Seborrhea is a scaly, oily skin condition on the scalp and face which can cause dandruff. It is also known as seborrheic eczema.
Its cause is known although the yeast, Malassezia, is probably involved. Aggravating factors may include stress, fatigue, seasonal changes, general poor health, nutritional deficiencies (especially biotin, vitamins B2 and B6, and too much vitamin A (in children).
Wikipedia points out that most seborrhea is probably an inflammation resulting from excessive malassezia yeast, which then produces toxic substances that irritate the skin. This is not proven, and it may be that the causes are more related to environmental, hormonal, and immune system factors.
Temporary or permanent hair loss is one possible result of seborrhea.
Dermatologists often recommend use of topical treatments such as shampoos and creams which include anti-fungal and other ingredients. However supplements such as probiotics lactobacillus casei, lacto ferrin, vitamin B7 (biotin), vitamins B2, B3, and B6, and zinc may be helpful.
Consult your dermatologist if you suspect you suffer from this condition.