Can hair loss be a sign of illness? When to get checked

Most people who lose hair are otherwise healthy. Pattern hair loss, for example, is mainly genetic. But hair can also be an early signal that something in the body is out of balance. A 2024 review in American Family Physician noted that hair loss can be caused by several systemic diseases, and that a careful history, examination and targeted blood tests may reveal malnutrition, autoimmune disease or hormone (endocrine) disease ¹.

Why illness can affect hair

Hair follicles are among the most active cells in the body. When the body is under strain, it may push many hairs into the resting phase at once. A few months later, they shed together. This is called telogen effluvium. DermNet explains that the shedding usually starts about 2 to 4 months after the trigger ². Because of this delay, many people do not connect their hair fall with an illness that happened months before.

Health problems that can show up in your hair

Thyroid problems

An underactive thyroid (hypothyroidism) can cause dry skin, dry hair or hair loss, along with tiredness, feeling cold, weight gain, constipation, low mood and irregular or heavy periods. Symptoms usually develop slowly ³. DermNet lists both an underactive and an overactive thyroid among the causes of telogen effluvium ². See thyroid problems and hair loss.

Iron deficiency anaemia

The NHS lists hair loss, noticed as more hair coming out when brushing or washing, as a less common symptom of iron deficiency anaemia. Common symptoms include tiredness, shortness of breath and pale skin ⁴. Heavy periods, pregnancy and a low-iron diet are frequent reasons for low iron in women. See anaemia, iron and hair loss.

Hormone conditions in women

Polycystic ovary syndrome (PCOS), now also called polyendocrine metabolic ovarian syndrome (PMOS), is a hormone condition that can affect hair growth. Its main symptoms include irregular periods, excess hair on the face or body, or hair loss on the head, weight gain, acne and difficulty getting pregnant ⁵. See PCOS and hair loss.

Fever and infections

High fevers and severe infections are well-known triggers of shedding a few months later ². In Malaysia, dengue is a common example. See hair loss after dengue. COVID-19 and other serious infections can do the same.

Autoimmune conditions

In alopecia areata, the immune system attacks the hair follicles, causing patchy hair loss ⁶. Mild cases often settle on their own ¹. Other autoimmune diseases can also affect hair, which is one reason a doctor may order blood tests ¹. See alopecia areata.

Poor nutrition and rapid weight loss

Crash dieting, eating disorders and weight-loss surgery can all lead to shedding ². See hair loss after weight loss.

Scalp infections

Ringworm of the scalp, a fungal infection, can cause patchy hair loss ⁷. It is treated with antifungal tablets ¹. See scalp ringworm.

Medicines and medical treatments

Many medicines, and especially chemotherapy, can cause hair loss. See medicines that can cause hair loss and chemotherapy and hair loss.

Signs that your hair loss needs a check-up

See a doctor if you notice:

• Sudden or heavy shedding, especially if you can see clumps of hair
• Bald patches, or patches that are red, scaly, itchy or painful
• Hair loss along with other symptoms, such as tiredness, feeling cold, weight changes, a racing heart, irregular periods or new excess body hair
• Hair loss after a serious illness, surgery or major weight loss that has not settled after several months
• Hair loss in a child
• Changes to your nails or eyebrows as well as your scalp hair

What a doctor may check

The American Academy of Dermatology explains that a dermatologist will ask how long you have had hair loss and whether it came on quickly, look closely at your scalp, nails and other areas with hair loss, and may gently pull on your hair to check how it is growing. If a disease, vitamin deficiency, hormone imbalance or infection is suspected, you may need a blood test or scalp biopsy ⁶.

Common blood tests include a full blood count, iron stores (ferritin) and thyroid function. Your doctor will decide which tests are right for you.

Sometimes there is more than one cause. The AAD gives the example of a woman who is shedding obviously after having a baby, but also has early hereditary hair loss that is less obvious ⁶.

Will my hair grow back?

Often, yes. The AAD notes that hair may grow back on its own after having a baby, recovering from a major illness or surgery, finishing cancer treatment or losing a lot of weight ⁶. Treating the underlying cause, such as correcting low iron or balancing thyroid levels, gives hair the best chance to recover. DermNet notes that telogen effluvium usually settles over time, though recovery can be slow ².

Do not ignore the emotional side

Hair loss is not "just cosmetic". The 2024 review found that people with moderate to severe hair loss are more likely to have anxiety and depression, and lower quality of life ¹. If your hair loss is affecting your mood, mention it to your doctor.

The bottom line

Hair loss is usually not a sign of serious illness, but it can be an early clue to thyroid problems, low iron, hormone conditions, infections or autoimmune disease. If your hair loss is sudden, patchy or comes with other symptoms, see a doctor. Finding and treating the cause is the most effective step you can take. See sudden hair fall.

Frequently asked questions

Can hair loss be a sign of illness?

Yes. A 2024 review noted that hair loss can be caused by several systemic diseases, including nutritional, autoimmune and hormone problems ¹.

Can thyroid problems cause hair loss?

Yes. The NHS lists dry hair or hair loss among the symptoms of an underactive thyroid ³.

What tests are done for hair loss?

A doctor examines your scalp and hair and may order blood tests or a scalp biopsy if a disease, deficiency, hormone problem or infection is suspected ⁶.

Why does hair fall months after being sick?

Illness can push many hairs into the resting phase. Shedding usually starts about 2 to 4 months later ².

Educational content. Not medical advice.