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Male pattern baldness usually begins at the front of your scalp, called your frontal hairline. The hairline gradually moves backward (recedes) and forms an "M" shape in most people with male pattern baldness, with the temples receding back more than the middle section of the scalp. Eventually the hair becomes thinner and shorter, and creates a horseshoe pattern around the sides of the head. The horseshoe pattern is a sign of advanced male pattern baldness, but some men can progress beyond this and become completely bald.  You can easily monitor your hairline by looking in the mirror and comparing what you see to photographs of your younger self. The "M" shape pattern is a characteristic feature of male pattern baldness because hair in the temples (and crown) appear to be the most sensitive to DHT levels. Some people don't exhibit an "M" pattern, but more of a half-moon pattern, where the entire front hairline recedes back in unison and does not leave a "widow's peak." In addition to thinning and receding of the frontal hairline, the same process can occur on the top (crown) of the head. Sometimes balding of the crown precedes a receding hairline, sometimes it follows after, and sometimes it happens concurrently. As noted, the hair follicles at the crown of the scalp seems to be more sensitive to DHT levels — much more so compared to the hair follicles above the ears or at the very back of the scalp.  To check the crown of your head, you'll need to hold a handheld mirror above your head while looking into your vanity mirror. Alternatively, get your partner or a friend to take a photo of your crown. Compare photos over time to judge the extent of your hair loss. A sign from the front that may indicate thinning and hair loss at your crown is a widening side or middle part. Some hair loss on a daily basis is normal and it typically grows right back, but aggressive male pattern balding leads to very noticeable amounts of permanent shedding. Keep your pillowcase clean and note how much hair you lose while sleeping (take pictures to document it). If it's much more than a dozen or so hairs per night, that might be cause for some concern. If you use a brush, make sure it's free of hair before you use it and then examine it after your normal brushing routine. Brushing triggers more hair loss naturally (especially if your hair is long), but more than a few dozen hairs is not normal and is a sign of male pattern balding.  If you have dark hair, use a light-colored pillowcase to highlight hair loss. Conversely, use a dark-colored pillowcase if you have light hair. Using a hair conditioner while washing your hair might lead to fewer tangles, which might lead to less hair loss from brushing or combing. If you wear a ponytail, consider letting it out at night while you sleep. The tightness can lead to more hair loss as you turn from side to side in the night. Keep in mind that the early phases of male pattern baldness mainly involve thinning and shortening of hair, and not necessarily hair loss. Although male pattern baldness is the most common cause of hair loss in men (by far), there are some other causes that you should be familiar with, such as: endocrine gland (pituitary, thyroid) disorders, malnutrition (especially protein), fungal infection, iron deficiency, taking too much vitamin A or selenium, over-medicating (particularly retinoids and anticoagulants), and cancer treatments (chemotherapy, radiation).  Severe loss of hair from all areas of the scalp within a very short period of time is not male pattern baldness. It's likely related to environmental toxicity (such as lead poisoning), over-medication, high doses of radiation, or extreme levels of emotional trauma (shock or fright). If your hair loss is patchy and involves widespread scaling that spreads over your scalp, then you likely have a ringworm infection. Other symptoms include broken hair, skin swelling, redness and oozing. Rapid hair loss or any hair loss that occurs with symptoms such as itching, burning, or tenderness of the scalp is probably due to an underlying condition other than (or in addition to) male pattern baldness. Certain hair treatments, such as applying hot oil, dyes, or chemicals meant to straighten hair, can cause scalp damage and permanent hair loss. To be completely sure if you are affected by male pattern baldness, make an appointment with a hair loss specialist, who is likely a dermatologist or physician with special training. Typical male pattern baldness is usually diagnosed based on the appearance and pattern of hair loss. However, your doctor will also ask about your extended family (particularly on your mom's side) and carefully examine your scalp under magnification (with a device called a densitometer) in order to assess the extent of miniaturization of the hair follicles.  Hair analysis or a scalp biopsy are not needed to properly diagnose your hair loss. Your doctor should let you know about all the potential treatments for male pattern balding, not just medications or hair transplant surgery.

Summary:
Monitor your hairline. Check the crown of your head. Look for hair on your pillow and hairbrush or comb. Distinguish between MPB and other causes of hair loss. Consult with a hair loss specialist.