The thyroid hormones affect nearly all tissues of the human body, including the skin. Skin tissues contain thyroid hormone receptors which, when occupied by thyroid hormones, stimulate cellular activity. Patients with hypothyroidism, or underactive thyroid disease, may experience skin symptoms that reflect diminished cellular skin activity in the absence of sufficient thyroid hormone. Some skin changes associated with hypothyroidism may cause itching.
Skin Dryness and Scaling
Up to 90 percent of patients with hypothyroidism experience skin dryness and scaling, according to the medical textbook "Werner & Ingbar’s The Thyroid." When examined under the microscope, the skin of patients with hypothyroidism exhibits thinning and increased keratin, a protein found in the uppermost layer of the skin and in the hair and nails. The increased keratin and impaired shedding of old cells gives the skin a scaly appearance.
Decreased production of certain fats in the skin may contribute to skin dryness with hypothyroidism. Skin dryness in patients with hypothyroidism often proves worst on the palms, soles, elbows and knees, although the entire skin surface is typically involved. Dryness and scaling of the skin commonly causes itching. In patients with severe hypothyroidism-related skin dryness and itching, persistent scratching may lead to abrasions and localized infection.
Some patients with hypothyroidism develop a severe form of skin scaling known as acquired ichthyosis. This condition represents a more advanced form of the scaling more commonly seen in patients with hypothyroidism. Itching frequently accompanies the scaling that characterizes acquired ichthyosis.
Patients with hypothyroidism may exhibit hives, or urticaria, which characteristically prove intensely itchy. In a 2006 article on the skin manifestations of hypothyroidism published in the "Indian Journal of Dermatology," Dr. Alka Dogra and colleagues report that approximately 16 percent of study patients exhibited hives. The authors note that the mechanism by which thyroid hormone deficiency provokes hives remains uncertain.
Dermatitis herpetiformis is a profoundly itchy, blistering rash that may occur in patients with hypothyroidism. Dermatitis herpetiformis — which is unrelated to the herpes viruses — typically affects the skin of the back, knees, elbows and buttocks on both sides of the body.