supplements help hair growth?
Biotin helps if there is a true deficiency, which is rare. High doses can interfere with some laboratory tests, so tell your doctor if you take it.
Can thyroid problems cause hair loss?
Yes. Both under- and over-active thyroid can cause diffuse thinning that often improves once thyroid levels are treated.
This content is for educational purposes only and is not medical advice. It is not intended to diagnose, treat, cure or prevent any disease. Speak with a qualified healthcare professional about your individual situation.
Individual results vary. Hair-growth products are used consistently over months, and outcomes depend on the cause and stage of hair loss.
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