Disease-related hair loss
A variety of diseases and health conditions can be the cause of hair loss. If you notice hair loss, especially if it's sudden or you're experiencing other health concerns, it's best to seek out a medical professional in order to rule out any of the following diseases.

Fungal Infections
Ringworm, which is the common name given to any fungal infection on the skin, can cause hair loss and is especially prevalent among children. Since it's contagious, ringworm is also found in warm, moist places where people congregate -- such as locker rooms. In addition to causing a scaly, itchy rash, fungal infections oftentimes lead to hair loss in the surrounding areas.
To properly diagnose a fungal infection, a dermatologist will need to microscopically view skin scrapings. If ringworm is diagnosed, an oral or topical antifungal medication will likely be prescribed.

Thyroid Disease
For hair to grow properly, a healthy thyroid gland is a requirement in order produce the correct amount of hormones. Thus, hyperthyroidism (an overactive thyroid) or hypothyroidism (an underactive thyroid) can cause problems for your hair. Depending on the exact type of thyroid condition, sufferers will likely notice thinning hair or even complete hair loss in rare cases.
Your primary care physician can easily diagnose a thyroid condition with a simple blood test and refer you to an endocrinologist for treatment. Fortunately, once the thyroid disease is controlled and hormone levelsare normalized, any hair thinning or hair loss should reverse.

Anemia, specifically iron-deficiency anemia, can lead to a variety of health issues including hair loss. Iron-deficiency anemia, as the name suggests, is caused by an insufficient amount of iron in the bloodstream due to anything from a poor diet to internal bleeding. While this type of anemia is more common among women, it can be found in men as well. In fact, leading health agencies estimate that there are more than a billion people who suffer from iron deficiency.
Once the anemia is controlled and the body absorbs enough iron, hair loss issues eventually subside over time. Other health issues caused by anemia include tiredness, depression, anxiety, poor appetite and constipation.

According to the Lupus Foundation of America, 45 percent of people with lupus suffer hair loss. Lupus is an autoimmune disease characterized by a hyperactive immune system. Other than hair, lupus affects the skin, joints, heart, kidneys and lungs.
Since hair loss is one of the first symptoms of lupus, it's important to talk to your doctor if you notice hair falling out in patches. Once the disease is brought under control, most patients will experience a reversal of the hair loss.