Skin rashes that develop on small sections of your body are usually caused by an external skin problem, such as an allergic reaction to a substance or material, localized skin inflammation or skin disease. Full body rashes, however, can often signal the presence of an internal problem. There are four main causes of full body rashes -- digestive system disorders, drug reactions, certain cancers and thyroid problems.
Digestive System Disorders
A number of digestive system problems can cause rashes to develop across the body. These disorders include celiac disease, liver disease, anemia related to iron deficiency and kidney problems.
Celiac disease is a disorder in which an individual's intestinal tract cannot absorb gluten protein from foods such as wheat, rye or barley. The immune system treats gluten as a dangerous foreign body and reacts by destroying the villi lining the intestines. The destruction of the villi causes the person to become deficient in certain nutrients, and to experience symptoms such as seizures, fatigue and a characteristic itchy rash covering the body.
Both liver and kidney problems can cause body rashes since both of these organs are responsible for filtering toxic substances from the bloodstream. If either aren't working properly, the substances can build up and cause persistent outbreaks across the body.
Iron deficiency anemia develops when the body has too few circulating red blood cells because of a lack of available iron. Again, the rash that can occur with this condition is thought to be due to a buildup of substances that the immune system reacts to.
There are a number of commonly prescribed medications that can cause allergic reactions that result in widespread rashes. These medications include both ingested and topical antifungal drugs, such as fluconazole, ketoconazole or clotrimazole; antibiotics like penicillin or cephalosporins; and certain pain medications such as any oxycodone drugs or duloxetine. Chemotherapy drugs may also cause a full body rash.
Immune System Cancers
Cancers that affect the immune system, particularly leukemia and lymphoma, can cause characteristic full body rashes. Individuals suffering from leukemia often have small red, rash-like spots on their skin caused by blood leaking from the capillaries that lie just under the skin. People with lymphoma do not develop these spots, but can suffer from extremely itchy, red skin.
A thyroid gland that produces too much of the thyroid hormone can lead to a condition known as Graves' dermopathy -- a widespread, lumpy skin rash. Similar to many of the other causes of full body rashes, this usually develops with the immune system attacks the thyroid gland, causing the balance of the hormones it produces to shift.