Xerosis: dry skin, frequently seen in the winter and also associated with older age, frequent bathing in hot showers or baths, and high-temperature and low-humidity environments. Other medical disorders
Diabetes mellitus, a group of metabolic diseases in which a person has high blood sugar
Hyperparathyroidism, overactivity of the parathyroid glands resulting in excess production of parathyroid hormone (PTH)
Iron deficiency anemia, a common anemia (low red blood cell or hemoglobin levels)
Cholestasis, where bile acids leaking into the serum activate peripheral opioid receptors, resulting in the characteristic generalized, severe itching
Malignancy or internal cancer, such as lymphoma or Hodgkins disease
Polycythemia, which can cause generalized itching due to increased histamines
Psychiatric disease ("psychogenic itch", as may be seen in delusional parasitosis)
Thyroid illness
Uraemia – the itching sensation this causes is known as uremic pruritus

Medication
Drugs (such as opioids) that activate histamine (H1) receptors or trigger histamine release
Chloroquine, a drug used in the treatment and prevention of malaria
Bile acid congeners such as obeticholic acid

Related to pregnancy
Gestational pemphigoid, a dermatosis of pregnancy
Intrahepatic cholestasis of pregnancy, a medical condition in which cholestasis occurs
Pruritic urticarial papules and plaques of pregnancy (PUPPP), a chronic hives-like rash

Other
Menopause, or changes in hormonal balances associated with aging
Terminal illness

Mechanism
Itch can originate in the peripheral nervous system (dermal or neuropathic) or in the central nervous system (neuropathic, neurogenic, or psychogenic). Dermal/pruritoceptive
Itch originating in the skin is known as pruritoceptive, and can be induced by a variety of stimuli, including mechanical, chemical, thermal, and electrical stimulation.
Itch (also known as pruritus) is a sensation that causes the desire or reflex to scratch. Itch has resisted many attempts to be classified as any one type of sensory experience. Itch has many similarities to pain, and while both are unpleasant sensory experiences, their behavioral response patterns are different. Pain creates a withdrawal reflex, whereas itch leads to a scratch reflex.Unmyelinated nerve fibers for itch and pain both originate in the skin; however, information for them is conveyed centrally in two distinct systems that both use the same nerve bundle and spinothalamic tract. Classification
Most commonly, an itch is felt in one place. If it is felt all over the body, then it is called generalized itch or generalized pruritus.If the sensation of itching persists for six weeks or longer, then it is called chronic itch or chronic pruritus. Chronic idiopathic pruritus or essential pruritus is a rare form of itch that persists for longer than six weeks, and for which no clear cause can be identified. Signs and symptoms
Pain and itch have very different behavioral response patterns. Pain elicits a withdrawal reflex, which leads to retraction and therefore a reaction trying to protect an endangered part of the body. Itch in contrast creates a scratch reflex, which draws one to the affected skin site. Itch generates stimulus of a foreign object underneath or upon the skin and also the urge to remove it. For example, responding to a local itch sensation is an effective way to remove insects from ones skin.