When i eat fish i get itchy scalp dry skin and thinning hair my doctor take prick test fish allergy but was negative i don't understand what is this?

Not all related: I can see itchy scalp as a possible allergy but eating fish cannot immediately cause you to have thinning hair and dry skin. This is related to a more chronic cause, did you get your thyroid checked? Did you see a dermatologist? You may have an unrelated skin condition.

May not be allergy: Allergic reactions generally occur immediately. Can get hives, angioedema, wheezing, runny nose, sneezing, nausea, vomitting, or diarrhea. Dry itchy skin is not a sign of an allergic reaction, but could be eczema flaring from ingestion of a triggering food.