Genetics, hormones, thyroid issues, nutritional deficiencies, and cancer treatment can all contribute to hair loss.

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We all lose about 100 hairs a day without noticing any obvious hair loss. If you have been noticing more “loose” hair than usual in your hairbrush or on your pillow when you wake in the morning, or your hair feels thinner when you wash and style it, genetics could be to blame. But there are also other reasons why hair loss occurs, including hormonal imbalances, illness, medications, and nutritional deficiencies.

Hair Loss from Male Pattern Baldness

Male pattern baldness is the main cause of hair loss in men—in fact, two out of three men notice that their hairline is receding at the temples by about age 60. This type of hair loss (officially known as androgenetic alopecia) is gradual, and even though it is generally referred to as male pattern baldness it can affect older women too, though the pattern of hair loss tends to be different in women (rather than hair loss at the temples, women tend to experience thinning of the hair all over the head).

It is unclear exactly what causes male pattern baldness, but hormones—specifically one called dihydrotestosterone—are thought to be the culprit. A gradual loss of hair results because male pattern baldness affects the size of hair follicles, effectively miniaturizing them. This means that the hair that grows from these follicles becomes progressively shorter and wispier, until it stops growing. If you have a close relative who has suffered from male pattern baldness you are more likely to develop this type of hair loss yourself.

A recent study found an association between male-pattern baldness and the risk for heart disease, particularly in younger men with this type of hair loss. In the study, presented at the 69th Annual Conference of the Cardiological Society of India, researchers found that young men with coronary artery disease had a higher prevalence of male-pattern baldness (49 percent versus 27 percent) compared to healthy controls. After the data was adjusted for age and other cardiovascular risk factors, male-pattern baldness was associated with a 5.6 times greater risk of coronary artery disease. The researchers suggested that men suffering from male pattern baldness should be monitored for coronary artery disease and advised on heart-healthy lifestyle changes such as healthy diet, exercise, and stress management.

Hormonal Hair Loss

Other types of hair loss have also been linked to hormones. For example, women often notice their hair becomes thinner after menopause, while new mothers typically lose excessive amounts of hair from about three months after they had their baby. The hormones estrogen and progesterone are likely the cause in both cases.

During menopause, levels of these hormones drop. Not only does this slow the growing cycle of your hair—it also tips your hormonal scale in favor of male hormones (called androgens) that are naturally present in your body. Much as they do in men with male pattern baldness, androgens shrink hair follicles, resulting in thinner hair. As if that isn’t bad enough, they also can trigger hair growth on your face. Women who suffer from a condition called polycystic ovary syndrome (PCOS) also often report hair loss along with hair growth on the face, and this is also due to an excess of male hormones.

In pregnant women, levels of estrogen and progesterone are unusually high—so high that you naturally lose much less hair than usual. Most moms-to-be will notice that their hair is particularly thick and lustrous while they are pregnant. Unfortunately, hormone levels fall to their normal level about three months postpartum. That’s when new moms start to notice excessive hair loss, though strictly speaking it isn’t hair loss: it’s your body “catching up” by shedding the hair you would otherwise have shed if you hadn’t been pregnant. So don’t panic if you’re a new mom noticing what appears to be hair loss—your hair’s natural loss-growth cycle will reestablish itself.

Health-Related Hair Loss

If you have an underactive thyroid (also called hypothyroidism) you may suffer from hair loss.

The thyroid is a small gland situated in your neck just below your larynx. Its function is to absorb iodine, an element that occurs mainly in salt, bread and fish, and convert it into thyroid hormones: thyroxine (T4) and triiodothyronine (T3). These hormones control your metabolism, influencing everything from the speed at which you digest fats and carbohydrates, to the rate at which your heart beats. If your T4 levels are too low, you are at risk of developing an underactive thyroid. Low T4 levels decrease your metabolic rate, causing body processes to slow down—common symptoms include fatigue, weight gain, sluggish bowel movements, increased sensitivity to cold—and hair loss.

You’re at increased risk for underactive thyroid if you have a family history of thyroid disease, have been treated for an overactive thyroid, and if you have had extensive surgery and/or radiotherapy to your neck. Some medications also can make you vulnerable—for example, the cancer drugs sunitinib (Sutent®) and imatinib (Gleevec®) may cause or worsen an underactive thyroid, as can amiodarone (Corderone®, Pacerone®), which is used to treat heart arrhythmias. If any of these factors apply to you and you’ve noticed excessive hair loss, mention it to your doctor.

Other health conditions that can result in hair loss include scalp infections like ringworm, fungal infections of the scalp, and autoimmune conditions such as lupus and type 1 diabetes.

Hair Loss from Medications and Medical Treatment

Medications that have been linked to hair loss include blood thinners, antihypertensive drugs (which treat high blood pressure), heart medications, antidepressants, and hormonal contraceptives.

Chemotherapy drugs used to treat cancer often cause hair loss, as can radiation therapy. Cancer cells divide quickly, and chemotherapy drugs are designed to prevent this division and/or kill cancer cells. Unfortunately, chemotherapy also destroys healthy cells that grow and divide rapidly—including cells found in hair follicles. There isn’t yet a medication to prevent this hair loss, but Food and Drug Administration recently cleared the expanded use of a cooling cap—the DigniCap Cooling System—to reduce it. In trials, 66 percent of women with breast cancer who were undergoing chemotherapy regimens associated with hair loss reported losing less than half their hair while using the cap.

The computer-controlled cap works by circulating liquid that cools the scalp during chemotherapy treatment. The cooling is intended to constrict blood vessels in the scalp, which reduces the amount of chemotherapy that reaches cells in the hair follicles. The cold temperature also decreases the activity of the hair follicles and slows down cell division, making them less likely to be affected by chemotherapy. The combined actions are thought to reduce the effect chemotherapy has on the cells, which may reduce hair loss.

The cap isn’t suitable for all chemotherapy regimens, and does cause side effects, including cold-induced headaches, neck and shoulder discomfort, chills and pain. Wearing it is not likely to cause chemotherapy drugs to miss some cancer cells in the scalp, according to the FDA—however, the long-term effects of scalp-cooling and risk of cancer spread have not been fully studied.

Nutrition and Hair Loss

A poor diet can cause hair loss, particularly if it is low in protein. Be sure you are getting enough by consuming sufficient dairy products, fish, meat, and eggs (if you are a vegetarian or vegan, pack your diet with soy protein, nuts, quinoa, leafy green vegetables, and beans).

Low iron levels can cause anemia, which is implicated in hair loss but can easily be reversed by taking an iron supplement. Other anemia symptoms include fatigue, headaches, dizziness and a pale complexion—if you have these anemia signs, ask your doctor to check your iron levels.

Another nutrition-related cause of hair loss is too much vitamin A, so if you take a daily multivitamin make sure you aren’t overdosing on A. The recommended daily amount differs depending on your age and other factors, so check with your doctor how much you should be consuming.