An unhealthy scalp can cause inflammation that makes it difficult for hair to grow. Skin conditions that lead to hair loss include seborrheic dermatitis (dandruff), psoriasis, and fungal infections such as ringworm.
The symptoms: Seborrheic dermatitis causes the scalp to shed its skin, so you'll notice greasy, yellowish scales on your shoulders or in your hair. It may be the result of yeast called Malassezia, hormonal changes, or excess oil in the skin.
Psoriasis, an autoimmune condition that causes excessive skin cell turnover, produces a very thick white scale on the scalp that can bleed if pulled off. With ringworm, a fungus you contract by touching an infected person or animal, you'll notice red patches on your scalp, which may be diffuse, Jakubowicz says.